MCAT - Bio/Biochem Foundations:: Metabolism

Glycolysis, the initial pathway of glucose catabolism, converts one molecule of glucose (a six-carbon sugar) into:

- **A)** One molecule of a six-carbon product, with no further breakdown
- **B)** Four molecules of a two-carbon product
- **C)** Two molecules of pyruvate (a three-carbon product), along with a net yield of 2 ATP and 2 NADH per glucose molecule
- **D)** Six separate one-carbon CO2 molecules directly

The early steps of glycolysis require an initial investment of ATP (phosphorylating glucose and later intermediates) before any ATP is generated; this energy investment phase serves to:

- **A)** Directly generate the majority of glycolysis's total ATP output
- **B)** Add phosphate groups to the sugar intermediates, which raises their internal energy content and helps 'trap' the sugar within the cell (since phosphorylated sugars cannot easily cross the plasma membrane) while also destabilizing the molecule in a way that facilitates its later breakdown into smaller, higher-energy intermediates during the subsequent payoff phase
- **C)** Immediately convert glucose directly into CO2 and water
- **D)** Have no functional purpose within the pathway, being an unnecessary vestige of no significance

Substrate-level phosphorylation, the mechanism by which ATP is directly generated during glycolysis (as opposed to oxidative phosphorylation), refers to:

- **A)** Direct enzymatic transfer of a phosphate group from a high-energy phosphorylated substrate intermediate directly onto ADP, forming ATP - occurring independent of the electron transport chain, membrane potential, or ATP synthase, and thus capable of proceeding even without oxygen present (anaerobically)
- **B)** ATP synthesis using energy derived from a proton gradient across a membrane, identical to oxidative phosphorylation
- **C)** A process that requires molecular oxygen as an obligatory direct participant
- **D)** A mechanism that can only occur within the mitochondrial matrix, never in the cytosol

Under anaerobic conditions (or in cells lacking mitochondria, like mature red blood cells), pyruvate produced by glycolysis is typically converted to lactate via lactic acid fermentation; this conversion serves the essential purpose of:

- **A)** Directly generating additional ATP beyond what glycolysis itself produces
- **B)** Consuming oxygen, which would otherwise accumulate to toxic levels
- **C)** Producing additional NADH for further use
- **D)** Regenerating NAD+ from the NADH produced earlier in glycolysis (by reducing pyruvate to lactate) - since glycolysis itself requires a continuous supply of NAD+ to proceed (as an electron acceptor at one specific step), and without a functioning electron transport chain (as under anaerobic conditions) to reoxidize NADH back to NAD+, this fermentation reaction becomes the essential mechanism sustaining a fresh NAD+ supply, allowing glycolysis to continue and keep generating ATP via substrate-level phosphorylation even without oxygen

Phosphofructokinase-1 (PFK-1), a key regulatory enzyme catalyzing one of the early, essentially irreversible steps of glycolysis, is considered the primary rate-limiting enzyme of the pathway because it is:

- **A)** The very first enzyme in the glycolytic pathway
- **B)** Allosterically regulated by multiple metabolic signals reflecting the cell's current energy status - notably, PFK-1 is INHIBITED by high ATP and citrate levels (signaling adequate energy/biosynthetic precursor availability) and ACTIVATED by high AMP levels (signaling low cellular energy status) - making PFK-1 activity, and thus overall glycolytic flux, responsive to the cell's real-time energy needs
- **C)** An enzyme that catalyzes a freely reversible reaction with no committed, rate-limiting character
- **D)** Completely unregulated by any metabolic signal, functioning at a constant, fixed rate regardless of cellular conditions

Before entering the citric acid (Krebs) cycle, pyruvate (produced by glycolysis in the cytosol) must first be transported into the mitochondrial matrix and converted into which molecule by the pyruvate dehydrogenase complex?

- **A)** Citrate, directly
- **B)** Oxaloacetate
- **C)** Acetyl-CoA - this oxidative decarboxylation reaction releases one carbon (as CO2) from the original three-carbon pyruvate, produces one NADH, and attaches the remaining two-carbon acetyl group to coenzyme A, forming acetyl-CoA, the molecule that actually enters and combines with oxaloacetate to begin the citric acid cycle
- **D)** Lactate

The citric acid (Krebs) cycle begins when acetyl-CoA's two-carbon acetyl group combines with the four-carbon molecule oxaloacetate to form:

- **A)** Citrate, a six-carbon molecule (the compound giving the cycle its alternative name, the citric acid cycle)
- **B)** Pyruvate directly
- **C)** A five-carbon intermediate
- **D)** Glucose, regenerating the original glycolytic substrate

For each single turn of the citric acid cycle (processing one acetyl-CoA molecule), the net yield of high-energy electron carriers and ATP/GTP produced directly is:

- **A)** 1 ATP, 1 NADH, 1 FADH2
- **B)** 3 ATP, no NADH or FADH2 produced at all
- **C)** 10 NADH, 10 FADH2
- **D)** 3 NADH, 1 FADH2, and 1 GTP (or ATP, depending on the specific tissue/enzyme isoform) generated via substrate-level phosphorylation - these reduced electron carriers (NADH, FADH2) are the cycle's primary energetic output, subsequently feeding into the electron transport chain to drive the large majority of ATP production via oxidative phosphorylation