Biochemistry Peptide Structure, Properties, and Protein Folding Study Notes

Peptide Structure and Properties

Origins of Key Biochemical Forces and Properties

Understanding the forces and properties that govern biomolecular structure and function is fundamental in biochemistry. These include:

• Intramolecular Forces: Forces within a molecule, such as covalent bonds, ionic bonds, and hydrogen bonds, that stabilize molecular structure.

• Intermolecular Forces: Forces between molecules, including hydrogen bonding, van der Waals interactions, and ionic interactions, which influence molecular interactions and assembly.

• Electronegativity: The tendency of an atom to attract electrons in a chemical bond, affecting bond polarity and reactivity.

• Equilibrium Constant (Keq): Describes the ratio of product to reactant concentrations at equilibrium for a reversible reaction.

• pH and pKa: pH measures hydrogen ion concentration; pKa is the acid dissociation constant, indicating the strength of an acid.

• Reducing and Oxidizing Agents: Reducing agents donate electrons; oxidizing agents accept electrons, driving redox reactions in biological systems.

Peptide Analysis: MTWQEHPNV

This peptide sequence consists of the following amino acids: Methionine (M), Threonine (T), Tryptophan (W), Glutamine (Q), Glutamic acid (E), Histidine (H), Proline (P), Asparagine (N), Valine (V).

• N- and C-Terminal Identification: The N-terminal is Methionine (M), and the C-terminal is Valine (V).

• Peptide Bonds: Peptide bonds link the carboxyl group of one amino acid to the amino group of the next, forming a planar structure due to partial double-bond character.

• Planarity of Peptide Bonds: All peptide bonds are planar; rotation is restricted around the peptide bond itself, but allowed around the adjacent α-carbon bonds (phi and psi angles).

• Ionizable Side Chains: In this sequence, Histidine (H), Glutamic acid (E), and possibly the N- and C-termini are ionizable at physiological pH.

• Polar Side Chains: Threonine (T), Glutamine (Q), Histidine (H), Glutamic acid (E), Asparagine (N) have polar side chains.

• Non-Polar Side Chains: Methionine (M), Tryptophan (W), Proline (P), Valine (V) are non-polar.

• Net Charge at Various pH: The net charge depends on the ionization state of the N-terminus, C-terminus, and side chains. At low pH, groups are protonated (positive charge); at high pH, deprotonated (negative charge).

Functional Residues in Peptides

• Phosphorylation: Typically occurs on serine, threonine, or tyrosine residues. In this peptide, threonine (T) can be phosphorylated, altering protein function and activity.

• Glycosylation: Common on asparagine (N) (N-linked) or serine/threonine (O-linked). Asparagine (N) in this peptide could be glycosylated, affecting folding and stability.

• Methylation/Acetylation: Lysine, arginine, and N-termini are common sites. This peptide does not contain lysine or arginine, but the N-terminus could be modified.

Charge of the Peptide at Different pH Values

The net charge of a peptide changes with pH due to the ionization of amino and carboxyl termini and side chains. To determine the net charge at specific pH values (e.g., 1.9, 3.7, 7.3, 9.8):

• At low pH (1.9): All groups are protonated; net charge is positive.

• At intermediate pH (3.7, 7.3): Some groups lose protons; net charge decreases.

• At high pH (9.8): Most groups are deprotonated; net charge is negative.

Equation for Net Charge:

$\text{Net charge} = \sum \text{(positive charges)} - \sum \text{(negative charges)}$

Protein Structure and Folding

Role of Proline in Protein Structure

Proline is unique among amino acids due to its cyclic structure, which restricts backbone flexibility and disrupts regular secondary structures.

• Secondary Structure Disruption: Proline often introduces kinks or turns in α-helices and β-sheets, preventing their regular formation.

• Breaking Tertiary Structure: Proline can disrupt tertiary interactions, affecting overall protein folding.

• Quaternary Structure: Proline-induced breaks can influence the assembly of multi-subunit proteins.

Example: In collagen, proline residues are essential for the triple helix structure, but in globular proteins, proline often marks the end of helices.

Protein Folding and Conformational Stability

Proteins can adopt multiple conformations, but only certain structures are stable under physiological conditions. Folding is driven by hydrophobic interactions, hydrogen bonding, van der Waals forces, and disulfide bonds.

• Folding Pathways: Proteins fold via intermediate states to reach their native conformation.

• Stable Conformations: Some proteins can exist in two or more stable conformations, a property known as conformational isomerism.

• Energy Landscape: The folding process is often depicted as a funnel-shaped energy landscape, with the native state at the lowest energy.

Equation for Gibbs Free Energy Change in Folding:

$\Delta G = \Delta H - T\Delta S$

where $\Delta G$ is the change in free energy, $\Delta H$ is the change in enthalpy, $T$ is temperature, and $\Delta S$ is the change in entropy.

Table: Amino Acid Properties Relevant to Peptide Analysis

Amino Acid	Side Chain Type	Ionizable?	Possible Modifications
Methionine (M)	Non-polar	No	Oxidation
Threonine (T)	Polar	No	Phosphorylation, O-glycosylation
Tryptophan (W)	Non-polar	No	Oxidation
Glutamine (Q)	Polar	No	Deamidation
Histidine (H)	Polar	Yes	Methylation
Glutamic acid (E)	Polar, acidic	Yes	Methylation
Proline (P)	Non-polar	No	Hydroxylation
Asparagine (N)	Polar	No	N-glycosylation
Valine (V)	Non-polar	No	None common

Additional info: The above notes expand on the brief questions by providing definitions, explanations, and context for each biochemical concept, as well as a table summarizing relevant amino acid properties for peptide analysis.