Which statement best defines substrate channeling and provides an example pathway?

Which statement best defines substrate channeling and provides an example pathway?

• A. Substrate channeling is the direct transfer of intermediates between enzymes, reducing diffusion losses; examples include the pyruvate dehydrogenase complex and purine biosynthesis enzyme clusters.
• B. Substrate channeling is the diffusion of products into the bloodstream.
• C. Substrate channeling requires impermeable membranes.
• D. Substrate channeling only occurs in prokaryotes.

Answer: (A)

Substrate channeling means the direct handoff of an intermediate from one enzyme to the next within a coordinated, often multi-enzyme assembly. This keeps the intermediate from diffusing away into the bulk solvent, which boosts the overall speed of the pathway and can protect unstable or reactive intermediates as they are transferred along the sequence. A classic example is the pyruvate dehydrogenase complex, where pyruvate is converted through a series of tightly associated enzymes and the acetyl group is shuttled along by a lipoamide arm from E1 to E2 and then to E3 without equilibrating with the surrounding solution. Similar organization occurs in purine biosynthesis enzyme clusters (sometimes called purinosomes), where successive steps are physically linked to pass intermediates directly between enzymes. The other statements don’t fit because channeling is not simply about releasing products into the bloodstream, it doesn’t require impermeable membranes, and it isn’t restricted to prokaryotes.

Substrate channeling means the direct handoff of an intermediate from one enzyme to the next within a coordinated, often multi-enzyme assembly. This keeps the intermediate from diffusing away into the bulk solvent, which boosts the overall speed of the pathway and can protect unstable or reactive intermediates as they are transferred along the sequence.

A classic example is the pyruvate dehydrogenase complex, where pyruvate is converted through a series of tightly associated enzymes and the acetyl group is shuttled along by a lipoamide arm from E1 to E2 and then to E3 without equilibrating with the surrounding solution. Similar organization occurs in purine biosynthesis enzyme clusters (sometimes called purinosomes), where successive steps are physically linked to pass intermediates directly between enzymes.

The other statements don’t fit because channeling is not simply about releasing products into the bloodstream, it doesn’t require impermeable membranes, and it isn’t restricted to prokaryotes.