Pyridoxal phosphate (PLP) is a form of vitamin B₆.

PLP serves as a coenzyme in the enzyme-catalyzed decarboxylation of meso-2,6-diaminopimelic acid, which is the last step in the biosynthesis of the amino acid lysine. The sequence of reactions involved in this process is shown in Fig. P20.54.

(a) In the first step of this reaction, a molecule of PLP forms an imine (Schiff base) adduct A with either of the amino groups of meso-2,6-diaminopimelate. Give the structure of adduct A.

(b) Carbon dioxide is lost from A to form a new imine intermediate X. Draw a curved-arrow mechanism for this decarboxylation reaction. This mechanism should lead you to the structure of X. Show that the formation of A provides the “electron sink” that is crucial to this decarboxylation reaction.

(c) Imine X is transformed by a series of acid–base reactions to a different imine derivative Y, which is identical to the imine that can be formed separately from PLP and one of the amino groups of lysine. Imine Y undergoes imine hydrolysis to PLP and lysine. Draw a structure for Y and a curved-arrow mechanism for its formation from X. (Assume acids and bases are present as necessary; these are provided by the enzyme.)

Transcribed Image Text:

-O-POCH₂. 0- CH O 0- CH3 pyridoxal phosphate (PLP) (zwitterion form) R¹ CH O IZ+ R³ abbreviated structure of PLP