Formation of Schiff's base is one of those mechanisms you may have blown past in your organic course, but which has an exagerated MCAT importance because of its importance within biochemistry. The MCAT writers tend to see the organic from the point of view of biochemistry. They can't help it. The MCAT is an exam for future doctors after all. Even though biochemistry isn't a formal requirement for the exam, biochemistry does watch over the test as a powerful influence in the background.
Read for comprehension. A recurring motif in metabolism is the formation of a Schiff's base (imine) between an aldehyde or ketone substrate and a lysine residue of an enzyme. In aldolase, for example, such a (protonated) Schiff's base stabilizes the formation of an enolate anion of dihydroxyacetone phosphate to undergo aldol condensation with glyceraldehyde 3-phosphate, or the reverse process in glycolysis (cleavage of fructose 1,6-bisphosphate).
Did you understand what you just read? Slow down if you didn't. It's not that hard to decode. This is the kind of thing an MCAT passage will throw at you. Not because you were supposed to know the facts already (so don't panic) but because you are supposed to know enough to put what you are reading into the context of fundamental principles, and they want to see if you are a good enough active reader to have reasonable comprehension. Learn to slow down and decode biochemical terminology. If you keep reading quickly at a conversational pace through a run of biochemical terminology, you will get pushed out of comprehension. Can you think of some MCAT questions that might arise out of this kind of passage. About Schiff's bases, enolates, or aldol condensation? (not about the biochemistry so much, except maybe to see if you understand what kind of amino acid lysine must be from the context) Let's continue . . .
Another example is the Schiff's base formed between pyridoxal phosphate (PLP), a derivative of vitamin B6, and the lysine residues of several enzymes. PLP contains an aldehyde moeity that is ideal for Schiff's base formation. The Schiff's base formed can act as an acid-base type catalyst in a context which excludes water in some types of enzyme activity. The protonated form of PLP enables the potential energy of negative charge densities to be lower that are involved with the catalytic intermediates in these reactions.