Biochemistry Exam Practice: Protein Structure, Function, and Enzyme Catalysis

Protein Structure and Primary Sequence

Primary Structure of Peptides

The primary structure of a protein refers to the linear sequence of amino acids joined by peptide bonds. This sequence determines the protein's unique characteristics and function.

• Peptide Bonds: Covalent bonds formed between the carboxyl group of one amino acid and the amino group of another.

• Example: The peptide shown is Cys-Ala-Gly-Glu-Ser-Arg.

• Directionality: The sequence is always written from the N-terminus (amino end) to the C-terminus (carboxyl end).

Peptide Bond Rotation and Hydrogen Bonding

Peptide bonds are planar and do not rotate freely, but the bonds adjacent to the alpha carbon can rotate, allowing for protein folding.

• Hydrogen Bonding: Hydrogen bonds can form between backbone atoms, stabilizing secondary structures such as alpha helices and beta sheets.

• Example: If the peptide is placed in a helix, hydrogen bonds would form between the carbonyl oxygen of one residue and the amide hydrogen of another, typically four residues apart.

Protein Structure: Helices and Membrane Association

Alpha Helix Structure

The alpha helix is a common secondary structure in proteins, stabilized by hydrogen bonds between backbone atoms.

• Helical Wheel: A diagram showing the orientation of amino acid side chains around the helix. Hydrophobic residues often cluster on one side, facilitating membrane association.

• Membrane Proteins: Helices embedded in membranes typically have hydrophobic side chains facing the lipid bilayer and hydrophilic side chains facing the aqueous environment.

Helix Side	Residue Type
Facing Membrane	Hydrophobic
Facing Solution	Hydrophilic

Protein Folding and Stability

Forces Driving Protein Folding

Protein folding is driven by several forces, including entropic and enthalpic contributions.

• Hydrophobic Effect: The burial of hydrophobic amino acids in the protein core increases entropy by releasing ordered water molecules.

• Electrostatic Interactions: Favorable interactions between charged side chains stabilize the folded structure.

pH Effects on Protein Structure

Changes in pH can alter the charge state of amino acid side chains, affecting protein folding and stability.

• Example: At low pH, acidic side chains are protonated, which can disrupt salt bridges and hydrogen bonds.

Hemoglobin Structure and Function

Oxygen Binding and Cooperativity

Hemoglobin is a tetrameric protein that binds oxygen cooperatively, meaning the binding of one oxygen molecule increases the affinity for subsequent oxygen molecules.

• Cooperative Binding: Results in a sigmoidal oxygen binding curve.

• Bohr Effect: Lower pH (higher H+ concentration) decreases oxygen affinity, shifting the curve to the right.

Condition	Curve Shift
Lower pH	Right
Higher pH	Left

Hemoglobin Mutations

Mutations in hemoglobin can affect its oxygen affinity and physiological function.

• Example: In Hb Providence, a mutation replaces Lys-82, altering the central cavity and affecting oxygen binding.

Enzyme Catalysis and Mechanisms

Enzyme Active Sites and Metal Ions

Many enzymes require metal ions for catalysis. The active site often contains amino acids that coordinate these ions.

• Example: Carboxypeptidase contains two histidine residues that coordinate a Zn2+ ion.

Enzyme Mechanisms

Enzymes catalyze reactions by lowering the activation energy, often through acid-base catalysis, covalent catalysis, or metal ion catalysis.

• Acid-Base Catalysis: Amino acid side chains donate or accept protons.

• Covalent Catalysis: Enzyme forms a transient covalent bond with the substrate.

• Metal Ion Catalysis: Metal ions stabilize charged intermediates or participate in redox reactions.

Reaction Coordinate Diagrams

Reaction coordinate diagrams illustrate the energy changes during a reaction. Enzymes lower the activation energy, increasing the reaction rate.

• Lower Activation Energy: Enzyme 1 with a lower peak in the diagram catalyzes the reaction faster than Enzyme 2.

Summary Table: Enzyme Effects on Reaction Rate

Enzyme	Activation Energy	Reaction Rate
Enzyme 1	Lower	Faster
Enzyme 2	Higher	Slower

Additional info:

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

• Key terms such as "primary structure," "cooperativity," and "catalysis" are defined for self-contained study.