Link: https://www.youtube.com/watch?reload=9&v=q6-FlFW01kM

Tip: When you see a carbonyl reaction, ask yourself if the carbonyl was attacked (more common) or if the carbonyl was the attacker. If you see a change in what was attached to the carbonyl carbon, the carbonyl carbon was attacked by a nucleophile and either an addition (aldehyde or ketone becomes an alcohol) or a substitution reaction (carbonyl with a new leaving group) took place. If the original carbonyl is intact with a new bond to the alpha carbon, the alpha carbon of the carbonyl actd as a nucleophile (carboxylate or enolate).

Note: In biochemistry, enzymes act as catalysts for any chemical reaction; and the class of enzyme that catalyzes aldol reactions is called, quite intuitively, 'aldolase'. Let’s now discuss the first step of Kreb’s cycle: acetyl coenzyme A (acetyl CoA) condenses with oxaloacetate to produce (S)-Citryl CoA through an aldol mechanism. Here, instead of an aldehyde or a ketone, a thioester acts as the nucleophilic partner