POTENTIAL ROLES OF CONSERVED AMINO-ACIDS IN THE CATALYTIC DOMAIN OF THE CGMP-BINDING CGMP-SPECIFIC PHOSPHODIESTERASE (PDE5)

The known mammalian 3':5'-cyclic nucleotide phosphodiesterases (PDEs) contain a conserved region located toward the carboxyl terminus, which constitutes a catalytic domain, To identify amino acids that are impo rtant for catalysis, we introduced substitutions at 23 conserved resid ues within the catalytic domain of the cGMP-binding cGMP-specific phos phodiesterase (cGBPDE; PDES). Wild-type and mutant proteins were compa red with respect to K-m for cGMP, k(cat) and IC50 for zaprinast, The m ost dramatic decrease in k(cat) was seen with H643A and D754A mutants with the decrease in free energy of binding (Delta Delta G(T)) being a bout 4.5 kcal/mol for each, which is within the range predicted for lo ss of a hydrogen bond involving a charged residue, His(643)?S and Asp( 754) conserved in all known PDEs and are sarong candidates to be direc tly involved in catalysis, Substitutions of His(603), His(607), His(64 7), Glu(672), Asp(714) also produced marked changes in k(cat) and thes e residues are likely to be important for efficient catalysis, The Y60 2A and E775A mutants exhibited the most dramatic increases in K-m for cGAMP, with calculated Delta Delta G(T) of 2.9 and 2.8 kcaj/mol, respe ctively, that these two residues are important for cGMP binding in the catalytic site, Zaprinast is a potent competitive inhibitor of cGB-PD E, but the key residues for its binding differ significantly from thos e that bind cGMP.