Biochemistry Multiple Choice Questions

While methionine also contains sulfur, it is involved in initiation and does not readily form disulfide bridges under physiological conditions. Cysteine contains a thiol (-SH) group, and two cysteine residues can oxidize to form a disulfide bond (-S-S-), which is crucial for the tertiary and quaternary structure of many proteins.

PFK-1 is the rate-limiting enzyme of glycolysis. High levels of ATP indicate that the cell has sufficient energy, signaling the need to slow down glycolysis. ATP acts as an allosteric inhibitor, shifting the enzyme to a less active state. Conversely, AMP and ADP activate PFK-1, while Fructose-2,6-bisphosphate is a potent activator.

In nucleotides, the nitrogenous base is always attached to the 1' carbon of the pentose sugar via a glycosidic bond. The phosphate group is attached to the 5' carbon, and the 3' carbon contains a hydroxyl group that forms the phosphodiester bond with the next nucleotide.

Riboflavin is the precursor for both FAD and FMN (Flavin Mononucleotide). These flavoproteins are essential redox carriers in metabolic reactions such as the Electron Transport Chain, fatty acid oxidation (beta-oxidation), and the Citric Acid Cycle.

Km is an inverse measure of the affinity of the enzyme for its substrate. It represents the substrate concentration at which the reaction rate is exactly half of Vmax. A low Km indicates high affinity, meaning the enzyme reaches half-maximal velocity at low substrate concentrations.

Beta-oxidation is the process by which fatty acid molecules are broken down in the mitochondria to generate Acetyl-CoA. Each cycle of beta-oxidation cleaves a two-carbon unit (acetyl-CoA) from the fatty acid chain. This Acetyl-CoA then enters the Citric Acid Cycle.

Glycogenesis is the anabolic process of glycogen synthesis, in which glucose molecules are added to chains of glycogen for storage. The reverse process, breaking glycogen down into glucose-1-phosphate, is called glycogenolysis.

A competitive inhibitor resembles the substrate and binds to the active site of the enzyme. By increasing the concentration of the substrate, the substrate can outcompete the inhibitor for the active site, effectively restoring the reaction velocity to Vmax.

NADH (and FADH2) are high-energy electron carriers produced by the Citric Acid Cycle and other pathways. They donate electrons to the Electron Transport Chain (ETC) complexes embedded in the inner mitochondrial membrane, driving proton pumping and ultimately ATP synthesis.

Myoglobin is a monomeric protein containing a single heme group, whereas hemoglobin is a tetramer. Consequently, their amino acid sequences (primary structures) are completely different, although both function to bind oxygen.

The Cori cycle (or lactic acid cycle) refers to the metabolic pathway in which lactate produced by anaerobic glycolysis in muscles is transported to the liver. The liver converts lactate back into glucose via gluconeogenesis, which is then released back into the blood for use by the muscles.

Both UAC and UAU are codons for the amino acid Tyrosine. Because the change in the DNA sequence does not alter the amino acid incorporated into the polypeptide chain, it is classified as a silent mutation.

DNA Polymerase III is the primary enzyme responsible for elongation in prokaryotes. It possesses 3' to 5' exonuclease activity, which allows it to remove incorrectly incorporated nucleotides immediately after they are added, ensuring high fidelity replication.

Aspartic acid (and Glutamic acid) have carboxyl groups in their side chains. The pKa of these groups is usually around 3.9 to 4.1. Since physiological pH is higher than the pKa, these groups will be deprotonated and carry a negative charge. Lysine and Arginine are positively charged, while Histidine varies but is often partially positive.

The biosynthesis of cholesterol begins with Acetyl-CoA. Two molecules of Acetyl-CoA condense to form Acetoacetyl-CoA, which then reacts with another Acetyl-CoA to form HMG-CoA. While HMG-CoA is a crucial intermediate (and the target of statins), the fundamental carbon unit is Acetyl-CoA.