## Learning Outcomes

- Identify the shared structure, stereochemistry, and acid-base behavior of the 20 proteinogenic amino acids.
- Classify amino acids by side-chain properties and predict their likely location in proteins.
- Use amino acid abbreviations, mutation notation, and side-chain chemistry in MCAT-style questions.
- Explain peptide bond formation, hydrolysis, planarity, and directional peptide sequences.
- Distinguish primary, secondary, tertiary, and quaternary protein structure.
- Predict how folding forces, solvation, disulfide bonds, heat, and solutes affect protein function.
- Apply pKa and pI reasoning to amino acid charge and separation questions.

## Medical College Admission Test (MCAT) Syllabus

For MCAT Foundational Concept 1A, proteins are tested as biomolecules whose chemical and structural properties determine their biological roles. Expect both recall questions, such as identifying an amino acid from a side chain, and passage questions that ask you to reason from sequence changes, pH, mutations, electrophoresis, or denaturation data.

- Structures, names, three-letter abbreviations, and one-letter abbreviations of the 20 proteinogenic amino acids
- Amino acid stereochemistry, including L-amino acids, glycine, and cysteine
- Side-chain classification: nonpolar, aromatic, polar, acidic, and basic amino acids
- Zwitterions, amphoteric behavior, pKa values, charge state, titration curves, and isoelectric point
- Peptide bond formation, hydrolysis, amide resonance, and peptide directionality
- Primary, secondary, tertiary, and quaternary protein structure
- Hydrophobic effect, hydrogen bonding, ionic interactions, disulfide bonds, and protein solvation
- Denaturation, conjugated proteins, and structure-function relationships in clinical or experimental contexts

## Test Your Knowledge

Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision.

1. Which amino acid is achiral, and which L-amino acid usually has an R absolute configuration?
2. A membrane-spanning alpha helix is enriched in which type of amino acid side chain?
3. Why does a peptide bond restrict rotation around the C–N bond?
4. At pH values below its pI, will an amino acid tend to carry a more positive or more negative net charge?
5. What level of protein structure changes when two folded monomers associate to form a dimer?

## Introduction

Proteins carry out a huge range of cellular functions: they form structural supports, transport molecules, recognize signals, regulate genes, move cells, and catalyze biochemical reactions. Their functions depend on their three-dimensional shapes, and those shapes depend on the chemical properties of their amino acid building blocks.

A single amino acid substitution can have major consequences. In sickle cell disease, hemoglobin differs from normal adult hemoglobin by a glutamate-to-valine substitution at position 6 in two beta chains. This replaces a negatively charged, hydrophilic side chain with a hydrophobic side chain, encouraging deoxygenated hemoglobin to aggregate. This example captures the central MCAT idea: protein structure and function are rooted in amino acid chemistry.

> **Key Term: amino acid**
> A molecule containing an amino group and a carboxyl group; on the MCAT, this usually refers to one of the 20 alpha-amino acids encoded in human proteins.

### The 20 proteinogenic amino acids: structure, stereochemistry, and side-chain chemistry

All standard proteinogenic amino acids share the same basic alpha-amino acid framework. The alpha carbon is bonded to four groups: an amino group, a carboxyl group, a hydrogen atom, and a variable side chain, often called the R group. The R group determines the amino acid’s chemical behavior and strongly affects protein folding, location, and function.

> **Key Term: R group**
> The variable side chain attached to an amino acid’s alpha carbon; it determines much of the amino acid’s polarity, charge, size, and reactivity.

For most amino acids, the alpha carbon is chiral because it has four different substituents. Glycine is the exception: its side chain is another hydrogen atom, so its alpha carbon is not chiral. Eukaryotic proteins use L-amino acids. In a Fischer projection, L-amino acids are drawn with the amino group on the left. Most chiral L-amino acids have S absolute configuration, but cysteine is the exception because sulfur in its side chain changes priority order, giving L-cysteine an R absolute configuration.

> **Test Tip:**
> Memorize this high-yield stereochemistry rule as a set: all protein amino acids are L; all are chiral except glycine; almost all chiral L-amino acids are S except cysteine, which is R.

The MCAT expects you to know names, structures, three-letter abbreviations, and one-letter abbreviations. The one-letter codes are common in mutations. For example, E6V means glutamic acid at position 6 has been replaced by valine.

Amino acids are commonly grouped by side-chain chemistry:

- **Nonpolar, nonaromatic:** glycine (Gly, G), alanine (Ala, A), valine (Val, V), leucine (Leu, L), isoleucine (Ile, I), methionine (Met, M), and proline (Pro, P). These are generally hydrophobic. Alanine, valine, leucine, and isoleucine have alkyl side chains. Methionine contains sulfur but is relatively nonpolar because the sulfur is in a thioether. Proline is cyclic, with its amino nitrogen built into a ring, reducing backbone flexibility.
- **Aromatic:** phenylalanine (Phe, F), tyrosine (Tyr, Y), and tryptophan (Trp, W). Phenylalanine is strongly hydrophobic. Tyrosine has a phenolic hydroxyl group, making it more polar than phenylalanine and able to be phosphorylated. Tryptophan has a large indole ring.
- **Polar, uncharged:** serine (Ser, S), threonine (Thr, T), asparagine (Asn, N), glutamine (Gln, Q), and cysteine (Cys, C). Serine and threonine have hydroxyl groups and are common phosphorylation sites. Asparagine and glutamine contain amide side chains. Cysteine contains a thiol and can form disulfide bonds.
- **Negatively charged, acidic:** aspartic acid/aspartate (Asp, D) and glutamic acid/glutamate (Glu, E). At physiological pH, their side-chain carboxyl groups are usually deprotonated and negatively charged.
- **Positively charged, basic:** lysine (Lys, K), arginine (Arg, R), and histidine (His, H). Lysine has a terminal amino group. Arginine has a guanidinium group with delocalized positive charge. Histidine has an imidazole ring with a side-chain pKa near physiological pH, so it is useful in acid-base chemistry in proteins.

> **Key Term: proteinogenic amino acid**
> One of the 20 standard amino acids encoded by the genetic code and incorporated into proteins during translation.

Hydrophobicity helps predict where residues occur in folded proteins. Strongly hydrophobic amino acids are often found in protein interiors or membrane-spanning regions. Charged and polar amino acids often appear on protein surfaces, where they can interact with water, ions, and other polar molecules.

A useful example is a transmembrane alpha helix. The portion crossing the lipid bilayer is likely enriched in hydrophobic side chains such as leucine, isoleucine, valine, phenylalanine, alanine, and methionine. Charged residues such as lysine, glutamate, and aspartate are less favorable within the hydrophobic core of a membrane unless they have a special functional role.

> **Exam Warning:**
> Do not classify every aromatic amino acid as equally hydrophobic. Phenylalanine is strongly hydrophobic, but tyrosine has a hydroxyl group and tryptophan has a nitrogen-containing ring. MCAT questions may test the mixed character of these side chains.

### Acid-base behavior, zwitterions, and isoelectric points

Amino acids are amphoteric: they can act as acids or bases. Their carboxyl group can donate a proton, and their amino group can accept a proton. In aqueous solution near neutral pH, free amino acids usually exist as zwitterions, with a positively charged amino group and a negatively charged carboxylate group.

> **Key Term: zwitterion**
> A molecule with both positive and negative formal charges but no net charge overall.
>
> **Key Term: amphoteric**
> Able to act as either an acid or a base by donating or accepting protons.

The charge state of an amino acid depends on pH relative to pKa values. A useful rule is:

- At low pH, groups are more protonated.
- At high pH, groups are more deprotonated.
- When pH equals pKa, half of that functional group is protonated and half is deprotonated.

For a simple amino acid without an ionizable side chain, the carboxyl group has a pKa near 2, and the amino group has a pKa near 9. At very acidic pH, the amino acid is fully protonated and carries a positive net charge. As pH rises past the carboxyl pKa, the carboxyl group loses H+ and becomes COO−, producing the zwitterion. As pH rises past the amino group pKa, the amino group loses H+ and becomes neutral NH2, giving the molecule a negative net charge.

The isoelectric point, pI, is the pH at which the molecule has zero net charge. For amino acids without ionizable side chains, pI is the average of the two relevant pKa values:

$$
pI=\frac{pK_{a,\ carboxyl}+pK_{a,\ amino}}{2}
$$

For glycine, using approximate pKa values of 2 and 9 gives a pI near 5.5 to 6.

For acidic or basic amino acids, there are three pKa values: the alpha-carboxyl group, the alpha-amino group, and the ionizable side chain. The pI is found by averaging the two pKa values that surround the neutral zwitterion. Acidic amino acids have low pI values because their side-chain carboxyl groups add an extra deprotonation step. Basic amino acids have high pI values because their side chains remain positively charged until higher pH.

> **Key Term: isoelectric point**
> The pH at which a molecule has no net electric charge.

For lysine, assume pKa values of about 2 for the carboxyl group, 9 for the alpha-amino group, and 10.5 for the side-chain amino group. Lysine is neutral between deprotonation of the alpha-amino group and deprotonation of the side chain, so:

$$
pI=\frac{9+10.5}{2}=9.75
$$

This explains why a basic amino acid has a pI well above 6.

> **Test Tip:**
> In charge questions, first compare pH with pI. If pH is below pI, the molecule is more protonated and tends to be net positive. If pH is above pI, it is more deprotonated and tends to be net negative.

Amino acid titration curves also appear on exams. The curve is relatively flat near pKa values because the molecule buffers added acid or base. The curve is steep near the pI because small additions of acid or base can cause a more rapid pH change when buffering capacity is low.

### Peptide bonds and the hierarchy of protein structure

Amino acids join through peptide bonds. A peptide bond forms when the amino group of one amino acid reacts with the carboxyl group of another amino acid, releasing water. This is a condensation or dehydration reaction. The reverse process, peptide bond hydrolysis, uses water and generally requires strong acid, strong base, or enzymatic catalysis.

> **Key Term: peptide bond**
> An amide bond linking the carboxyl carbon of one amino acid residue to the amino nitrogen of the next residue in a peptide or protein.

A peptide has directionality. The N-terminus contains the free amino end, and the C-terminus contains the free carboxyl end. Sequences are conventionally written from N-terminus to C-terminus. A dipeptide has two amino acid residues, a tripeptide has three, an oligopeptide has a small number, and a polypeptide has many residues. Proteins are large, folded, functional polypeptides.

Peptide bonds are amide bonds and show resonance. The nitrogen lone pair can delocalize toward the carbonyl group, giving the C–N bond partial double-bond character. Because of this, rotation around the peptide C–N bond is restricted, and the atoms involved in the peptide bond are relatively planar. Rotation is much freer around the single bonds on either side of the peptide bond, helping the protein backbone fold into allowed conformations.

> **Exam Warning:**
> Peptide bond formation is not the same as protein folding. Peptide bonds define primary structure. Folding creates higher-order structure through noncovalent interactions and, in some cases, disulfide bonds.

Protein structure is described at four levels:

**Primary structure** is the linear amino acid sequence. It is held together by covalent peptide bonds. A change in primary structure can alter every higher level of structure, as in the glutamate-to-valine substitution in sickle cell hemoglobin.

**Secondary structure** refers to local, repeating backbone conformations, mainly alpha helices and beta-pleated sheets. These are stabilized by hydrogen bonds between backbone carbonyl oxygens and amide hydrogens, not primarily by side-chain hydrogen bonding. Alpha helices are coiled structures; beta sheets align strands side by side and can be parallel or antiparallel.

**Tertiary structure** is the overall three-dimensional shape of a single polypeptide chain. It is stabilized by hydrophobic interactions, hydrogen bonds, ionic interactions, van der Waals forces, and disulfide bonds. The hydrophobic effect is especially significant: nonpolar side chains cluster away from water, while polar and charged groups tend to face the aqueous environment.

Quaternary structure\*\* is the arrangement of multiple polypeptide subunits. Hemoglobin, for example, has four subunits. If a receptor tyrosine kinase exists as an inactive monomer and then forms a dimer after ligand binding, the highest level of structure changes from tertiary to quaternary.

> \*\*Key Term: disulfide bond
> A covalent bond formed by oxidation of two cysteine thiol groups, producing a cystine linkage that can stabilize protein structure.

Disulfide bonds are common stabilizing features, especially in extracellular proteins, where the oxidizing environment favors their formation. In the reducing environment of the cytosol, free cysteine thiols are more common.

### Folding, localization, function, and denaturation

Protein folding is strongly tied to the surrounding solvent. In aqueous environments, hydrophobic residues tend to move inward, while hydrophilic residues face outward. This creates a solvation layer of water molecules around polar and charged surface groups. In membrane proteins, the pattern can reverse for membrane-spanning regions: hydrophobic residues face lipid tails, while polar or charged residues may line channels or binding sites.

The chemical properties of side chains also determine function. Charged residues can bind ions or form salt bridges. Histidine can accept or donate protons near physiological pH, so it often appears in enzyme active sites. Serine, threonine, and tyrosine can be phosphorylated, changing charge and signaling state. Cysteine can form disulfide bonds or participate in redox chemistry. Glycine is small and flexible, allowing tight turns or close packing; collagen, with its tightly packed triple helix, contains a high proportion of glycine. Proline is rigid and often introduces bends or disrupts regular alpha helices.

Some proteins are conjugated, meaning they contain a non-polypeptide component needed for function or localization. Examples include glycoproteins with carbohydrate groups, lipoproteins with lipid components, and hemoproteins such as hemoglobin with heme groups. Such modifications can help direct proteins to organelles, anchor them in membranes, or supply cofactors needed for activity.

> **Key Term: conjugated protein**
> A protein that contains a non-amino-acid component, such as a lipid, carbohydrate, metal ion, or prosthetic group, needed for structure, localization, or function.

Denaturation is the loss of higher-order protein structure. A denatured protein generally loses biological activity because active sites, binding surfaces, and structural features depend on correct folding. Primary structure usually remains intact during denaturation unless peptide bonds are hydrolyzed.

Heat denatures proteins by increasing molecular motion. At high enough temperatures, thermal energy can overcome interactions that stabilize secondary, tertiary, and quaternary structure. Cooking egg whites is a familiar example: albumin unfolds and aggregates, changing from clear and fluid to white and solid.

Solutes can also denature proteins. Urea disrupts stabilizing interactions, including hydrogen bonding and hydrophobic packing. Detergents such as sodium dodecyl sulfate, also called SDS, bind and solubilize proteins, disrupting noncovalent interactions and often giving proteins a strong negative charge for electrophoretic separation. Reducing agents can break disulfide bonds by converting cystine linkages back into cysteine residues.

> **Test Tip:**
> Denaturation usually affects secondary, tertiary, and quaternary structure, not primary structure. If an answer choice says heat breaks peptide bonds under ordinary biological denaturation conditions, be skeptical.

MCAT passages may connect denaturation to function. If an enzyme is heated and loses activity, the best explanation is often that the active site’s shape or chemical environment has changed. If SDS is used in a gel, it denatures proteins and helps separate them mainly by size because SDS masks native charge differences. If reducing conditions are added, disulfide-linked subunits may separate.

The main exam skill is prediction: from side-chain identity to protein location, from pH to charge, from mutation to folding, and from denaturant to loss of activity. When uncertain, reduce the question to chemistry. Ask: Is the side chain charged, polar, hydrophobic, bulky, flexible, rigid, aromatic, sulfur-containing, or able to form special covalent bonds?

## Key Point Checklist

This article has covered the following key knowledge points:

- Standard protein amino acids have an alpha carbon bonded to an amino group, carboxyl group, hydrogen atom, and R group.
- The R group determines most amino acid chemical properties and strongly affects protein folding and function.
- All protein amino acids are L; all are chiral except glycine; most chiral L-amino acids are S except cysteine, which is R.
- Nonpolar residues tend to be buried in protein interiors or exposed to lipid in transmembrane regions.
- Charged and polar residues often occur on protein surfaces and interact with water, ions, or other polar molecules.
- Amino acids are amphoteric and commonly exist as zwitterions near neutral pH.
- The pI is the pH at which net charge is zero; below pI molecules tend to be more positive, and above pI they tend to be more negative.
- Peptide bonds form by condensation and are cleaved by hydrolysis.
- Peptide bonds are planar and rotation-restricted because amide resonance gives the C–N bond partial double-bond character.
- Primary structure is amino acid sequence; secondary structure is backbone hydrogen bonding; tertiary structure is a full single-chain fold; quaternary structure is multiple-subunit assembly.
- Disulfide bonds form between cysteine residues and can stabilize protein structure.
- Denaturation disrupts higher-order structure and usually removes function while leaving peptide bonds intact.

## Key Terms and Concepts

- amino acid
- R group
- proteinogenic amino acid
- zwitterion
- amphoteric
- isoelectric point
- peptide bond
- disulfide bond
- conjugated protein


Foundational concept 1: Biomolecules - 1A: Structure and function of proteins and constituent amino acids