Metabolism MCQ

Metabolism questions on the INBDE reward a map, not memorized trivia: know where each pathway runs, what it yields, and the one enzyme that controls its rate. Then connect it to dentistry, because the same glucose metabolism that fuels your cells is what oral bacteria use to make the acid that causes caries, and the fasting and fed switches explain ketone breath, healing, and the diabetic patient.

Glycolysis & Gluconeogenesis

• Glycolysis runs in the cytoplasm of every cell, converting glucose to pyruvate for a net 2 ATP and 2 NADH. PFK-1 is the committed, rate-limiting step.
• Hexokinase (everywhere, low Km, inhibited by its product G6P) versus glucokinase (liver and pancreas, high Km, not product-inhibited) sets how tissues trap glucose.
• Without oxygen, pyruvate is reduced to lactate to regenerate NAD+ so glycolysis can continue; the liver recycles that lactate back to glucose (Cori cycle).
• Gluconeogenesis is essentially glycolysis in reverse in the liver, using four bypass enzymes (pyruvate carboxylase, PEPCK, fructose-1,6-bisphosphatase, glucose-6-phosphatase) to make glucose during fasting.

Krebs Cycle & Oxidative Phosphorylation

• Pyruvate enters the mitochondrion and is converted to acetyl-CoA by pyruvate dehydrogenase, which needs thiamine (B1) and other B-vitamin cofactors.
• The Krebs cycle (matrix) oxidizes acetyl-CoA to CO2, capturing energy as NADH and FADH2; isocitrate dehydrogenase is rate-limiting.
• The electron transport chain (complexes I to IV) pumps protons across the inner membrane; ATP synthase then uses that gradient to make ATP (chemiosmosis). Roughly 2.5 ATP per NADH and 1.5 per FADH2.
• Cyanide and carbon monoxide block complex IV; oligomycin blocks ATP synthase; uncouplers (such as 2,4-dinitrophenol) dissipate the gradient as heat. All collapse aerobic ATP production.

Amino Acid Metabolism

• There are nine essential amino acids the body cannot make and must obtain from diet; the rest are synthesized from metabolic intermediates.
• Transamination (by ALT and AST, using vitamin B6 / pyridoxal phosphate) moves amino groups onto carbon skeletons that feed glycolysis or the Krebs cycle.
• The liver converts the ammonia from amino acid breakdown into urea (the urea cycle); liver failure lets ammonia rise and impair the brain.
• Phenylketonuria (PKU) is loss of phenylalanine hydroxylase, so phenylalanine cannot become tyrosine and accumulates; treatment is a low-phenylalanine diet and avoiding aspartame, which contains phenylalanine.

Lipid Metabolism & Ketone Bodies

• Fatty acids are broken down by beta-oxidation in the mitochondria after the carnitine shuttle (CPT-1) carries them in; fatty acid synthesis runs separately in the cytoplasm.
• When insulin is low (fasting, uncontrolled diabetes), the liver turns acetyl-CoA into ketone bodies (acetoacetate, beta-hydroxybutyrate, and acetone) as fuel for the brain and muscle.
• Excess ketones lower blood pH (ketoacidosis), and volatile acetone gives the breath its fruity smell, classically in diabetic ketoacidosis.
• Cholesterol synthesis is controlled by HMG-CoA reductase, the enzyme statins inhibit.