What would happen to the proton gradient and ATP production after a drug has poisoned the enzyme that combines acetyl CoA and oxaloacetate to form citrate

The proton gradient becomes weaker

Reduction in the amount of ATP produced.

Explanation:

The combination of acetyl CoA and oxaloacetate drives the formation of nicotinamide adenine dinucleotide + hydrogen (NADH). Poisoning the enzyme that aids this combination will result to lesser production of NADH which would lead to weakening the proton gradient and the reduction in the amount of adenosine triphosphate (ATP) produced.