Cell biology and biochemistry
Practice questions with explanations

D. They accumulate in the acidic canaliculus, are converted to the active sulfenamide, and bind irreversibly to the pump

The requirement for acid activation explains why they must be given before a meal when pumps are active, and irreversible binding explains why the duration of effect far exceeds the plasma half-life, since new pump synthesis is required. It also explains why efficacy builds over several days rather than being maximal on day one.

C. The mitochondrion

Oxidative phosphorylation on the inner mitochondrial membrane generates most cellular ATP, with the electron transport chain creating a proton gradient that drives ATP synthase. Drugs and toxins that uncouple this gradient, such as dinitrophenol, or inhibit the chain, such as cyanide, cause energy failure. Mitochondrial toxicity also underlies some adverse effects of antiretrovirals and of metformin-associated lactic acidosis.

D. Vitamin K

Vitamin K is the cofactor for gamma-glutamyl carboxylase, which creates the calcium-binding sites on clotting factors II, VII, IX and X and on proteins C and S. Warfarin blocks the reductase that regenerates the active form of vitamin K, so the factors are made but non-functional. Vitamin C is a cofactor in collagen hydroxylation, and B12 in methylmalonyl-CoA conversion.