A vibrational structure of 7,8-dihydrobiopterin bound to dihydroneopterin aldolase

Dihydroneopterin aldolase (DHNA) catalyzes the conversion of 7,8-dihydroneopterin to 6-hydroxymethyl-7,8-dihydropterin and glycolaldehyde. An inhibitor of the enzyme, 7,8-dihydrobiopterin, free in solution and bound in its complex with the enzyme has been studied by Raman difference spectroscopy. By using isotopically labeled 7,8-dihydrobiopterin and normal mode analyses based on ab initio quantum mechanic methods, we have positively identified some of the Raman bands in the enzyme-bound inhibitor, particularly the important N5=C6 stretch mode. The spectrum of the enzyme-bound inhibitor shows that the pK(a) of N5 is not significantly increased in the complex. This result suggests that N5 of 7,8-dihydroneopterin is not protonated before the bond cleavage of 7,8-dihydroneopterin during the DHNA-catalyzed reaction as has been suggested. Our results also show that the N5=C6 stretch mode of 7,8-dihydrobiopterin shifts 19 cm^-1 upon binding to DHNA. Various possibilities on how the enzyme can bring about such large frequency change of the N5=C6 stretch mode are discussed.