Free BACE Chemistry and Biochemistry Questions and Answers 2 — Questions and Answers
Question 1: What is the chemical basis for enzyme catalysis?
- Enzymes change the equilibrium constant of reactions making unfavorable reactions thermodynamically possible
- Enzymes lower the activation energy of reactions increasing the reaction rate without being consumed (Correct answer)
- Enzymes increase the temperature of the microenvironment around substrates to accelerate reactions
- Enzymes supply energy (ATP) directly to drive otherwise endergonic reactions
Correct answer: Enzymes lower the activation energy of reactions increasing the reaction rate without being consumed
Enzymes lower the activation energy (Ea) of biochemical reactions by stabilizing the transition state, dramatically increasing the reaction rate without being permanently altered.
Enzymes work by binding substrates in the active site and stabilizing the transition state through electrostatic interactions, covalent catalytic intermediates, and proper substrate orientation. The activation energy is lower than the uncatalyzed reaction. This does NOT change the equilibrium — the reaction proceeds to the same endpoint but faster. Enzyme-catalyzed reactions can be 10^6 to 10^14 times faster than the uncatalyzed reaction.
Question 2: What is the difference between an agonist and an antagonist in the context of receptor pharmacology?
- An agonist binds and activates a receptor; an antagonist binds but blocks receptor activation (Correct answer)
- An agonist inhibits receptor signaling; an antagonist activates receptor signaling
- An agonist is a small molecule; an antagonist is always a large protein
- An agonist is natural; an antagonist is always synthetic
Correct answer: An agonist binds and activates a receptor; an antagonist binds but blocks receptor activation
Agonists bind receptors and trigger their biological response; antagonists bind receptors but block the response, often competing with natural agonists.
Agonists mimic the natural ligand (e.g., insulin receptor agonists for diabetes treatment). Antagonists compete with natural ligands and block signaling (e.g., monoclonal antibodies like trastuzumab block HER2 receptor activation in cancer). Partial agonists activate but at less than maximum efficacy. Understanding agonist/antagonist profiles is essential in designing therapeutic biologics and screening assays.
Question 3: What is the Michaelis-Menten constant (Km) a measure of?
- The maximum rate of an enzyme-catalyzed reaction when substrate is saturating
- The substrate concentration at which the reaction rate is half of Vmax reflecting the affinity of the enzyme for its substrate (Correct answer)
- The rate at which an enzyme-substrate complex dissociates back to free enzyme and substrate
- The optimal pH at which an enzyme has maximum activity
Correct answer: The substrate concentration at which the reaction rate is half of Vmax reflecting the affinity of the enzyme for its substrate
Km is the substrate concentration at which reaction rate = Vmax/2. A low Km indicates high affinity (enzyme is half-saturated at low substrate concentration); a high Km indicates low affinity.
In Michaelis-Menten kinetics: v = (Vmax x [S]) / (Km + [S]). When [S] = Km, v = Vmax/2. Low Km: enzyme achieves half-maximum rate at low substrate concentration (high affinity). High Km: high substrate concentration needed for half-maximum rate (low affinity). In competitive inhibition, apparent Km increases. In non-competitive inhibition, apparent Vmax decreases while Km is unchanged.
Question 4: What type of chemical bond holds the two strands of the DNA double helix together?
- Covalent phosphodiester bonds
- Hydrogen bonds between complementary base pairs (Correct answer)
- Ionic bonds between negatively charged phosphates and positive ions
- Disulfide bonds between cysteine residues in the DNA structure
Correct answer: Hydrogen bonds between complementary base pairs
The two DNA strands are held together by hydrogen bonds between complementary base pairs (A-T: 2 hydrogen bonds; G-C: 3 hydrogen bonds).
Hydrogen bonds are non-covalent interactions. In DNA: A pairs with T via 2 hydrogen bonds; G pairs with C via 3 hydrogen bonds. The overall stability of the double helix also comes from base stacking interactions. GC content determines melting temperature — higher GC content means higher Tm because G-C pairs have 3 hydrogen bonds vs 2 for A-T.
Question 5: What is the primary structure of a protein?
- The three-dimensional folded conformation of the complete polypeptide chain
- The linear sequence of amino acids linked by peptide bonds (Correct answer)
- The coiled or sheet-like local structure formed by hydrogen bonding of the backbone
- The arrangement of multiple polypeptide subunits into a functional complex
Correct answer: The linear sequence of amino acids linked by peptide bonds
Primary structure is the sequence of amino acids in the polypeptide chain, connected by peptide bonds — it is the most fundamental level of protein structure.
Protein structure has four hierarchical levels: (1) Primary: amino acid sequence; (2) Secondary: local regular structures stabilized by backbone hydrogen bonds (alpha helices, beta sheets); (3) Tertiary: overall 3D fold of the single polypeptide chain; (4) Quaternary: arrangement of multiple polypeptide chains in multisubunit proteins. The primary structure determines all higher levels of structure through the principles of thermodynamic protein folding.
Question 6: What is the function of NADH and FADH2 in cellular respiration?
- They directly phosphorylate ADP to ATP in the cytoplasm
- They are electron carriers that donate electrons to the mitochondrial electron transport chain driving ATP synthesis (Correct answer)
- They are cofactors required for glycolysis reactions in the cytoplasm
- They transport carbon dioxide from cells to the lungs for exhalation
Correct answer: They are electron carriers that donate electrons to the mitochondrial electron transport chain driving ATP synthesis
NADH and FADH2 are reduced electron carriers that transfer high-energy electrons to the mitochondrial electron transport chain, driving proton pumping and ATP synthesis.
During the citric acid cycle and glycolysis, NAD+ and FAD are reduced to NADH and FADH2 by accepting electrons from oxidized substrate molecules. At the inner mitochondrial membrane, NADH donates electrons at Complex I, and FADH2 donates at Complex II. Electrons flow through the ETC to Complex IV where oxygen is the final electron acceptor. The proton gradient drives ATP synthase. Each NADH yields ~2.5 ATP; each FADH2 yields ~1.5 ATP.
Free BACE Chemistry and Biochemistry Questions and Answers 2
What is the chemical basis for enzyme catalysis?