Schiff Bases in Biological Systems

In many cases, the cofactors of biologically important proteins are linked to their apoproteins by Schiff bases. In this review we discuss three classes of proteins in which Schiff bases are crucial for their function and catalytic mechanisms. First, we focus on retinylidene proteins, in which the cofactor retinal is fixed to the apoprotein by a protonated Schiff base. Light induced retinal isomerization triggers a photocycle or a cascade of structural changes in which deprotonation of the Schiff base is a crucial step. We compare the photocycles of archaeal type rhodopsins, channelrhodopsins and the light induced cascades of visual rhodopsins. Pyridoxal phosphate (PLP) dependent enzymes are summarized in the second section. In these proteins, the cofactor PLP is linked via a Schiff base to the side chain of a lysine. In most cases, hydrolysis of this linkage is critical for substrate binding. Finally, aldolases, that catalyze aldol cleavage during glycolysis, contain a lysine residue at their active center. Formation of a protonated Schiff base linkage to a carbonyl group of the substrate is the first step of the catalytic mechanism. This will be discussed in the last section.