Recognition of allylic substrates in Sulfolobus acidocaldarius geranylgeranyl diphosphate synthase: analysis using mutated enzymes and artificial allylic substrates.

We examined the substrate specificity of two mutated geranylgeranyl diphosphate synthases, I-9 and I-11, with respect to several artificial substrates. These mutated enzymes have replacements in the amino acid sequences from positions 170 to 173, which are thought to be a part of the putative substrate binding region. The wild-type enzyme catalyzes the condensation of IPP with a series of (2E)-3-methyl-2-alkenyl diphosphates to give products with carbon numbers between 14 and 21. On the other hand, the mutated enzymes show lower activities for artificial substrates with short alkyl chains than those of the wild-type enzyme though the carbon numbers of the products are similar to those in the case of the wild-type. The mutated enzyme I-11 never accepts artificial substrates shorter than C8. Analysis of additional mutated enzymes revealed that the characteristics of the mutated enzymes arise from a few substitutions within positions 171 to 173. These results indicate that the amino acids in the positions 171 to173 of the geranylgeranyl diphosphate synthase from Sulfolobus acidocaldarius are involved in recognition of short allylic substrates, such as dimethylallyl diphosphate, but not in recognition of the chain length of the products.