Question 1

Which of the following is the most likely mechanism of action of a pharmacological agent that prevents ATP generation in isolated mitochondria?
A) activation of cytochrome c
B) decrease of NADH generation by NADH dehydrogenase
C) disruption of pH gradient across mitochondrial membrane
D) increase of transport of pyruvate through the mitochondrial membrane
E) inhibition of glyceraldehydes-3-phosphate dehydrogenase

Accepted Answer

Disruption of the pH gradient across the mitochondrial membrane would prevent ATP generation by uncoupling oxidative phosphorylation, as the proton gradient is essential for ATP synthase function.  The normal flow of electrons, through the proteins of the oxidative phosphorylation machinery, is coupled with the establishment of a proton gradient across the inner mitochondrial membrane. The establishment of this pH gradient is the chemiosmotic potential that is coupled with the production of ATP. Any disruption in this PMF, such as with the addition of an uncoupling agent like dinitrophenol, results in a loss of the energy needed for ATP synthesis but does not prevent oxygen consumption.

Question 2

According to the chemiosmotic theory of how oxidation and phosphorylation are coupled, which of the following processes produces the chemical gradient that drives the generation of ATP from ADP?
A) electron transport causes H+ to penetrate the inner mitochondrial membrane
B) electron transport ejects H+ into the intermembrane space
C) FADH2 is regenerated from FAD
D) NADH is regenerated from NAD+
E) osmotic forces cause H+ to bind with FAD in the mitochondrial matrix

Accepted Answer

Electron transport ejects H+ into the intermembrane space, creating a proton gradient that drives ATP synthesis.  The normal flow of electrons, through the proteins of the oxidative phosphorylation machinery, is coupled with the establishment of a proton gradient across the inner mitochondrial membrane. As electrons flow from one complex to another the energy released is used to pump protons out of the matrix into the inner membrane space. The establishment of this proton gradient is the chemiosmotic potential that is coupled with the production of ATP.

Question 3

In a study of the regulation of body weight, the administration of exogenous leptin to mice is found to increase the activity of proton leak channels located in the inner membrane of mitochondria in adipose tissue cells. Which of the following is most likely to increase as a result of the increased transport of protons through these mitochondrial leak channels? A) heat production B) mitochondrial NADH concentrations C) mitochondrial synthesis of ATP D) NADH reduction from NAD₊ E) pH of the mitochondrial matrix

Accepted Answer

The increased activity of proton leak channels leads to a dissipation of the proton gradient across the mitochondrial membrane, resulting in increased heat production as energy is released as heat instead of being used for ATP synthesis.

Question 4

In newborns, brown fat plays an important role in the generation of heat through which of the following actions?
A) blocking the action of hormone-sensitive lipase
B) increasing the rate of thyroxine synthesis
C) inducing of shivering by leptin
D) promoting proton leakage into mitochondria without ATP formation
E) uncoupling gluconeogenesis from triglyceride hydrolysis

Accepted Answer

Brown fat generates heat through non-shivering thermogenesis by promoting proton leakage into mitochondria, which uncouples oxidative phosphorylation, leading to heat production without ATP formation.  The uncoupling of proton flow releases the energy of the electrochemical proton gradient as heat. This process is a normal physiological function of brown adipose tissue (BAT). Newborn babies contain BAT in their neck and upper back that serves the function of nonshivering thermogenesis. The mitochondria in BAT contain a protein called uncoupling protein 1 (UCP1; also called thermogenin). UCP1 acts as a channel in the inner mitochondrial membrane to control the permeability of the membrane to protons. When norepinephrine is released in response to cold sensation, it binds to β-adrenergic receptors on the surface of brown adipocytes triggering the activation of adenylate cyclase. Activated adenylate cyclase leads to increased production of cAMP and the concomitant activation of PKA with the result being phosphorylation and activation of hormone-sensitive lipase. The released free fatty acids bind to UCP1 triggering an uncoupling of the proton gradient and the release of the energy of the gradient as heat.

Quiz 17: Mitochondrial Functions and Oxidative Phosphorylation

Which of the following is the most likely mechanism of action of a pharmacological agent that prevents ATP generation in isolated mitochondria?

According to the chemiosmotic theory of how oxidation and phosphorylation are coupled, which of the following processes produces the chemical gradient that drives the generation of ATP from ADP?

In newborns, brown fat plays an important role in the generation of heat through which of the following actions?