THE CRYSTAL-STRUCTURE OF PHOSPHORIBULOKINASE FROM RHODOBACTER-SPHAEROIDES REVEALS A FOLD SIMILAR TO THAT OF ADENYLATE KINASE

The essential photosynthetic enzyme phosphoribulokinase (PRK) is respo nsible for the conversion of ribulose 5-phosphate (Ru5P) to ribulose 1 ,5-bisphosphate, the substrate for the CO2 fixing enzyme ribulose 1,5- bisphosphate carboxylase/oxygenase (Rubisco). We have determined the s tructure of the octameric bacterial form of PRK to a resolution of 2.5 Angstrom. The protein is folded into a seven-member mixed beta-sheet surrounded by alpha-helices, giving the overall appearance of the nucl eotide monophosphate family of kinases. Homology with the nucleotide m onophosphate kinases suggests a number of amino acid residues that are likely to be important in catalysis and suggests the roles of some am ino acid residues that have been mutated prior to the determination of the structure. Further, sequence identity across eukaryotic and proka ryotic species and a calculation of the buried surface area suggests t he identity within the octamer of a dimer conserved throughout evoluti on. The width of the groove leading to the active site is consistent w ith an oriented molecule of thioredoxin controlling the oxidation stat e of two cysteines that regulate activity in the eukaryotic enzymes. A lthough neither Asp 42 nor Asp 169 can be definitively assigned as the catalytic base, the crystal structure suggests the location of a ribu lose 5-phosphate binding site and suggests a role for several of the c onserved basic residues.