Rate-Limiting Steps, USMLE

Question 1

Rate limiting step for glycogenolysis

Answer 1

Glycogen phosphorylase

Question 2

Rate limiting step for HMP shunt

Answer 2

G6PD

Question 3

Rate limiting step for De novo pyrimidine synthesis

Answer 3

Carbamoyl phosphate synthetase II

Question 4

Rate limiting step for De novo purine synthesis

Answer 4

Glutamine-PRPP aminotransferase

Question 5

Rate limiting step for glycolysis

Answer 5

Phosphofructokinase-1

Question 6

Rate limiting step for Gluconeogenesis

Answer 6

fructose-1,6-bisphosphatase

Question 7

Rate limiting step for TCA cycle (krebb cycle)

Answer 7

Isocitrate dehydrogenase

Question 8

Rate limiting step for glycogen synthesis

Answer 8

glycogen synthase

Question 9

Rate limiting step for Urea cycle/synthesis

Answer 9

carbamoyl phosphate synthetase I (mitochondria)

Question 10

Rate limiting step for Fatty acid synthesis

Answer 10

Acetyl-coA carboxylase (ACC)

Question 11

Rate limiting step for fatty acid oxidation

Answer 11

Carnitine acyltransferase I

Question 12

Rate limiting step for ketogenesis

Answer 12

HMG-coA synthase

Question 13

Rate limiting step for cholesterol synthesis

Answer 13

HMG-CoA reductase

Question 14

Rate limiting step for heme synthesis

Answer 14

Aminolevulinate synthase (ALA synthetase)

Question 15

Rate limiting step for bile acid synthesis

Answer 15

7-alpha-hydroxylase

Question 16

Gluconeogenesis enzyme that is stimulated by cortisol, epinephrine, and glucagon when energy levels from beta oxidation are adequate

Answer 16

Phosphoenolpyruvate carboxykinase

Question 17

Ezyme bridges glycolysis with the tricarboxylic acid cycle and requires co-factors from the electron transport chain (NAD/FAD; If hypoxic = no cofactors –> no ezyme activity)

Answer 17

Pyruvate dehydrogenase ( Pyruvate + co A —(enzyme used) –> acetyl-co a)

Question 18

Rate-limiting enzyme: Uric acid metabolism

Answer 18

Xanthine oxidase

Question 19

Catecholamine synthesis

Answer 19

Tyrosine hydroxylase