Chapter 3: Bioenergetics, Enzymes & Metabolism

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Welcome to the Deep Dive, the shortcut to being truly well -informed.

Today, we are pulling back the curtain on, well, the fundamental process of life itself,

how a cell turns chaos into order.

We're jumping straight into the engine room.

Our source is chapter three of carp cell and molecular biology, and it's all about bioenergetics, enzymes, and metabolism.

And the whole thing starts with this incredible paradox.

Just think about it for a second.

You, listening right now, might be on a high protein diet.

Or you could be vegan or low carb.

The wildly different from person to person.

Exactly.

And yet,

despite that constant flux that variability in what you eat, every single one of your cells maintains this astonishingly stable, constant internal environment.

That's the central question the chapter poses right at the start.

The title of the whole section is, you are not what you eat.

If our diets are so different, but our cells look and function the same, there has to be this incredibly complex and tightly controlled process that funnels everything down.

It takes all these raw materials and reduces them to a single essential currency.

And that currency is, of course, ATP, adenosine triphosphate.

We need something like 2000 kilocalories of energy a day just to keep the lights on, and every bit of it gets processed through this one molecule.

So the big takeaway isn't that you are what you eat.

It's that you are what your enzymes make you.

That vast web of reactions, taking all that food and turning it into you and ATP, that's defined by these tiny molecular machines.

So that's our mission for this deep dive.

We're going to map that whole system out.

We'll start with the hard rules of physics bioenergetics, which tells us what's even possible.

Then we get into the enzymes, which dictate how fast things happen.

And finally, we'll trace the actual flow of energy through metabolism using glucose oxidation as our main example.

And we'll be following the source material exactly, describing all the key figures and concepts as we go.

It's a system of incredible organization.

The book uses this perfect metaphor, Giuseppe Arsamboldo's painting, Rudolf II, as vertumnus.

Oh, the one where the face is made of fruits and vegetables.

That's the one.

It's a collection of all these completely separate, unrelated pieces, you know, the amino acids, the sugars, the fats and metabolism organizes them into this complex, recognizable living whole.

So let's start building that portrait.

Okay, let's start with the fundamental physics,

bioenergetics.

It's basically the study of how living things transform energy.

And energy is, well, just the capacity to do work to make something happen.

And you can't talk about that without talking about thermodynamics.

These are the absolute laws.

They predict which way a reaction will go and whether it's going to need energy to get started.

But this is a key point.

They tell us nothing about how fast it will happen or the mechanism that's for the enzymes.

Exactly.

So the first rule is the first law of thermodynamics, the law of conservation of energy.

Pretty simple, right?

Energy can't be created or destroyed.

It can only be converted.

The technical term is transduced and energy transduction is happening literally everywhere.

A wind turbine converts mechanical energy to electrical.

A battery converts chemical to electrical.

And in biology, it's just, it's amazing.

The source gives a bunch of examples in figure 3 .1.

You've got the chemical energy and ATP being converted to mechanical energy for muscle contraction.

Or an electric eel converting that same chemical energy into, what, 500 volts of electricity?

Or a firefly, which I think is the coolest, converting it directly into light by luminescence.

But the big one, the one that powers almost all life on earth, is photosynthesis.

That's the ultimate transduction, converting light energy from the sun into stable chemical energy and sugar.

So to study this stuff, scientists divide the universe into two parts.

The system, which is what you're looking at, say, a single cell or a leaf.

And the surroundings, which is everything else.

The energy inside that system is its internal energy, or E.

And the change in that energy, E, is defined by the heat exchanged, Q, and the work done, W.

The formula is E, E, you'll Q, W.

A reaction that loses heat is exothermic.

One that gains heat is endothermic.

Figure 3 .2 in the text goes a great example with a leaf.

During the day, the leaf is the system.

It's absorbing sunlight, taking in CO2, so its internal energy is increasing.

It's an endothermic process.

Then at night, it flips.

It starts oxidizing those carbohydrates it made, releasing energy to power its own processes.

Its internal energy decreases, and it becomes exothermic, releasing heat.

So the first law is about accounting for the energy.

But the second law of thermodynamics is what gives life direction.

It tells us that events always proceed downhill, from a state of higher energy to a state of lower energy.

It's why perpetual motion machines are possible.

You always lose some energy.

And that lost energy is related to this concept of entropy, right?

That's the measure of disorder.

Exactly.

The second law says that for any spontaneous event, the total entropy or disorder of the universe must increase.

The amount of energy that becomes unavailable for work is actually quantified as T, where T is the temperature.

The book uses a great visual in Figure 3 .3,

a sugar cube dissolving in water.

You start with this highly ordered crystalline structure.

And it spontaneously dissolves into a completely disordered random solution.

That's a huge increase in entropy.

But what if you wanted to reverse it to get that crystal back?

You'd have to put in a lot of energy.

You'd have to evaporate the water, for instance.

And that process, turning liquid water into highly disordered water vapor, actually increases the entropy of the surroundings even more than you decrease the entropy of the sugar.

The universe's net disorder still goes up.

So this is always the big question in biology.

How are we as these incredibly ordered complex systems not violating the second law?

We seem to be fighting against entropy.

We aren't violating it at all.

We are masters of increasing the entropy of our surroundings.

We maintain our own high internal order, our complex DNA sequences, our folded proteins, by constantly taking in ordered low entropy food like glucose.

And breaking it down into highly disordered, high entropy waste products like CO2 and water, and releasing a lot of heat.

Right.

We are little pockets of water that only exist by creating a much bigger mess of disorder around us.

Okay.

So to bring both laws together, we use the Gibbs free energy equation, KG AOS Chi.

And this AG, the change in free energy, is the real predictor.

It's the energy that's actually available to do work.

It's the bottom line.

If AG is negative, the reaction is spontaneous.

It's exergonic.

It proceeds downhill.

If AG is positive, it's non -spontaneous.

It's endergonic, and you have to put energy in to make it happen.

So that HST's term, the enthalpy, that's mostly about the energy and the chemical bonds themselves.

Yes, the change in the total energy content.

So let's look at the classic ice water example to see how it all balances.

When water freezes into ice, it forms more stable hydrogen bonds.

That's favorable, so the H is negative.

Ice is more ordered than liquid water.

So the entropy change, AG, is also negative, which is unfavorable.

Exactly.

So which one wins?

It all depends on the temperature, the T, and the equation.

As table 3 .1 shows, if the temperature is low, like below zero degrees Celsius, that favorable negative H term is bigger than the unfavorable TST term.

The result is a net negative H, and freezing is spontaneous.

But if you raise the temperature, the TST term gets bigger and eventually overwhelms the H.

The T becomes positive, and freezing is no longer spontaneous.

The second law wins.

Okay, so let's apply this to chemical reactions.

The law of mass action says that reactions proceed toward equilibrium, a state where the forward and reverse reaction rates are equal.

And the ratio of products to reactants at that point is the equilibrium constant, Keek.

Right.

If Keek is greater than one, the reaction favors making products.

The text also points out a specific type, the dissociation

This is for when two things, say A and B, bind to form a complex, AB.

And Kd is just the concentration of the three parts divided by the concentration of the complex.

Yes.

And it's super useful in research because it's an inverse measure of affinity.

A really small Kd means you have a lot of the complex and not much of the free stuff.

It means the two molecules have a very tight binding affinity.

Okay, so to compare different reactions on an equal footing, we use the standard free energy change, or G degree.

This is the EG get under very specific standardized conditions.

25 degrees Celsius, pH of 7 .0s, and all concentrations at a whopping 1 .0 molar.

Which never happens in a cell.

Never.

But it gives us a fixed baseline for comparison.

And it's directly related to the KC.

As table 3 .2 shows, if a reaction's K is greater than one, its D degrees is automatically negative.

It's a handy reference.

But this next part, this is what I think is the most mind blowing concept in this section.

The difference between that standard D degrees and the actual operational DG inside a living cell.

This is absolutely critical.

Because concentrations in the cell are not 1 .0 molar, we have to use the full equation.

DG degrees plus 2 .303 RT log of the product -reactant ratio.

That second term is a correction factor for the real -world conditions inside the cell.

So let's use the most important example.

ATP hydrolysis.

That's ATP breaking down into phosphate.

Its standard free energy change, the drudgy degrees, is already very favorable.

It's never in a 7 .3 kilocalories per mole.

And part of the reason for that is you're relieving the electrostatic repulsion.

ATP has three negatively charged phosphate groups all crammed together.

They really want to get away from each other.

But now, let's factor in the real cellular concentrations, which are shown in figure 3 .5.

ATP is kept high, around 10 millimolar.

ADP is low, maybe 1 millimolar.

And phosphate is around 10 millimolar.

So that concentration ratio in the equation is very small, which makes the logarithm a large negative number.

And what does that do to the actual BB?

It makes it way more negative.

The actual usable energy the cell gets from ATP hydrolysis isn't saying in a 7 .3.

It's closer to negative 11 .5 kilocalories per mole.

So the cell is actively manipulating concentrations to squeeze almost 60 % more power out of every single ATP molecule.

That's it.

It's holding the entire system incredibly far from equilibrium.

And that extra energy is what powers everything.

So how does it use that power to drive the reactions that are unfavorable, the endergonic ones?

Two main ways.

First, you can just remove the product of a reaction as soon as it's made.

That keeps the concentration ratio favorable and can be enough to pull a reaction forward, even if its energy is positive.

But the more important way is coupling.

Yes.

You take the highly exergonic energy from ATP hydrolysis and directly link it to an endergonic reaction you need to run.

The synthesis of the amino acid glutamine is the classic example.

Right.

Making glutamine is endergonic.

It has an EG degree of plus 3 .4 kilocalories per mole.

It won't happen on its own.

But if you couple it to ATP hydrolysis with its negative 7 .3 kilocalories per mole, the overall combined reaction has a very favorable D degrees of negative 3 .9.

And the key to making that link work is the common intermediate.

Exactly.

The glutamic acid doesn't just react with ammonia.

First, ATP transfers a phosphate to the glutamic acid, creating a high energy intermediate called glutamyl phosphate.

Then in a second step, ammonia kicks off the phosphate to form glutamine.

The energy is transferred directly through that intermediate.

And all of this only works because the cell is fighting to maintain disequilibrium.

If it ever reached equilibrium, the concentration of ADP would be about 10 million times higher than ATP.

And at that point, the cell would have zero capacity to do any work.

It would be dead.

The fact that the cell keeps the ATP to ADP ratio high, maybe 10 or 100 to 1, is the definition of being alive.

And figure 3 .6 just lists all the kinds of work ATP does.

Creating electrical charge separation, concentrating solutes, sliding muscle filaments, activating proteins through phosphorylation.

It's everything.

Which leads to the final point in this section.

A living cell is an open system.

Matter and energy are constantly flowing through it.

So it never reaches equilibrium.

Correct.

Instead, it exists in a steady state.

As figure 3 .7 illustrates, a steady state is when the concentrations of all the intermediates in a pathway remain constant, but the individual reactions themselves are not at equilibrium.

There's a constant flow.

It's a dynamic, ordered disequilibrium.

True equilibrium only happens at death.

Okay, so that's the what can happen of thermodynamics.

Now we move to the what does happen and how fast.

The enzymes.

These are the biological catalysts.

The history is pretty cool.

For a long time, people thought something like fermentation needed a whole living cell, some kind of vital force.

Until the Bickner brothers in 1897.

Right.

They ground up yeast cells, made this yeast juice, and showed it could still ferment sugar into alcohol.

They proved the catalysts were just chemicals, which they called enzymes.

And most enzymes are proteins, though we now know about catalytic RNA molecules called ribozymes.

Many of them are also conjugated proteins, meaning they need a non -protein helper to function.

This could be a metal ion called a cofactor, or a complex organic molecule, a coenzyme, which are often derived from vitamins like NAD plus or FAD.

The properties of enzymes are just staggering.

You only need a tiny amount.

They are totally reusable.

They're incredibly specific for their target molecule, the substrate.

And this is so important.

They have zero effect on the thermodynamics.

They don't change the EG.

They only change the rate.

And when you say change the rate, we're not talking about a small boost.

We're talking about increases of 10 to the eighth to 10 to the 13th fold.

Some are even faster.

The book mentions one OMP decarboxylase that speeds up a reaction that would normally take 78 million years and makes it happen in a fraction of a second.

So how do they do that?

It's all about overcoming the activation energy or EA.

Right.

The EA is that energy barrier you have to climb to get to the transition state, that unstable fleeting moment when chemical bonds are actually breaking and forming.

Figure 3 .8 shows this as a kind of hump on the energy graph.

So it's like just heating things up gives more molecules enough kinetic energy to randomly make it over that hump.

Exactly.

But as figure 3 .9 shows, an enzyme doesn't do that.

An enzyme grabs the substrate and physically lowers the height of the hump itself.

It makes the transition state easier to reach.

And it does that by binding more tightly to the unstable transition state than it does to the stable starting substrate.

It stabilizes that fleeting high energy arrangement.

All of this catalytic action happens at a very specific place on the enzyme called the active site.

The enzyme and substrate form an ES complex, usually through non -covalent bonds.

Looking at figure 3 .11, the active site isn't just a simple pit.

It's often a deep hydrophobic cleft or crevice.

And it's formed by amino acids that might be really far apart in the primary chain but are brought together by the protein's tertiary folding.

And that very specific 3D geometry is what gives enzymes their incredible specificity.

So let's get into the nitty gritty.

Figure 3 .12 shows three main mechanisms of catalysis.

What's the first one?

The first is substrate orientation.

If a reaction involves two different substrates, the enzyme's active site will bind both of them and hold them in the absolute perfect position relative to each other for the reaction to occur.

It eliminates the randomness.

It takes away the entropic cost of getting them to meet just right.

Precisely.

The second mechanism is changing reactivity.

The amino acid side chains in the active site can be acidic or basic.

They can donate or accept protons, which changes the electron distribution in the substrate and makes it much more reactive.

We see this in the example of chymotrypsin in Figure 3 .13.

This is an enzyme that cuts peptide bonds.

Yes, and it does it in two steps.

In its active site, a serine residue is made much more reactive because a nearby histidine pulls a proton off of it.

This super reactive serine then attacks the substrate, forming a temporary covalent bond.

That's the intermediate.

That's the intermediate.

And the first half of the substrate is released.

Then, in step two, a water molecule comes in, helped by that same histidine, and it breaks the covalent bond, releasing the second product, and regenerating the enzyme so it's ready to go again.

It's a beautiful little machine.

Okay, and the third mechanism.

Inducing strain.

This is linked to the idea of induced fit.

The old model was lock and key, but we now know that when a substrate binds, the enzyme often changes its shape.

It clamps down on it.

It climbs down.

As it does, it can physically bend or stretch the substrate, putting strain on specific bonds and pushing it toward the geometry of the transition state.

Figure 3 .14, showing hexokinase binding to glucose, is the perfect visual for this.

You can see the protein's two lobes closing around the glucose molecule, like a jaw.

It aligns all the chemical groups perfectly.

In modern techniques like time -resolved crystallography, let us actually see this happening.

Scientists can use these incredibly powerful x -ray beams and laser flashes to essentially create a molecular movie of the enzyme in action.

They can capture these structural changes that happen in picoseconds.

Yeah.

The book describes an experiment with myoglobin, shown in Figure 3 .16.

They knock a carbon monoxide molecule off with a laser and then watch how the protein structure shifts in response.

Residues like phenylenine and histidine literally rotate and move within 100 picoseconds.

It shows that proteins are not rigid structures.

They're constantly in motion, and that motion is essential for their function.

OK, let's talk about enzyme kinetics.

This is the math of how fast enzymes work.

If you plot the initial reaction velocity, V, against the substrate concentration, S, you get this classic hyperbolic curve shown in Figure 3 .1 sieve.

It starts up fast, then levels off.

It levels off because the enzyme becomes saturated.

All the active sites are full and working as fast as they can.

That maximum rate is called Vmax.

And from Vmax, you can calculate the turnover number, or K -cap.

Which is the maximum number of reactions a single enzyme molecule can perform per second.

And as you can see in Table 3 .3, these numbers can be huge, hundreds of thousands per second for some enzymes.

The other key number is the Michaelis constant, KRM.

That's the substrate concentration you need to get to half of Vmax.

KM is a really useful measure of an enzyme's affinity for its substrate.

A high KMM means you need a lot of substrate to get the enzyme working, so that indicates a low affinity.

A low KMM means high affinity.

And to determine these values more accurately, scientists use a Lineweaver -Burk plot.

Right.

As shown in Figure 3 .18, it's a mathematical trick.

You plot the reciprocals, 1 over V versus 1 over S, and it turns that hyperbola into a straight line.

The intercepts of that line give you very precise values for Vmax and KM.

We should also mention, as Figure 3 .1 timeshows, that enzyme activity is highly sensitive to things like pH and temperature.

They have optimal ranges.

Which is why putting food in the refrigerator works.

You're not killing the microbes.

You're just lowering the temperature to dramatically slow down the activity of their spoilage enzymes.

Alright, let's wrap this section with enzyme inhibitors.

These are molecules that are critical for regulation and, of course, for medicine.

They fall into two main camps,

irreversible and reversible.

Irreversible inhibitors bind super tightly, often covalently, and they just kill the enzyme.

Penicillin is a great example.

It irreversibly inhibits a bacterial enzyme needed to build their cell walls.

And the feature box on the growing problem of antibiotic resistance really drives home how important this is.

It's a genuine public health crisis.

There's just not much financial incentive for drug companies to develop new antibiotics, so our arsenal is dwindling.

And these drugs usually target one of three bacterial enzyme systems.

The first, as you said, is cell wall synthesis.

Penicillin hits the transpeptidase enzyme.

But another drug, vancomycin, works differently.

It doesn't bind the enzyme.

It binds to the Dialis substrate itself, so the enzyme can't grab it.

But bacteria evolve.

MRSA is a huge problem.

And now we're seeing vancomycin resistance, where bacteria have acquired genes to make a different peptide that vancomycin can't recognize.

What's the second target?

Genetic systems.

Things like bacterial ribosomes or the enzymes for DNA replication.

The drug Cipro, Aquinalone, inhibits an enzyme called DNA gyrase, which bacteria need to unwind their DNA.

Metabolic enzymes.

Sulfa drugs, for instance, are mimics of a molecule called PEBA.

They block an enzyme that bacteria need to make folic acid.

We get folic acid from our diet, so the drug is specific to the bacteria.

And bacteria fight back with all sorts of mechanisms.

They can evolve enzymes like mutlactamase to destroy penicillin.

They can install pumps to spit the drug out.

Or they can mutate the target enzyme so the drug doesn't bind as well.

It's a constant arms race.

It's why treating something like HIV, whose reverse transcriptase enzyme mutates incredibly fast, requires a cocktail of different drugs.

Okay, so what about reversible inhibitors?

These bind more loosely.

The first type is competitive inhibitors, shown in Figure 3 .20.

They look like the substrate, so they compete for the same spot in the active site.

But if you just swamp the system with enough of the real substrate, you can outcompete the inhibitor and still reach the original VMAX.

Correct.

Kinetically, as you can see in Figure 3 .21, a competitive inhibitor increases the apparent KM.

It makes the affinity seem lower.

But VMAX is unchanged.

The drug Captopril for high blood pressure is a competitive inhibitor of an enzyme called ACE.

And the other type is non -competitive inhibitors.

These guys don't bind at the active site.

They bind somewhere else, at an allosteric site.

This binding changes the enzyme shape and makes it less effective.

So adding more substrate does nothing to stop it.

Nothing.

It's not a competition.

Kinetically, a non -competitive inhibitor lowers the VMAX.

It's like you have less functional enzyme, but it doesn't change the KM of the enzymes that are still working.

An HIV drug, Tepronivir, is a non -competitive inhibitor.

All right, now we get into the map itself.

Metabolism.

We split all these reactions into two big categories.

Catabolism and anabolism.

Catabolism is about breaking things down disassembly.

It releases energy, ATP, and raw materials.

Anabolism is about building things up synthesis.

And it costs energy.

And as figure 3 .22 shows, these pathways have a certain flow.

Catabolic pathways are convergent.

You start with lots of different molecules.

Fats, proteins, carbs.

And they all get funneled down into just a few common intermediates.

While anabolic pathways are divergent, they start from those few common precursors and build out into the thousands of different molecules a cell needs.

Central to all of this are redox reactions, oxidation, and reduction.

It's all about moving electrons.

In organic molecules, we can think of it in terms of bonds to carbon.

Right.

As figure 3 .23 shows, a carbon bonded to hydrogen is in a reduced state.

A carbon bonded to oxygen is in an oxidized state.

And the more reduced a molecule is, like a fatty acid or methane, the more chemical energy it holds.

The final most oxidized state of carbon is CO2.

So our main example is the oxidation of glucose.

If you burn it completely, you get a massive 686 kilocalories per mole, which is enough energy to make up to 36 ATP.

To capture that energy efficiently, the cell does it in many small controlled steps.

And the first 10 steps are called glycolysis.

This pathway is anaerobic, no oxygen needed, and it happens in the cytosol.

It takes one glucose and breaks it into two molecules of pyruvate.

And looking at the energetics of the pathway in figure 3 .25, there's a really important regulatory feature.

Of the 10 reactions, only three are far from equilibrium.

They have large negative pi by g values.

So those are steps 1, 3, and 10.

These are the irreversible steps, and they're the ones that give the whole pathways forward direction.

They pull everything along.

And because they're irreversible, they become the key control points for regulation.

Exactly.

Now, glycolysis actually starts with an energy investment.

The cell has to spend two ATPs up front to phosphorylate the sugar.

This does two things, right?

It activates the molecule, makes it more reactive, and it also traps it.

The phosphorylated sugar can't get back out through the cell membrane.

Correct.

The energy payback comes later.

Specifically, when glyceraldehyde -3 -phosphate gets oxidized, this step generates energy in two ways, as shown in figure 3 .26.

First, it's a redox reaction.

The substrate is oxidized, and the electrons are transferred to the coenzyme NAD, plus AD, reducing it to NADH.

And NADH, which is derived from the vitamin niacin, is the cell's main currency of reducing power for catabolism.

It's a high -energy molecule that will carry those electrons off to the electron transport chain to make a lot of ATP later.

Okay, what's the second way energy is made?

Subscrate -level phosphorylation.

This is where a high -energy phosphate group is transferred directly from a substrate molecule, like 1 .3 -bisphosphoglycerate, straight to an ADP molecule to make an ATP.

No oxygen involved.

And the reason this works thermodynamically is something called phosphate transfer potential.

Yeah, if you look at figure 3 .28, it's a scale of how willingly different molecules give up their phosphate.

Intermediates like phosphenolpyruvate are way higher up the scale than ATP.

They have a much more negative D of hydrolysis.

So they have a really low affinity for their phosphate group.

Exactly, which means they can easily donate it downhill to ADP to make ATP.

So after all is said and done, what's the net profit from glycolysis?

You spend two ATP and you make four, so the net gain is two ATP for every one glucose that's converted to two pyrodates.

But glycolysis is anaerobic, which creates a problem.

You're constantly turning NAD +, into NADH.

If there's no oxygen around for the NADH to dump its electrons onto.

You run out of NAD +, and if you run out of NAD +, glycolysis grinds to a halt.

So the solution is fermentation.

Yes, fermentation's only job, as shown in figure 3 .29, is to regenerate NAD +, from NADH so that glycolysis can keep going.

It does this by dumping the electrons from NADH back onto pyruvate.

In our muscles, that turns pyruvate into lactate.

In yeast, it turns it into ethanol and CO2.

It solves the immediate NAD +, problem, but it's incredibly inefficient.

You're throwing away more than 90 % of the energy that was in the glucose molecule.

Now we have to make a clear distinction between NADH and another very similar molecule, NADPH.

They look almost identical, but they have completely separate jobs.

NADH is for catabolism.

It's carrying electrons to be cashed in for ATP.

NADPH, which has an extra phosphate group, is the cell's primary source of reducing power for anabolism for building things.

So why have two different pools?

Why not just use one?

It's all about regulation and separation.

When you're building complex molecules like fats, you need high -energy electrons, and that's what NADPH provides.

By having two different coenzymes and different sets of enzymes that only recognize one or the other, the cell can keep its breakdown and build up pathways from interfering with each other.

Given how little ATP is actually in a cell at any one time, its production and use has to be incredibly tightly regulated.

And as we said, that regulation happens at those three irreversible glycolytic steps.

And the cell uses two main tools.

The first is covalent modification.

This usually means a protein kinase enzyme comes along and attaches a phosphate group to the target enzyme, changing its shape and switching it on or off.

And the second tool is allosteric modulation.

Right.

This is where a small molecule, a modulator, binds to an allosteric site on the enzyme, not the active site.

And that binding changes the protein's conformation and affects its activity.

The classic example of this is feedback inhibition, shown in figure 3 .30.

It's the most logical way to regulate.

If you have a long metabolic pathway, the final end product of that pathway will often come back and allosterically inhibit the very first enzyme that's unique to that pathway.

As soon as you have enough of the product, you stop making more right at the source.

You don't waste any energy or resources.

Exactly.

This regulation is also critical for controlling opposing pathways, like breaking down glucose, glycolysis, versus making new glucose, gluconeogenesis.

Since glycolysis has those three irreversible steps, you can't just run it backwards to make glucose.

Right.

Gluconeogenesis has to use a different set of enzymes to bypass those three roadblocks, as you can see in figure 3 .31.

These bypass reactions are themselves highly favorable.

And having different enzymes for the forward and reverse directions allows for independent reciprocal control.

Precisely.

A molecule like ATP, which signals high energy, will inhibit the key catabolic enzyme in glycolysis.

But a molecule like AMP, which signals low energy, will activate that same enzyme.

The opposite is true for the anabolic gluconeogenesis enzymes.

And the master energy sensor in the cell is a kinase called AMP -activated protein kinase, or AMPK.

When the ratio of AMP and ADP to ATP gets high, that's a red alert for low energy.

AMPK gets activated and it immediately starts phosphorylating a whole host of enzymes.

It shuts down energy -consuming anabolic pathways and cranks up energy -producing catabolic pathways.

AMPK is so central to metabolism that it's a major drug target.

Metformin, which is a really common drug for type 2 diabetes, works in part by activating AMPK.

And this all leads into the human perspective box on caloric restriction and longevity.

It's a fascinating link.

Yeah, the data from model organisms like yeast, worms, and rats is pretty clear.

Reducing their caloric intake by, say, 20 or 30 percent significantly increases their average and maximum lifespan.

The survival curve for nematode worms in the chapter's figure is striking.

The calorie -restricted worms just live longer and healthier.

Now, the results in primates have been a bit more mixed on extending maximum lifespan.

But there's a consensus that it improves all the key health markers.

Better insulin sensitivity, lower blood glucose.

It highlights this deep, deep connection between how we manage our metabolism and the aging process itself.

Okay, let's take a quick look at plants.

Their metabolism has an extra layer of regulation because it's completely governed by the light -dark cycle.

They have to undergo this massive metabolic shift twice a day.

During the day, it's all about photosynthesis and storing energy.

At night, they have to switch over to burning those stores to survive.

But what's really amazing is that it's not just a reaction to light.

It's anticipatory.

Plants have internal circadian rhythms.

It's a protein -based clock that cycles on a roughly 24 -hour period, even if you keep the plant in constant darkness.

And that clock gets synchronized or entrained by the actual sunrise and sunset.

The advantage is pure efficiency.

It takes time to build all the protein machinery for photosynthesis.

The internal clock allows the plant to anticipate sunrise and start making all that stuff ahead of time.

So the second the sun comes up, it's ready to go at maximum capacity.

That's incredible.

Okay, for our last section, let's bring all this molecular detail right into a clinical application,

using metabolism to find tumors.

This is the engineering linkage, using PETE scanning.

The problem with things like x -rays is they just see physical density.

But what if a tumor is metabolically different?

Which they often are.

This is the Warburg effect.

Many cancer cells have a dramatically increased rate of glycolysis.

They just guzzle glucose like crazy.

So we can exploit that.

In positron emission tomography, or PETE scanning, we inject the patient with a radioactive tracer called FDG.

It's basically a glucose molecule with a radioactive fluorine atom attached.

So the cancer cells, with their high metabolic rate, gobble it up.

They gobble it up.

And the first enzyme of glycolysis, hexokinase, phosphorylates it, just like it would with glucose.

But here's the trick.

The FDG molecule is slightly different.

And the next enzyme in the pathway can't recognize it.

So it gets trapped inside the cell.

It's a one -way ticket.

So the highly metabolic cancer cells accumulate huge amounts of this radioactive tracer.

How do we see it?

The radioactive fluorine, 18F, emits a particle called a positron.

When that positron immediately collides with a nearby electron, they annihilate each other and release two gamma rays that shoot off in exactly opposite directions.

Ah, and the PET scanner is designed to detect those paired simultaneous gamma rays.

Exactly.

A computer can then triangulate the origin point of those rays with incredible precision.

And as you can see in the brain scan in figure 3 .33, the tumor lights up like a Christmas tree because of its intense metabolic activity.

It gives doctors information that goes way beyond just the tumor's size.

So we've really covered a lot of ground today.

We started with the absolute laws of physics with AGG and discovered that the actual power of ATP in the cell is this massive mannecavabinum 0 .5 kilobolcal mol, all because the cell maintains a state far from equilibrium.

Then we saw how enzymes are these molecular wizards, increasing reaction rates by unbelievable amounts by lowering activation energy, using things like induced fit and transition state stabilization.

We followed the energy through glycolysis, pointing out those three key irreversible steps for control and the different jobs of NEDH and NEDPH.

And then we saw how it's all controlled from feedback inhibition all the way up to the master sensor AMPK, which connects metabolism to everything from antibiotic resistance to imaging tumors with the Warburg effect.

And the big picture that emerges is that our bodies, our very lives, are defined by these incredibly ancient, highly conserved metabolic pathways.

Glycolysis is basically the same in us as it is in a bacterium.

Which brings us to a final provocative thought for you to consider.

If the core machinery glycolysis, the TCA cycle, these fundamental catabolic and anabolic pathways are shared across almost all life on earth,

then what percentage of our genome is actually dedicated just to creating the subtle species -specific tweaks, the small variations in enzymes and the unique regulatory networks that make a yeast cell fundamentally different from a human cell?

How much variation does it really take to create all the complexity we see around us?

Something to explore on your own.

Thank you for joining us for this deep dive into bioenergetics, enzymes, and metabolism.