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

(D)ihydroneopterin aldolase (DHNA) catalyzes the conversion of 7,8-dihhydro neopterin to 6-hydroxymethyl-7,8-dihydropterin and glycolaldehyde. An inhib itor 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 analyse s based on ab initio quantum mechanic methods, we have positively identifie d some of the Raman bands in the enzyme-bound inhibitor, particularly the i mportant N5=C6 stretch mode. The spectrum of the enzyme-bound inhibitor sho ws that the pK(alpha) of N5 is not significantly increased in the complex. This result suggests that N5 of 7,8-dihydroneopterin is not protonated befo re the bond cleavage of 7,8-dihydroneopterin during the DHNA-catalyzed reac tion as has been suggested. Our results also show that the N5=C6 stretch mo de 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.