MCAT - Bio/Biochem Foundations:: Biomolecules

A peptide bond linking two amino acids is formed via which type of reaction?

- **A)** Hydrolysis, releasing a water molecule's worth of energy
- **B)** Dehydration (condensation) synthesis, in which the carboxyl group of one amino acid reacts with the amino group of another, releasing a water molecule
- **C)** Oxidation-reduction, with no water involved
- **D)** Phosphorylation of the amino acid backbone

A protein's primary structure refers specifically to:

- **A)** The linear sequence of amino acids joined by peptide bonds along the polypeptide chain
- **B)** Local, repeating folding patterns like alpha helices and beta sheets
- **C)** The overall three-dimensional folded shape of the entire protein
- **D)** The assembly of multiple separate polypeptide subunits into a functional complex

Secondary structure in proteins, such as the alpha helix and beta pleated sheet, is stabilized primarily by:

- **A)** Disulfide bonds between cysteine residues
- **B)** Hydrophobic interactions between nonpolar side chains exclusively
- **C)** Hydrogen bonds between backbone carbonyl oxygen and amide hydrogen atoms (the peptide backbone itself), forming regular, repeating local structural patterns
- **D)** Ionic bonds between charged side chains

Tertiary structure, the overall three-dimensional folded shape of a single polypeptide chain, is stabilized by which combination of interactions among side chains (R groups)?

- **A)** Only hydrogen bonds along the backbone, identical to secondary structure
- **B)** Only peptide bonds
- **C)** Only van der Waals forces, with no other contribution
- **D)** Hydrophobic interactions (nonpolar side chains clustering away from water in the protein core), hydrogen bonds, ionic bonds (salt bridges) between charged side chains, and covalent disulfide bonds between cysteine residues

Quaternary structure refers specifically to proteins that:

- **A)** Consist of only a single polypeptide chain, regardless of its folding
- **B)** Are composed of two or more separate polypeptide chains (subunits) that associate together to form a single functional protein complex
- **C)** Have no secondary or tertiary structure at all
- **D)** Exist only as denatured, unfolded chains

Denaturation of a protein refers to:

- **A)** Breaking peptide bonds within the primary sequence
- **B)** A process that always destroys the protein permanently and irreversibly, with no exceptions
- **C)** Loss of a protein's native secondary, tertiary, and/or quaternary structure (e.g., due to heat, extreme pH, or certain chemicals) without necessarily breaking the primary sequence's peptide bonds - often resulting in loss of biological function, though some proteins can renature and refold correctly if the denaturing condition is removed
- **D)** The normal, required first step of protein synthesis

Enzymes increase the rate of biochemical reactions primarily by:

- **A)** Lowering the activation energy required for the reaction to proceed, without changing the overall free energy difference (ΔG) between reactants and products
- **B)** Changing the equilibrium position of the reaction, favoring products over reactants
- **C)** Providing energy input that would otherwise be unavailable
- **D)** Permanently consuming themselves in the reaction, like a typical reactant

Competitive enzyme inhibitors work by:

- **A)** Binding to a site distinct from the active site, changing the enzyme's shape
- **B)** Binding directly to the enzyme's active site, structurally resembling the natural substrate and thereby competing with it for access to that site
- **C)** Permanently and irreversibly destroying the enzyme's structure
- **D)** Only functioning on enzymes that have no active site