Chapter 5: Structures of the Major Compounds of the Body

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Have you ever stopped to think about the invisible building blocks inside you?

These incredibly

molecular structures,

their shapes, their functions.

They literally dictate everything.

How your body works, how medicine interacts.

Well, it's this amazing hidden world.

It really is an entire universe of organized complexity operating constantly.

So today,

our mission, our deep dive, is into the structures of the major compounds of the body.

We're drawing from Mark's basic medical biochemistry.

A fantastic resource.

Think of this as your guide to the essential architecture of life will unpack carbohydrates, lipids, nitrogen compounds, focusing on their names, shapes,

and crucially, what they do.

And how they're all linked.

Exactly.

We're aiming for those aha moments,

guiding you step by step through the patterns that explain health and disease too.

That's right.

And for anyone wanting to really grasp biochemistry, understanding these basic structures, these functional groups, it's like learning the alphabet.

You just have to know it.

Absolutely.

It's critical for making sense of metabolism, clinical situations, everything.

And we'll walk you through it, making it clear even without diagrams, you'll be able to picture it.

Okay.

Let's start with that alphabet then.

The language of biochemistry.

Functional groups.

So most organic molecules, they're mainly carbon and hydrogen, right?

A basic skeleton.

Correct.

That's the backbone.

But the really interesting part, it seems, is the other stuff.

Oxygen, nitrogen, phosphorus, sulfur,

and how they're arranged in these specific functional groups.

That's what gives each molecule its unique personality.

Its personality, its purpose, its reactivity.

Yes.

These aren't just decorations, they're the active parts.

They determine, well, pretty much everything.

Characteristics, solubility, the reactions it'll undergo.

And the names even give clues.

Often, yes.

Like, ul in ethanol tells you it's an alcohol group.

Or win in acetone.

That signals a ketone group.

It's a direct hint about the molecule's structure.

And tiny differences in these groups.

They can have huge consequences.

The source mentions methanol versus ethanol, wood alcohol versus drinking alcohol.

A classic example.

The only difference is changing a methyl group, CH3, to an ethyl group, CH3CH2.

That's it.

That is literally it, structurally.

Methanol is way more toxic.

It causes blindness, slow heart rate, coma, seizures,

just from that one small change.

It's a really stark reminder, isn't it, how critical structure is, down to the smallest detail.

So how do we talk about the state of these groups?

You mentioned oxidized and reduced.

Right.

It's a key concept.

Oxidation generally is losing electrons.

Biochemically, you often see this as gaining oxygen or losing hydrogen.

Okay.

Reduction is the opposite, gaining electrons, which often means gaining hydrogen or losing oxygen.

So think of a spectrum.

An alcohol group can be oxidized to an aldehyde or a ketone, which can then be further oxidized to a carboxyl group.

Each step is more oxidized.

Got it.

More oxygen or less hydrogen equals more oxidized.

Basically, yes.

And we see this clinically.

The case of Diane A with diabetic ketoacidosis, DKA, her liver was overproducing ketone bodies, things like hydroxybutyrate, acetoacetate, and acetone.

Now ketone bodies is a bit of a misnomer for hydroxybutyrate.

Oh.

Because structurally, it's an alcohol.

It has hydroxyl group, not a ketone group.

Acetoacetate does have a keto group, and it's more oxidized than hydroxybutyrate because its key carbon has one less hydrogen.

Ah, I see the connection to oxidation state.

Exactly.

And the acetone, that's volatile.

It explains that sweet sort of fruity smell on the breath of someone in DKA, like Diane.

Fascinating.

Okay, so beyond oxidation, groups can also carry charge.

Yes.

Acidic groups like carboxylates, phosphates, sulfates, they tend to release a proton, leaving behind a negative charge, an anion.

And you mentioned the AIDS suffix is a clue for that.

Often, yeah.

Carboxylate, phosphate, sulfate, see the pattern?

Got it.

And then you have basic groups, usually containing nitrogen like in ammins.

Think of dopamine.

These can accept a proton, acquiring a positive charge.

So positive and negative charges.

And this relates to polarity.

Absolutely.

Polarity is huge.

Polar bonds like carbon -oxygen or carbon -nitrogen have unequal electron sharing, creates partial positive and negative charges, little magnets almost.

Non -polar bonds like carbon -carbon or carbon -hydrogen share electrons pretty much equally, no significant charge separation.

And this polarity dictates solubility.

Profoundly.

Water itself is polar, a dipolar molecule, so it loves interacting with other polar or charged molecules.

Anything water soluble or hydrophilic needs those polar or charged groups to hang out with water, forming a hydration shell.

Like sugars.

Sugars are a great example, loaded with polar hydroxyl groups.

But hydrophobic water -fearing compounds like lipids have large non -polar sections.

Water essentially pushes them away, trying to maximize its own hydrogen bonding.

So they clump together, like oil and water.

Exactly.

They form droplets or layers.

It's all driven by these polarity rules.

And this isn't just about dissolving.

It affects reactions, too.

Definitely.

Those partial charges are key for reactivity.

They guide how molecules interact.

You have electrophiles, which are electron -poor, seeking out electron -rich nucleophiles.

It's like molecular attraction.

Like magnets finding each other?

Kind of.

And this attraction drives the formation of crucial bonds.

Esters, thioesters, amides, phosphosters.

It's the engine of biochemistry.

Can you give an example of how polarity affects a real molecule's function?

Sure.

Think about cholesterol.

It's mostly non -polar, right?

Largely water -insoluble fits into membranes.

Now compare it to colic acid, which is made from cholesterol.

Colic acid gets a carboxyl group and three hydroxyl groups added, all arranged on one side.

So it gets a polar face.

Precisely.

A hydrophilic surface.

This transforms it, allows it to form bile salts.

And what do bile salts do?

They emulsify fats in your gut.

They make the non -polar fats mix with the watery environment, a total change in function just by adding polar groups.

That's a great illustration.

Now quickly on naming.

Sometimes you see numbers, sometimes Greek letters.

Right, nomenclature.

You can number carbon starting from the most oxidized end.

Or you use Greek letters alpha, beta, gamma, starting from the carbon next to the most oxidized group.

So three hydroxybutyrate is the same as?

Dry hydroxybutyrate.

Beta is the second letter, corresponding to the third carbon if you count the carboxyl carbon as one.

And this matters clinically.

It can.

You mentioned dog hydroxybutyrate, GHB, the date rape drug.

That gamma tells you the hydroxyl group is on the gamma carbon, the fourth one from the carboxyl.

Structure matters.

Okay, that foundation in functional groups is crucial.

Let's move to the next big players.

Carbohydrates, sugars.

Not just sweet, right?

Not at all.

Vital energy, key structural components.

So what defines a simple sugar, a monosaccharide?

Okay, simplest forms.

Usually a chain of three or more carbons.

One carbon has a carbonyl group if it's an aldehyde, it's an aldose sugar.

If it's a ketone, it's a ketose.

Like glucose and fructose.

Exactly.

Glucose is an aldohexose, six carbon aldose.

Fructose is a ketohexose, six carbon ketose.

And the other carbons typically have hydroxyl OH groups, hence the O suffix for sugars.

And you mentioned small tweaks matter.

What about D and L sugars?

Ah, chirality or handedness.

If a carbon has four different groups attached, it's asymmetric or chiral.

This leads to mirror image forms, D and L.

Like your left and right hands.

Perfect analogy.

And our bodies are incredibly specific.

We primarily use D sugars, like D glucose, also called dextrose, the stuff in IV drips.

L glucose exists, but our enzymes don't really work with it.

Wow.

And what are epimers?

They're a specific type of isomer.

They have the same formula, same atoms, but differ in the arrangement around just one asymmetric carbon.

D glucose and D galactose, for instance, only differ at carbon four.

And enzymes can tell that tiny difference apart.

Oh yes.

Specific enzymes can interconvert them, which is vital for metabolism.

These sugars aren't usually straight chains of water, are they?

Not primarily, no.

They mostly form rings, five or six -membered rings.

This happens when the carbonyl group reacts with the hydroxyl group on the same molecule.

And this creates the anomeric carbon.

Correct.

A new chiral center.

The hydroxyl group on this anomeric carbon can point down, alpha, a -simo position, or up, beta, our position, relative to the ring structure.

Alpha down, beta up.

Usually drawn that way, yes.

And in solution, these alpha and beta forms can actually flip back and forth.

It's called mutarotation.

That is, until the anomeric carbon forms a bond with something else, locking it in place.

And sugars can get modified.

Substituted sugars.

Yes, absolutely.

Adding groups changes their function dramatically.

Put a phosphate group on glucose, you get glucose 6 -phosphate, it traps the glucose inside the cell.

Or add an amino group, you get amino sugars like glucosamine or galactosamine, often acetylated.

Add sulfate groups.

These modified sugars are building blocks for complex molecules like proteoglycans.

Proteoglycans.

They sound important.

They are.

They form these huge branched structures, almost like a bottle brush.

Lots of negatively charged groups, they trap water.

Essential for your joints, cartilage, extracellular matrix, eye fluid.

So sugars aren't just simple fuel.

They build complex machinery.

Definitely.

And they can be chemically modified in other ways, too oxidized or reduced.

Oxidize the aldehyde carbon, you get an onyc acid, like gluconic acid.

Oxidize the other end, the terminal hydroxyl, you get a uronic acid, like glucuronic acid, important for detoxification.

And reduction.

Reduce the aldehyde, you get a polyol, like sorbitol.

Reduce one of the hydroxyl groups down to just a hydrogen, you get a deoxy sugar.

The most famous is deoxyribose.

The sugar in DNA.

The very same.

Missing in oxygen compared to ribose, hence deoxy.

You mentioned a reducing sugar test earlier.

How did that work with these structures?

Ah, right.

That old test detected sugars with a free aldehyde group or one that could easily like in the open chain structure during mutarotation.

That aldehyde group could reduce copper ions in the test region, causing a color change.

So it detected the potential to donate electrons.

Exactly.

Even ketoses could sometimes react if they could rearrange into an aldose form, a process called tautomerization.

It wasn't perfectly specific, but useful before modern enzyme tests helped spot things like fructose in urine.

Okay, so how do sugars link together to make bigger structures?

Through glycosidic bonds.

The anomeric carbon reacts with an out -OH or an at -NH group on another molecule.

And an O -glycosidic bond.

Right.

If it links to a nitrogen, like adenine linking to radose and ATP, that's an N -glycosidic bond.

If it links to an oxygen, like one sugar linking to another, or a sugar linking to a protein, that's an O -glycosidic bond.

Like in lactose.

Milk sugar.

Perfect example.

Lactose is galactose linked to glucose via a beta -one -year -quan bond.

Two sugars make a disaccharide.

And more sugars.

Three to maybe twelve is an oligosaccharide.

Hundreds or thousands make a polysaccharide.

Think starch, or glycogen, or a glucose storage molecule.

Glycogen is a huge branched polysaccharide made of glucose units linked by alpha -one -year bonds, mainly with alpha -one spawns at the branch points.

Complex stuff from simple sugars.

Alright, let's shift gears.

Away from the water -loving carbs to the water -fearing lipids.

The hydrophobic world, yes.

Defined primarily by their dislike of water.

Started with the basics.

Fatty acids.

Right.

Long hydrocarbon chains.

Usually straight chains in humans.

One end has a methyl group, that's the omega end.

The other end has a carboxyl group, dash COH, the acidic part.

They vary in length.

Yes.

And whether they have double bonds.

Most common in humans are even -numbered chains, 16 to 20 carbons long.

Satuated means no double bonds, like palmitic acid, C16, or stearic acid, C18.

Unsaturated means one or more double bonds.

And the structure affects how they behave.

Like melting point.

Absolutely.

Longer chains mean higher melting points.

But double bonds, especially cis double bonds, put kinks in the chain.

Kinks?

Yeah, they bend the chain so the molecules can't pack together as tightly.

This lowers the melting point.

More double bonds, more kinks, lower melting point.

So unsaturated fats are more likely to be liquid.

Like vegetable oils.

Exactly.

Compared to saturated fats like butterfat, which packs neatly and is solid, this fluidity is crucial for our cell membranes.

They need to be flexible, not rigid.

How do we name the unsaturated ones?

Delta and omega systems.

Two main ways.

The delta system counts carbons from the carboxyl end, useful chemically.

So oleic acid is 18 .1, rhodon 9, 1, double bond, starting at carbon 9.

The omega system counts from the methyl end.

This is important nutritionally, telling us about essential fatty acids our bodies can't make, like linoleic acid in omega 3 or arachidonic acid in omega 6.

And arachidonic acid is important.

Very.

It's a 20 carbon omega 6 fatty acid, 20 .4, a 5, 5, 8, and 11, 14.

That's the precursor for icosanoids, prostaglandins, thromboxans, leukotrains, powerful local hormones.

You mentioned cis double bonds.

Is there another kind?

Yes, trans.

It's about the geometry around the double bonds.

Cis means the hydrogen atoms or the main carbon chains are on the same side.

This causes that kink.

It's the common form in nature.

And trans?

Trans means they're on opposite sides.

The chain stays straighter, more like a saturated fat.

These are less common naturally, but are produced during artificial hydrogenation of oils.

Ah, trans fats.

That rings a bell.

Right.

Hydrogenation was used to make liquid oils more solid, improve shelf life, texture like making margarine.

But consuming trans fats was linked to increase heart disease risk.

So they're regulated now?

Yes.

FDA mandated labeling, and many places have banned artificial trans fats in foods, a major public health issue stemming from lipid structure.

Okay.

Beyond fatty acids, what other lipids are key?

Ethylglycerols.

Also known as glycerides.

You have a glycerol backbone, a simple three carbon alcohol.

Fatty acid is a mono acylglycerol.

Two is di.

Three is tri - Triglycerides.

That's how we store fat, right?

Primarily yes.

Usually mixed triglycerides, meaning different fatty acids at the three positions,

stored in adipose tissue.

Then phospholipids, like in membranes.

Correct.

Phosphoacetylglycerols.

Again, glycerol backbone.

Fatty acids at carbons one and two.

But at carbon three, you have a phosphate group and usually something else attached to the phosphate, like choline.

Phosphadilcholine.

Best of them, that's the one.

A major membrane phospholipid.

Crucially, it's amphipathic.

It has both worlds.

The fatty acid tails are hydrophobic, nonpolar, and the phosphate choline head group is hydrophilic, polar charged.

This dual nature is perfect for forming cell membranes, a bilayer with tails inward, heads outward.

Clever design.

What about sphingolipids?

They sound different.

They are.

Key difference.

No glycerol backbone.

They're built on an amino alcohol called sphingosine.

Fingosine.

Derived from serine and palmitate.

Attach a fatty acid to sphingosine's amino group, you get a ceramide.

Okay.

Then add other things.

Attach sugars, you get glycolipids, like cerebrosides or gangliosides important in nerve cells.

Attach a phosphorcholine group, you get sphingomyelin.

Sphingomyelin.

Sounds like myelin sheath.

Exactly.

Crucial component of the myelin sheath that insulates nerve axons.

Also found in cell membranes.

So different backbone, but similar roles and structure.

And finally, steroids.

Like cholesterol.

Right.

Characterized by that specific four ring structure, the steroid nucleus.

Cholesterol is the big one in animals.

Found in membranes, helps regulate fluidity.

And it's a precursor.

Yes.

The starting point for synthesizing all steroid hormones, like testosterone, estrogen, cortisol, and also those amphipathic bile salts we talked about earlier.

They're all built from cholesterol, which itself is made from simpler five carbon units called isoprene units.

From carbs and lipids, let's turn to nitrogen.

Vital element.

Nitrogen containing compounds.

Absolutely essential.

Starting with the obvious, amino acids.

Building blocks of proteins.

Correct.

And in human proteins, they are always L amino acids.

The L refers to the stereochemistry.

The alpha means the amino group is on the carbon, right next to the carboxyl group.

Always L alpha in us.

For protein synthesis, yes.

But amino acids are also precursors for other things, like neurotransmitters, HOM, nucleotides.

And there are other types, like beta or gamma amino acids, such as GABA, gamma aminobutyric acid, which is a neurotransmitter itself.

And you mentioned bacteria use D amino acids.

Some do, yeah.

Particularly in their cell walls.

It's an interesting difference from us.

Theoretically, it's a target for antibiotics, inhibit D amino acid metabolism.

You might harm bacteria, but not human cells.

Clever idea.

What about nitrogen in rings?

Very important.

Heterocyclic rings containing nitrogen.

We call them nitrogenous bases.

Key types are purines, like adenine and guanine, and pyrimidines.

Cytosine, thymine, uracil.

Also puridine rings found in vitamins like niacin B3 and pyridoxin B6.

You often see spinyne at the end.

Yeah.

Adenine, guanine, thymine, puridine often include nitrogens in the ring.

Unlike a simple benzene ring, which is pretty unreactive, these nitrogen -containing rings can hydrogen bond shuffle electrons around.

Makes them very useful biologically.

And these bases form nucleosides and nucleotides.

Exactly.

Take a base, purine or pyrimidine, and attach it to a sugar, ribose or deoxyribose, via an N -glycosidic bond, that's a nucleoside, adenine plus ribose, adenosine.

Okay.

Now, add one or more phosphate groups to the sugar part of the nucleoside, that's a nucleotide, adenosine plus three phosphates, adenosine triphosphate ATP.

The energy currency.

The very same, built from these components.

You also mentioned tautomers earlier with the reducing sugar tests.

Do they pop up here, too?

They do.

Uric acid is a good example.

It's the breakdown product of purines.

It exists in different tautomeric forms where hydrogens and double bonds shift around.

And this matters.

Big time.

Think about Lottite's case of gout.

At normal body pH, around 7 .4, uric acid mostly exists as its dissociated form, urate.

Urate is not very soluble in water, so if levels get too high, it can crystallize out as monosodium urate, especially in cooler areas like joints.

The big toe is classic.

Causing the pain and inflammation of gout.

Precisely.

And in the acidic environment of urine, it can form uric acid kidney stones.

So caudomerism and pH affected solubility and clinical outcome.

Treatments like allopurinol work by inhibiting uric acid production.

Wow, chemistry in action.

Okay, last topic.

Free radicals.

Often painted as the bad guys.

They often are, biologically speaking, a radical is any molecule with a single unpaired electron.

Highly reactive.

A free radical is just one that exists independently.

Out of their form.

Various ways.

Radiation can knock an electron off a molecule or even split water into things like the hydroxyl radical.

Extremely reactive.

That dot means unpaired electron.

Yes.

And that hydroxyl radical can then snatch an electron from something else, like a lipid in a cell membrane.

Damaging it and creating a new radical starts a chain reaction.

Even some pollutants like nitrogen dioxide NO2 in smog are radicals.

So clinically, they're significant.

Hugely.

They're implicated as agents of cell death and destruction linked to many chronic diseases, heart disease, diabetes, arthritis, emphysema, and also acute injuries like stroke, heart attack, radiation damage.

Our bodies have defenses.

We do.

Antioxidant systems.

But excessive free radical production can overwhelm them.

Understanding their structure helps us appreciate why they're so damaging.

What a tour.

From functional groups to free radicals.

It really drives home how these tiny structural details, a double bond here, a polar group there, orchestrate the entire symphony of life.

It really does.

It shows the elegance and efficiency.

Understanding these structures, these carbohydrates, lipids, nitrogen compounds, even radicals.

It's not just memorization.

No, it's seeing the patterns.

Exactly.

Seeing the fundamental principles that explain how things work, both in health and when things go wrong in disease.

It connects everything.

So a final thought for everyone listening,

given how these tiny, almost invisible structural differences have such massive biological effects,

what other biochemical details, maybe ones we haven't even touched on, do you think are quietly shaping your health right now?

Something to ponder.

Definitely.

Keep exploring, keep asking questions, and keep connecting those biochemical dots.