Biochemistry Study Notes: Protein Structure, Function, and Enzyme Catalysis EXAM 2

Protein Structure and Function

Primary, Secondary, and Tertiary Structure

Proteins are complex biomolecules whose structure determines their function. The organization of protein structure is hierarchical:

• Primary Structure: The linear sequence of amino acids in a polypeptide chain.

• Secondary Structure: Local folding patterns such as α-helices and β-sheets, stabilized by hydrogen bonds.

• Tertiary Structure: The overall three-dimensional shape of a single polypeptide, stabilized by interactions including hydrophobic effects, hydrogen bonds, ionic interactions, and disulfide bonds.

Example: The diagram of a helical wheel shows the distribution of hydrophobic and hydrophilic residues in an α-helix, with polar and nonpolar sides.

Amino Acid Properties and Classification

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

• Polar, Negatively Charged: Aspartate, Glutamate

• Polar, Positively Charged: Lysine, Arginine, Histidine

• Nonpolar: Alanine, Valine, Leucine, Isoleucine, Methionine, Phenylalanine, etc.

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

Example: Amino acids with charged side chains are often found on the surface of proteins, interacting with the aqueous environment.

Peptide Bond and Protein Backbone

The peptide bond is formed between the carboxyl group of one amino acid and the amino group of another, resulting in a planar, rigid structure due to resonance.

• Peptide bond: $\text{CO-NH}$ linkage, partial double-bond character

• Backbone dipole: The peptide backbone has a dipole moment, with the N-terminus being partially positive and the C-terminus partially negative.

Protein Techniques and Analysis

Hemoglobin Variants and Oxygen Binding

Hemoglobin is a tetrameric protein responsible for oxygen transport in blood. Variants arise from single amino acid substitutions, affecting function:

• HbS (sickle-cell): Substitution of Val for Glu on the surface, leading to aggregation and sickling.

• HbC: Substitution of Lys for Glu, affecting solubility.

• Hb Pro: Substitution of Pro for a residue in a helix, disrupting secondary structure.

Oxygen Binding Curve: Hemoglobin exhibits cooperative binding, shown by a sigmoidal curve. Variants can shift the curve, indicating changes in oxygen affinity.

Chaperones and Protein Folding

Molecular chaperones assist in the proper folding of proteins and prevent misfolding and aggregation, especially under cellular stress.

• Chaperones bind to unfolded or misfolded proteins, facilitating correct folding.

• They are crucial for maintaining protein homeostasis.

Enzyme Catalysis and Kinetics

Enzyme Function and Catalytic Strategies

Enzymes accelerate chemical reactions by lowering the activation energy ($E_a$) through various strategies:

• Acid-base catalysis: Transfer of protons to stabilize intermediates.

• Covalent catalysis: Formation of transient covalent bonds with substrates.

• Transition state stabilization: Enzyme active site stabilizes the transition state.

• Proximity/orientation effects: Enzyme brings substrates together in the correct orientation.

• Metal ion catalysis: Metal ions participate in catalysis by stabilizing charges or activating substrates.

Reaction Coordinate Diagram

The energy profile of an enzyme-catalyzed reaction shows a lower activation energy compared to the uncatalyzed reaction:

• Activation energy ($E_a$): The energy barrier that must be overcome for a reaction to proceed.

• Enzymes lower $E_a$, increasing reaction rate.

Equation:

$k = A e^{-E_a/RT}$

where $k$ is the rate constant, $A$ is the pre-exponential factor, $E_a$ is activation energy, $R$ is the gas constant, and $T$ is temperature.

Enzyme Mechanism Example: Asp32 in Peptide Catalysis

In serine proteases, Asp32 acts as a general base, removing a proton from a water molecule to activate it for nucleophilic attack on the peptide bond:

• Step 1: Water is deprotonated by Asp32.

• Step 2: Activated water attacks the carbonyl carbon of the peptide bond.

• Step 3: Peptide bond is cleaved.

Protein Structure: Disulfide Bonds and Helices

Disulfide Bonds

Disulfide bonds are covalent linkages between the sulfur atoms of two cysteine residues, stabilizing protein tertiary and quaternary structure.

α-Helix Structure

The α-helix is a right-handed helical structure stabilized by hydrogen bonds between backbone amides:

• Each turn contains approximately 3.6 residues.

• Hydrogen bonds form between the carbonyl oxygen of residue $i$ and the amide hydrogen of residue $i+4$.

Protein-Ligand Interactions

Effectors and Hemoglobin

Effectors modulate hemoglobin's oxygen affinity:

Effector	Effect
BPG (2,3-bisphosphoglycerate)	Stabilizes the T-state, decreasing oxygen affinity
CO	Stabilizes the R-state, increasing oxygen affinity
Low pH (high [H+])	Stabilizes the T-state, decreasing oxygen affinity (Bohr effect)

Summary Table: Protein Structure and Interactions

Interaction Type	Role in Protein Structure
Hydrophobic interactions	Stabilize tertiary structure by driving nonpolar residues to the protein interior
Hydrogen bonds	Stabilize secondary and tertiary structures
Disulfide bonds	Stabilize tertiary and quaternary structures
Ionic bonds	Stabilize tertiary structure, especially on protein surfaces

Additional info:

• Some context and explanations have been expanded for clarity and completeness.

• Equations and tables have been formatted for study purposes.