Multifunctional role of His159in the catalytic reaction of serine palmitoyltransferase.

Serine palmitoyltransferase (SPT) belongs to the fold type I family of the pyridoxal 5'-phosphate (PLP)-dependent enzyme and forms 3-ketodihydrosphingosine (KDS) from l-serine and palmitoyl-CoA. Like other alpha-oxamine synthase subfamily enzymes, SPT is different from most of the fold type I enzymes in that its re face of the PLP-Lys aldimine is occupied by a His residue (His(159)) instead of an aromatic amino acid residue. His(159) was changed into alanine or aromatic amino acid residues to examine its role during catalysis. All mutant SPTs formed the PLP-l-serine aldimine with dissociation constants several 10-fold higher than that of the wild type SPT and catalyzed the abortive transamination of l-serine. These results indicate that His(159) is not only the anchoring site for l-serine but regulates the alpha-deprotonation of l-serine by fixing the conformation of the PLP-l-serine aldimine to prevent unwanted side reactions. Only H159A SPT retained activity and showed a prominent 505-nm absorption band of the quinonoid species during catalysis. Global analysis of the time-resolved spectra suggested the presence of the two quinonoid intermediates, the first formed from the PLP-l-serine aldimine and the second from the PLP-KDS aldimine. Accumulation of these quinonoid intermediates indicated that His(159) promotes both the Claisen-type condensation as an acid catalyst and the protonation at Calpha of the second quinonoid to form the PLP-KDS aldimine. These results, combined with the previous model building study (Ikushiro, H., Fujii, S., Shiraiwa, Y., and Hayashi, H. (2008) J. Biol. Chem. 283, 7542-7553), lead us to propose a novel mechanism, in which His(159) plays multiple roles by exploiting the stereochemistry of Dunathan's conjecture.