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Schiff base

Actually another name for an imine functional group, in biochemistry a Schiff base is formed from the condensation of an amine group with the carbonyl group of an aldehyde or ketone, as shown in the figure at left, below.

Chemical equation for Schiff base formation and protonation   Since the nitrogen of the resulting bond has basic character (indicated by the lone pair shown explicitly on the nitrogen atom in the figure), it can take up a proton to form the conjugate acid of the Schiff base, or the protonated Schiff base. The formation of a Schiff base and its functionality are important in the mechanism of a number of enzymes, including that of aldolase and those, like aminotransferases, using pyridoxal phosphate as a cofactor. In general, the primary amine is the ε-amino group of a lysine residue.
Another important example of the biological function of a Schiff base is provided by bacteriorhodopsin, a light-driven proton pump. The reversible protonation of the Schiff base is central to the proton pumping mechanism of this remarkable energy-transducing molecule.

The figure below shows a plausible scheme for the formation of a Schiff base in the context of the active site of an enzyme. Basic groups on the enzyme are represented as :B-Enz, and acidic groups as +HB-Enz. It is important for the amine in this mechanism to be in its unprotonated, basic form, since it is the nucleophile attacking the electrophilic carbonyl carbon. A protonated amine would not act as a nucleophile. The intermediate produced in the first step is termed a carbinolamine. Note also that the scheme shown results in the direct production of the protonated form of the Schiff base.

Scheme 1: Schiff base formation, general acid-base catalysis.

Scheme for enzyme catalyzed formation of a Schiff base
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© 2014 Jeff D Cronk, Gonzaga University