Mammalian protein geranylgeranyltransferase-I: substrate specificity, kinetic mechanism, metal requirements, and...

Protein geranylgeranyltransferase-I (PGGT-I) catalyzes the transfer of the 20-carbon prenyl group from geranylgeranyl pyrophosphate to the cysteine residue near the C-termini of a variety of eukaryotic proteins. Kinetic analysis of homogenous PGGT-I from bovine brain reveals that the reaction follows a sequential pathway in which either prenyl donor or acceptor can bind first to the enzyme and that the reaction operates at steady-state rather than at rapid equilibrium. Substrate inhibition by prenyl acceptor but not by prenyl donor suggests that geranylgeranyl pyrophosphate binding first to free enzyme is the kinetically preferred pathway. This is supported by isotope trapping experiments which show that the ternary complex goes on to products faster than the release of geranylgeranyl pyrophosphate from the complex. The KM for the interaction of geranylgeranyl pyrophosphate with PGGT-I is markedly affected by the structure of the prenyl acceptor bound to the enzyme. A detailed analysis of the substrate specificity of PGGT-I reveals that peptides which contain a C-terminal leucine are preferred (kcat/KM = 1-5 x 10(5) M-1 s-1) to those that end in serine (kcat/KM = 2-4 x 10(3) M-1 s-1) or phenylalanine (kcat/KM = 0.5 x 10(3) M-1 s-1). PGGT-I also catalyzes the farnesylation of peptides that have a C-terminal leucine; kcat for farnesylation and KM for farnesyl pyrophosphate are similar to those for geranylgeranylation, but the KM for the peptide is 30-fold higher. Geranyl pyrophosphate is utilized by PGGT-I but is a poor substrate. Optimal activity of PGGT-I is obtained in the presence of micromolar amounts of Zn2+ and mM amounts of Mg2+. Mn2+ or Cd2+ but not Co2+ can substitute for Zn2+ and for Mg2+. Metals are not required for tight-binding of geranylgeranyl pyrophosphate to PGGT-I, and the measured dissociation equilibrium constant for this binary complex is 16 nM. Photoaffinity analogues of geranylgeranyl pyrophosphate and farnesyl pyrophosphate were prepared and shown to exclusively label the beta-subunit. The implication of the results for the substrate specificity of protein prenylation in cells is briefly discussed.