Chapter 7: Enzymes: Mechanism of Action

Welcome to Last Minute Lecture.

This free chapter overview is designed to help students review and understand key concepts.

These summaries supplement, not replace, the original textbook and may not be redistributed or resold.

For complete coverage, always consult the official text.

We are jumping right into what are basically the absolute fundamental machines of life, enzymes.

Right.

These are the molecular superheroes, you know, the catalysts that allow every single biochemical reaction from synthesizing DNA to just getting energy from your coffee to happen on a time scale that actually sustains life.

And our mission in this deep dive is to move past the, well, the simple definition.

We're going to unpack the sheer genius of their mechanism, how they are regulated and critically why knowing these details is so central to understanding disease.

Okay.

So let's start right there with the biomedical why.

Almost all enzymes are proteins, but we should probably give a nod to the outliers.

Oh yeah, the catalytic RNA molecules or ribozymes,

a huge clue in evolutionary history.

But in the human body, why do these enzyme changes matter so much?

Because disease is so often a direct result of changes in enzyme quantity or, you know, their activity.

Think about genetic defects, nutritional problems, or just severe tissue damage.

The machinery is broken.

Machinery is broken.

Or in some cases, it's actively sabotaged.

Yeah.

Right?

Like with bacterial toxins.

A classic terrible example is vibrio cholerae.

When that pathogen releases its toxin, it's not just random damage.

The toxin is actually an enzyme itself.

It transfers.

It does something specific.

Very specific.

It performs ADP ribosylation.

So it covalently modifies a host G protein and essentially locks that protein into an on state that one single enzymatic act just completely overwhelms the body's regulatory balance.

Wow.

So that immediately links the mechanism that transfers activity directly to a devastating clinical outcome.

But beyond the body, the power of enzymes is, I mean, it's everywhere.

The renin we use for cheese, the proteases and amylases and laundry detergent breaking down stains.

It's chemistry on demand.

And the central theme, no matter where they are, is radical efficiency paired with radical specificity.

We're talking about enhancing reaction rates by factors of, what, 10 to the sixth or more?

A million times faster.

Or more.

And they do this without being consumed or permanently changed.

They're the ultimate reusable tools.

And that specificity, the stereospecificity, is almost mind boggling.

How can an enzyme choose between two molecules that are just mirror images of each other, like a D sugar versus an L sugar?

It all comes down to structure.

Specifically, something called the three points of attachment model.

Okay.

Imagine the enzyme's active site is shaped to bind the substrate at three very distinct nonsymmetrical points.

If the enzyme can only bind one of those stereoisomers that way, it guarantees that only one specific side of that molecule is exposed for the reaction.

So even if the starting material, say, isn't chiral, locking it in that way predetermines the final product's chirality.

Exactly.

Take pyruvate.

You lock it in with those three points, and it can only be reduced in a way that produces exclusively L -lactate.

You never get a random or racemic mixture.

And that precision is what lets the cell run thousands of different reactions at once without them all interfering.

It's the difference between a high -tech assembly line and just a chaotic workshop.

So since that specificity is so critical, let's talk about how we even catalog this huge library of enzymes.

I know the early names were confusing.

Peptin.

Trypsin.

It was chaos.

Total chaos.

That's why the IUB system, the International Union of Biochemistry, became necessary.

It gives every enzyme a unique name and an EC number.

And that number is based on the type of reaction.

Exactly.

Not just what molecule it acts on.

So the EC number is like a map.

If we take hexokinase, its number is EC 2 .7 .1 .1.

That first digit, the 2, tells you what to transphrase.

It moves a functional group.

Right.

And transphrases, along with ligases, which are class 6, they're the ultimate regulators.

How so?

Well, transphrases are often adding phosphoryl groups from ATP, which drives all the signal transduction.

And ligases join molecules together using ATP, building things like DNA.

The classification immediately tells you where that enzyme sits in the cell's chain of command.

Okay, let's talk about the assistants, the little helper molecules that give enzymes their chemical power.

What's the real difference between a prosthetic group, a cofactor, and a coenzyme?

Start with prosthetic groups.

These are permanent.

Tightly, stably bound to the protein, sometimes covalently,

they define the enzyme.

Like FAD or FMN?

Or metal ions?

Like iron or zinc?

If an enzyme has a tightly bound metal ion like that, we call it a metalloenzyme.

And those ions are critical.

They act as Lewis acids or bases, making the substrate much more reactive.

And cofactors are the opposite.

Pretty much.

They're transient, they bind weakly, and then dissociate after the reaction.

These are usually metal ions that are just present in the environment, and we call the enzyme metal -activated.

They aren't part of its permanent structure.

And then the most famous ones, coenzymes.

The delivery service.

The perfect term for them.

They're molecular shuttles designed to be recycled.

Many of them are derivatives of B vitamins like nicotinamide and NAD and NADP.

And their unique job is to - Two things, really.

First, they stabilize highly reactive chemical species the cell couldn't handle otherwise, like the hydride ion carried by NADH.

Second, they act as stable carriers for small groups, like the acetyl group on coenzyme A.

They basically increase the contact points for transferring these little molecular pieces around.

Which brings us to the main stage, the active site.

Emil Fischer first noticed that enzymes were more stable, more heat -resistant when their substrates were around.

Yeah.

That suggested the formation of a stable enzyme -substrate complex.

That active site, that little cleft or pocket, is where all the magic happens.

But it's not just a pocket, right?

No, not at all.

It's a chemical incubator.

It shields the substrates from water and creates the perfect local environment.

It can be hydrophobic, acidic, basic, to push that reaction forward way more efficiently than the rest of the cell ever could.

And they use four key strategies to get that speed.

Let's start with number one, catalysis by proximity.

It seems obvious, but just by binding two substrates in the active site, the enzyme creates this tiny zone of super high local concentration.

And orients them perfectly.

Perfectly.

And just that geometry alone gives you a rate enhancement of at least a thousand -fold.

Number two is acid -base catalysis.

I've heard of general versus...

...practical difference.

So specific acid -base catalysis just involves the protons or hydroxide ions already in the solution.

So the rate only depends on the overall pH.

But general is more powerful.

Way more powerful.

Because the rate depends on all the acids and bases present, including the amino acid side chains right there on the enzyme.

This lets the enzyme donate or accept a proton at the exact time and place it's needed.

The third one is, I think just brilliant conceptually, catalysis by strain.

This is where the enzyme uses its binding energy to do some heavy lifting.

How so?

For reactions where a bond needs to be broken, the enzyme doesn't just bind the substrate.

It physically distorts it, forcing it into a strain shape that actually resembles the high -energy transition state.

So it's already halfway to breaking.

Exactly.

That mechanical strain weakens the target bond and makes it much easier to break.

The enzyme basically pays the energy cost up front.

Which means we could design an inhibitor that looks like that strain shape.

Absolutely.

That's the whole idea behind transition state analogs.

They're molecules that perfectly mimic that high -energy intermediate.

The enzyme is evolved to bind that state so tightly, so these analogs are incredibly powerful inhibitors.

And the last one, number four, is covalent catalysis.

The enzyme actually gets involved in the reaction.

It becomes a temporary reactant.

It forms a transient -covalent bond with the substrate, creating a whole new lower -energy reaction pathway.

Of course, the enzyme has to be restored at the end.

And this often leads to that ping -pong mechanism.

Frequently.

That's where the first product is released before the second substrate even binds.

This all moves us so far beyond the old lock -and -key idea.

Oh, miles beyond.

It's Koshland's induced fit model.

The key word is dynamism.

Like a hand in a glove.

A perfect analogy.

The substrate induces a change in the enzyme's shape, which in turn facilitates the chemical transformation.

It's harnessing that binding energy to do active work.

Let's see this in a couple of key examples.

First,

acid -base catalysis in HIV proteins.

Right.

This is an aspartic protease.

It uses two conserved aspartyl residues.

One acts as a general base.

It rips a proton from a nearby water molecule, making that water super -nuclear -philic.

So it can attack the peptide bond?

It attacks the peptide bond.

Then the second aspartate acts as a general acid, donating a proton to break down the resulting high -energy intermediate.

It's this beautifully choreographed proton shuttle.

Okay.

And for covalent catalysis, the classic example is chymechipsin, the serine protease with its famous charge -relay network.

That's the one.

The trio.

ASP102, his 57, and serine 195.

They aren't close in the linear sequence, but in the folded protein they're positioned perfectly.

So what do they do?

His 57 acts as a shuttle.

It increases the nucleophilicity of serine 195's oxygen, letting it attack the peptide bond and form a covalent acyl enzyme intermediate.

So the enzyme is temporarily stuck to part of the substrate.

How does it get restored?

That seems risky.

That's the genius of the relay network.

Once the front product leaves, the network shifts its focus.

It now activates a water molecule, turning it into a powerful hydroxide ion that attacks that acyl enzyme bond.

And that breaks it?

Breaks it.

And the final proton shuttle restores serine 195 back to its original state, ready for the next cycle.

It's built for flawless self -restoration.

And mechanisms like that, they're often conserved across whole families of enzymes, which tells us something about their evolution.

Absolutely.

Enzymes like chymotrypsin and trypsin are homologs.

They arose from gene duplication.

They use the same core mechanism, but they've evolved to recognize different substrates.

And we know they're related because those key catalytic residues are conserved across Exactly.

And a related idea is isozymes.

Okay, what's the difference?

Isozymes catalyze the exact same reaction, but they're physically distinct proteins.

They usually arise from gene duplication, too, but they allow for tissue -specific adaptation.

Different tissues might need the same reaction, but regulated in a different way.

Isozymes make that possible.

That incredible efficiency, thousands of molecules a second, is exactly what makes them such powerful tools in the lab.

You're amplifying their presence.

We use that amplification for everything.

In drug discovery, high throughput screening, or HTS, uses robotics to assay thousands of potential inhibitors at once.

You just need an assay that produces a colored or fluorescent product.

And we can even use them to detect things that aren't enzymes, right, with the lysis.

Correct.

The enzyme -linked immunoassay.

You link an antibody, which recognizes your target protein, to a reporter enzyme, like alkaline phosphatase.

The antibody binds the target, and the enzyme then produces a really strong signal, letting you quantify something that's otherwise invisible.

We see that same trick in clinical chemistry all the time, especially with NAD -dependent dehydrogenases.

They're the workhorses of optical assays.

Because the reduced forms, NADH and NADPH,

absorb light strongly at 340 nanometers.

And the oxidized forms don't.

The oxidized forms, NAD plus an NADP plus A, don't.

So you just monitor the change in absorbance at 340, and that tells you the rate of enzyme activity.

And if your enzyme of interest, say hexokinase, doesn't produce an absorbing product, you can just link it to one that does.

That's a coupled assay, exactly.

You add an excess of glucose -6 -phosphate dehydrogenase.

It uses the product from the hexokinase reaction and makes NADPH.

Since it's in excess, the rate of NADPH production is governed entirely by your target, the hexokinase.

Let's bring those techniques into the clinic.

When cells die from an injury, they release enzymes into the plasma and these act as biomarkers.

In the timing, the diagnostic window is critical.

Cytoplasmic enzymes show up fast.

Ones from organelle show up later.

Historically, we use things like AST, ALT, and creatine kinase, or CK.

Right, focusing on the CKMB isozyme for heart attacks.

We did, because it was enriched in the myocardium.

But the Kergold standard is way more precise.

We moved away from measuring enzyme activity to measuring structural proteins.

Why the shift?

Specificity.

CKMB activity is found in the heart, but also in small amounts elsewhere, which can give you false positives.

The current standard, cardiac troponins I and T, were structural proteins that are highly specific to heart muscle.

So if you see them in the blood, you know where they came from.

You know exactly where they came from.

Their presence is a direct, sensitive, and definitive indicator of myocardial damage.

And they stay elevated for days, which gives you a much wider and more specific diagnostic window.

Enzymes are also key tools for genetic studies.

Oh, absolutely.

Think of restriction endonucleuses.

They're like chemical scissors that cut DNA at specific sequences.

And any deviations in the resulting fragment patterns, called RFLPs, can be used to identify genetic markers.

And of course, PCR,

the polymerous chain reaction.

Which relies entirely on a thermostable DNA polymerase to amplify tiny amounts of DNA.

It's essential for everything from pathogen detection to genetic screening.

And what about in research?

If you want to study an enzyme that's really rare, you can't just purify it from tissue.

No, you'd never get enough.

So we turn to recombinant studies.

We clone the gene and express huge quantities of the enzyme, usually in E.

coli.

But then you have a purification nightmare.

How do you find your one protein in that sea of bacterial proteins?

The affinity tags.

You add a short DNA sequence to the gene.

So the protein gets made with an extra little tag on it, like a polyhistidyl or his tag.

Which binds to what?

It binds strongly and specifically to an immobilized metal, like nickel.

So you run your whole self -valicate over a nickel column.

Everything washes through, except your tagged protein.

Then you can wash it off and you've got a pure sample.

And then you can use techniques like site -directed mutagenesis.

So systematically change individual amino acids, say, swapping out a crucial serine for an alanine to definitively prove its role in catalysis or binding.

OK, so to summarize this whole deep dive,

enzymes are stereospecific, hyper -efficient catalysts.

They use these conserved dynamic mechanisms, proximity, acid base, strain,

covalent modification, all wrapped up in the induced fit model to make life's reactions happen.

And we covered their critical role in diagnostics, from using NADPH to track reaction rates in the lab, all the way to the definitive role of troponin as a biomarker for a heart attack.

It's an immense, intricate system, but let's close on the ultimate evolutionary catalyst, the ribozyme.

Yes.

The discovery that RNA could perform catalysis just fundamentally shifted biology.

For decades, everyone assumed proteins were the only catalysts.

But the RNA World Hypothesis suggests RNA came first.

The hypothesis is that RNA was the first biologic macromolecule, able to act as both an information carrier, like DNA, and a catalyst, like a protein.

And the most enduring piece of evidence for that is the core of the ribosome itself.

Exactly.

We now know that the ribosomal RNA, not the protein parts, is what's primarily responsible for forming the peptide bonds.

The ribosome is a ribozyme.

So here's the thought to leave everyone with.

If RNA was the first catalyst, if it provided the blueprint for all this more complex protein machinery we've just discussed,

does the fact that the ribosome is still a ribozyme show us that sometimes the simplest, most fundamental design is just too essential, too integral to life's very beginnings to ever be fully replaced?

A powerful thought on the persistence of ancient biochemistry.

Thank you for joining us on this deep dive into the powerful molecular mechanisms that govern life.

We hope this exploration helps you connect the intricate chemistry to every aspect of cellular and clinical outcome.

Keep questioning.

Keep exploring.