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MCAT · Biological and Biochemical Foundations · Biochemistry
Metabolism and bioenergetics is part of Biochemistry in the MCAT Biological and Biochemical Foundations section. The questions below are written in the style of the real test, each with the answer and a worked explanation.
Question types you will see here include glycolysis regulation, aTP and reducing-equivalent accounting, electron transport inhibitors and uncouplers and fed-to-fasted metabolic switch.
Common traps: treating hexokinase as the main control in liver; reversing the effect of citrate or AMP on PFK-1; missing tissue differences (liver versus muscle).
Question 1
A reaction with ΔG°′ = +20.0 kJ/mol is coupled to the hydrolysis of ATP to ADP and Pi (ΔG°′ = −30.5 kJ/mol). What is ΔG°′ for the coupled reaction?
−50.5 kJ/mol
−30.5 kJ/mol
−10.5 kJ/mol
+10.5 kJ/mol
Answer: C. −10.5 kJ/mol
Standard free-energy changes of coupled reactions add: +20.0 + (−30.5) = −10.5 kJ/mol, so the coupled process is favorable under standard conditions. The value −50.5 kJ/mol comes from subtracting +20.0 instead of adding it.
Question 2
During intense exercise, skeletal muscle converts pyruvate to lactate. What is the main purpose of this reaction?
To regenerate NAD⁺ so that glycolysis can continue
To convert pyruvate into glucose within the muscle
To produce additional ATP directly from pyruvate
To make FADH₂ for the electron transport chain
Answer: A. To regenerate NAD⁺ so that glycolysis can continue
Lactate dehydrogenase oxidizes NADH to NAD⁺, which glyceraldehyde-3-phosphate dehydrogenase needs to keep glycolysis running when oxidative capacity is limited. The lactate dehydrogenase reaction itself makes no ATP; the benefit is the continued ATP production by glycolysis.
Question 3
Which enzyme is used in gluconeogenesis but not in glycolysis?
Hexokinase
Pyruvate carboxylase
Aldolase
Phosphoglycerate kinase
Answer: B. Pyruvate carboxylase
Pyruvate carboxylase converts pyruvate to oxaloacetate, the first step in bypassing the irreversible pyruvate kinase reaction. Aldolase and phosphoglycerate kinase catalyze reversible reactions used in both pathways, and hexokinase is used only in glycolysis.
Question 4
The uncoupler 2,4-dinitrophenol is added to isolated, respiring mitochondria that have substrate and ADP. Which set of changes is expected?
Oxygen consumption rises, ATP synthesis falls and more energy is released as heat.
Oxygen consumption falls, ATP synthesis falls and the proton gradient rises.
Oxygen consumption rises, ATP synthesis rises and less heat is released.
Oxygen consumption is unchanged, because the electron transport chain is blocked by the uncoupler.
Answer: A. Oxygen consumption rises, ATP synthesis falls and more energy is released as heat.
A is correct: an uncoupler lets protons leak back across the inner membrane, so the chain runs faster without making ATP, and the energy is lost as heat. B: the gradient falls and oxygen use rises with an uncoupler. C: ATP synthase has no gradient to use and heat rises. D: an uncoupler leaves the chain intact and speeds it.
Question 5
In liver, glucagon acting through PKA phosphorylates the bifunctional enzyme PFK-2/FBPase-2. A mouse is engineered so this enzyme cannot be phosphorylated and its kinase activity stays dominant. During a fast, how will hepatic metabolism compare with that of a normal mouse?
Glycolysis will fall further, because fructose-2,6-bisphosphate drops
Glycolysis stays higher and gluconeogenesis lower, as F-2,6-BP remains
Glycolysis will rise, because accumulating citrate activates PFK-1
Nothing will change, because hexokinase sets the liver's glycolytic flux
Answer: B. Glycolysis stays higher and gluconeogenesis lower, as F-2,6-BP remains
Normally phosphorylation turns PFK-2 off and FBPase-2 on, lowering fructose-2,6-bisphosphate. Here F-2,6-BP stays high, which keeps activating PFK-1 and inhibiting FBPase-1 (B). A describes the normal response, D wrongly makes hexokinase the liver's control point, and C reverses citrate, which inhibits PFK-1.
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