Amino Acids, Peptides, Proteins, and Enzymes: Structured Study Notes

Amino Acids

Structure and Function

Amino acids are the monomeric units of proteins and serve as precursors for neurotransmitters. Each amino acid contains an amino group, a carboxyl group, a hydrogen atom, and a distinctive side chain (R group) attached to a central (α) carbon, which is chiral except in glycine.

• General Structure: $\mathrm{H_2N{-}CHR{-}COOH}$

• Zwitterion: At neutral pH, amino acids exist as zwitterions, carrying both positive and negative charges.

• Precursor Role: Some amino acids are precursors for neurotransmitters (e.g., tryptophan for serotonin).

Classification of Amino Acids

Amino acids are classified based on the properties of their side chains:

• Nonpolar Aliphatic: Glycine, Alanine, Valine, Leucine, Isoleucine, Methionine, Proline

• Aromatic: Phenylalanine, Tyrosine, Tryptophan

• Polar Uncharged: Serine, Threonine, Cysteine, Asparagine, Glutamine

• Positively Charged (Basic): Lysine, Arginine, Histidine

• Negatively Charged (Acidic): Aspartic acid, Glutamic acid

Abbreviations and Codes

Amino acids are represented by three-letter and one-letter codes for convenience in protein sequence notation.

Amino Acid	Three-letter	One-letter
Alanine	Ala	A
Arginine	Arg	R
Asparagine	Asn	N
Aspartic Acid	Asp	D
Cysteine	Cys	C
Glutamine	Gln	Q
Glutamic Acid	Glu	E
Glycine	Gly	G
Histidine	His	H
Isoleucine	Ile	I
Leucine	Leu	L
Lysine	Lys	K
Methionine	Met	M
Phenylalanine	Phe	F
Proline	Pro	P
Serine	Ser	S
Threonine	Thr	T
Tryptophan	Trp	W
Tyrosine	Tyr	Y
Valine	Val	V

Enantiomers and Optical Activity

Amino acids (except glycine) exist as L- and D-enantiomers, which are nonsuperimposable mirror images. Proteins are composed almost exclusively of L-amino acids.

• Optical Activity: L- and D- forms rotate plane-polarized light in opposite directions.

• Biological Relevance: L-amino acids are predominant in nature.

Spectroscopic Properties

Aromatic amino acids (tryptophan, tyrosine, phenylalanine) absorb ultraviolet light, which is useful for protein quantification and analysis.

• Absorption Peaks: Tryptophan absorbs most strongly at ~280 nm.

• Application: UV spectroscopy for protein concentration measurement.

Acid-Base Properties

Amino acids can act as acids and bases due to their amino and carboxyl groups. Their ionization state depends on the pH of the environment.

• Isoelectric Point (pI): The pH at which the amino acid has no net charge.

• Protonation States: At low pH, both groups are protonated; at high pH, both are deprotonated.

pKa Values of Amino Acids

The pKa values of the carboxyl and amino groups, as well as the side chains, determine the ionization state of amino acids.

• Carboxyl group: pKa ≈ 2–3

• Amino group: pKa ≈ 9–10

• Side chain pKa: Varies by amino acid

Equation: $pK_a = -\log(K_a)$

Titration Curves for Amino Acids

Titration curves show the buffering regions and pKa values of amino acids. The Henderson-Hasselbalch equation is used to calculate pH:

• Equation: $pH = pK_a + \log\left(\frac{[A^-]}{[HA]}\right)$

• Isoelectric Point: $pI = \frac{pK_{a1} + pK_{a2}}{2}$

Practical Influence of Acid-Base Properties

The acid-base properties of amino acids affect their buffering capacity, protein purification, and separation techniques such as electrophoresis.

• Buffering: Amino acids help maintain pH in biological systems.

• Electrophoresis: Separation based on charge at different pH values.

Essential Amino Acids

Essential amino acids cannot be synthesized by the human body and must be obtained from the diet.

Essential Amino Acids
Arginine (Arg)
Histidine (His)
Isoleucine (Ile)
Leucine (Leu)
Lysine (Lys)
Methionine (Met)
Phenylalanine (Phe)
Threonine (Thr)
Tryptophan (Trp)
Valine (Val)

• Deficiency: Can lead to malnutrition diseases such as Kwashiorkor.

Uncommon Amino Acids

Some amino acids are found only in specific proteins or are not incorporated into proteins but have biological roles (e.g., hydroxyproline in collagen, ornithine in the urea cycle).

Peptides and Peptide Bond

Formation and Structure

Peptides are short chains of amino acids linked by peptide bonds, formed by condensation (removal of water) between the amino group of one amino acid and the carboxyl group of another.

• Peptide Bond: Has partial double-bond character due to resonance, restricting rotation.

• N-terminus: Free amino group

• C-terminus: Free carboxyl group

Conformation and Isomerism

Peptide bonds can exist in trans and cis configurations, with the trans form being more stable due to reduced steric interference.

Cysteine and Cystine

Cysteine residues can form disulfide bonds (cystine) through oxidation, which stabilizes protein structure.

• Equation: $2\mathrm{Cysteine} \xrightarrow{oxidation} \mathrm{Cystine} + 2H^+ + 2e^-$

Types of Peptides

• Oligopeptides: 2–15/20 residues

• Polypeptides: >15/20 residues

• Proteins: Polypeptides with more than 50 residues

Charge of Peptides in Solution

The net charge of a peptide depends on the pKa values of its R groups and the pH of the solution.

Peptide Production

Peptides are often produced by the hydrolysis of precursor proteins via specific proteases.

Proteins

Definition and Importance

Proteins are polypeptides with a defined sequence and number of amino acid residues, conferring unique and specific biological functions. Their properties depend on both the amino acid composition and sequence.

Structural Organization Levels

Proteins exhibit hierarchical structural organization:

• Primary Structure: Linear sequence of amino acids

• Secondary Structure: Local folding patterns (α-helix, β-sheet, β-turns)

• Tertiary Structure: Three-dimensional arrangement of all atoms

• Quaternary Structure: Arrangement of multiple polypeptide subunits

Primary Structure

The primary structure is the unique sequence of amino acids in a polypeptide chain (e.g., insulin, β-lactoglobulin).

Secondary Structure

• α-Helix: Right-handed, stabilized by H-bonds, Ala favors formation, Pro and Gly disrupt

• Collagen Triple Helix: Three polypeptide chains, left-handed, repetitive X-Pro-Gly or X-Hyp-Gly sequences

• β-Pleated Sheet: Can be parallel or antiparallel, stabilized by H-bonds, Ala and Gly are frequent

• β-Turns (Loops): Change direction, join α-helices and β-sheets, Pro and Gly are common

Tertiary Structure

The tertiary structure is the overall three-dimensional shape of a single polypeptide chain, stabilized by electrostatic interactions, hydrogen bonds, non-polar interactions, and disulfide bonds.

Protein Conformations

• Fibrous Proteins: Structural roles (e.g., keratin, collagen)

• Globular Proteins: Functional roles (e.g., myoglobin, enzymes)

Quaternary Structure

Quaternary structure refers to the arrangement of multiple polypeptide subunits, which may be identical or different (e.g., hemoglobin).

• Functions: Lower synthesis energy, easier substitution, complex regulation (allosteric behavior)

• Important: Avoid denaturation to preserve function

Domains and Motifs

Domains are distinct functional and structural units within a protein. Motifs are recurring supersecondary structures (e.g., αα unit, β-hairpin, Greek key).

Enzymes, Vitamins, and Cofactors

Enzymes as Biocatalysts

Enzymes are proteins that catalyze biochemical reactions, increasing reaction rates without being consumed.

• Characteristics: Specificity, regulation, hydrophilicity, occurrence in various cellular locations

• Example: Catalase catalyzes the decomposition of hydrogen peroxide: $2\mathrm{H_2O_2} \rightarrow 2\mathrm{H_2O} + \mathrm{O_2}$

Nomenclature

• Descriptive: Substrate + "ase" (e.g., sucrase)

• Descriptive (Type of Reaction): Substrate + reaction type + "ase" (e.g., alcohol dehydrogenase)

• Non-descriptive: Traditional names (e.g., trypsin, papain)

Enzyme Classification

Enzymes are classified by the type of reaction they catalyze:

1. Oxidoreductases: Catalyze redox reactions

2. Transferases: Transfer functional groups

3. Hydrolases: Catalyze hydrolytic cleavage

4. Lyases: Remove/add groups to double bonds

5. Isomerases: Catalyze isomerization

6. Ligases: Join two molecules

Cofactors and Coenzymes

Many enzymes require non-protein components for activity:

• Cofactor: Inorganic ions or organic molecules required for enzyme activity

• Coenzyme: Organic cofactors (often derived from vitamins)

• Holoenzyme: Complete, catalytically active enzyme with its cofactor

• Apoenzyme: Inactive enzyme without its cofactor

Vitamins

• Water-Soluble: Function as coenzymes, not stored in the body

• Fat-Soluble: Vitamins A, D, E, K; stored in the body, roles in vision, bone formation, antioxidants, blood clotting

Enzyme Mechanisms

• Lock-and-Key Model: Substrate fits exactly into the active site

• Induced Fit Model: Enzyme changes shape to accommodate substrate (e.g., glucose-hexokinase)

Activation Energy and Transition State

Enzymes lower the activation energy required for reactions, stabilizing the transition state and increasing reaction rates.

Zymogens

Zymogens are inactive enzyme precursors that require activation (e.g., by proteolytic cleavage).

Factors Affecting Enzyme Activity

• Temperature: Optimal range for activity

• pH: Each enzyme has an optimal pH

Enzyme Kinetics

Rate of Reaction

The rate of an enzymatic reaction is defined as the amount of product formed per unit time.

Michaelis-Menten Kinetics

The Michaelis-Menten equation describes the relationship between substrate concentration and reaction rate:

$v = \frac{V_{max}[S]}{K_m + [S]}$

• Vmax: Maximum reaction rate

• Km: Substrate concentration at half-maximal velocity

Important Concepts

• International Unit (IU): Amount of enzyme that catalyzes the conversion of 1 μmol of substrate per minute

• Turnover Number (kcat): Number of substrate molecules converted per enzyme molecule per unit time

Enzyme Inhibition

• Competitive Inhibition: Inhibitor competes with substrate for active site; increases Km, Vmax unchanged

• Uncompetitive Inhibition: Inhibitor binds only to enzyme-substrate complex; decreases both Km and Vmax

• Noncompetitive Inhibition: Inhibitor binds to enzyme or enzyme-substrate complex; Vmax decreases, Km unchanged

Type	Km	Vmax
Competitive	Increases	Unchanged
Uncompetitive	Decreases	Decreases
Noncompetitive	Unchanged	Decreases

Summary

This guide covers the foundational concepts of amino acids, peptides, proteins, and enzymes, including their structure, classification, properties, and roles in biochemistry. Understanding these topics is essential for further study in protein function, enzyme kinetics, and metabolic regulation.