USMLE Biochemistry Exam Questions And Answers

USMLE Biochemistry Exam Questions And Answers What is the rate-limiting enzyme of glycolysis? - ANSWER- phosphofructokinase-1 (PFK-1) What is the rate-limiting enzyme of gluconeogenesis? - ANSWER- fructose-1,6-bisphosphatase What is the rate-limiting enzyme of the TCA cycle? - ANSWER- isocitrate dehydrogenase What is the rate-limiting enzyme of glycogen synthesis? - ANSWER- glycogen synthase What is the rate-limiting enzyme of glycogenolysis? - ANSWER- glycogen phosphorylase What is the rate-limiting enzyme of the HMP shunt? - ANSWER- glucose-6-phosphate dehydrogenase (G6PD) What is the rate-limiting enzyme of de novo pyrimidine synthesis? - ANSWER- carbamoyl phosphate synthetase II (CPS II) What is the rate-limiting enzyme of de novo purine synthesis? - ANSWER- glutamine-PRPP amidotransferase What is the rate-limiting enzyme of the urea cycle? - ANSWER- carbamoyl phosphate synthetase I (CPS I) What is the rate-limiting enzyme of fatty acid synthesis? - ANSWER- acetyl-CoA carboxylase What is the rate-limiting enzyme of fatty acid oxidation? - ANSWER- carnitine acyltransferase I What is the rate-limiting enzyme of ketogenesis? - ANSWER- HMG-CoA synthase What is the rate-limiting enzyme of cholesterol synthesis? - ANSWER- HMG-CoA reductase How many ATP are formed in the aerobic metabolism of glucose? - ANSWER- malate-aspartate shuttle (heart/liver): 32 ATP glycerol-3-phosphate shuttle (muscle): 30 ATP Compared to glucokinase, what are the Km and Vm of hexokinase? - ANSWER- low Km (high potency), low Vmax What are the cofactors required by the pyruvate dehydrogenase complex? - ANSWER- pyrophosphate (B1, thiamine, TPP) FAD (B2, riboflavin) NAD (B3, niacin) CoA (B5, pantothenate) Lipoic acid What reaction in the TCA cycle requires the same cofactors as the pyruvate dehydrogenase complex? - ANSWER- alpha-ketoglutarate dehydrogenase, which converts alpha-ketoglutarate to succinyl-CoA Poisoning by what substance would result in vomiting, rice water stools, and garlic breath? - ANSWER- arsenic What are the directly ketogenic amino acids? - ANSWER- lysine and leucine What is deficient in a patient with increased pyruvate and alanine, lactic acidosis, and neurologic defects? How can this be treated? - ANSWER- pyruvate dehydrogenase complex deficiency treat with increased intake of ketogenic nutrients (high fat content, increased lysine and leucine) What are the four possible outcomes of pyruvate metabolism? Which of these are made in the cytosol? Which are made in the mitochondria? - ANSWER- Mitochondria: oxaloacetate, acetyl-CoA Cytosol: lactate, alanine Which complex in the electron transport chain is inhibited by amytal (barbiturates), rotenone, and MPP (from opiate synthesis)? - ANSWER- Complex I Which complex in the electron transport chain is inhibited by antimycin A? - ANSWER- Complex II