Enzymes that exploit imines--one way or the other.

In enzymology, as in organic chemistry, many transformations revolve around the chemistry of the carbonyl group. A carbonyl group is electrophilic how can it be made more so? Hydrogens next to a carbonyl group have a reduced pK, how can this reactivity be promoted? Nature has found several ways to address these mechanistic challenges, the most important of which is the use of imines. Studies on two enzymes which exploit imines are described: one in which the carbonyl component is supplied by substrate and the second, less usual case, in which the carbonyl group is part of the protein backbone.