Role of amino acid residues on the GS region of Stichopus arginine kinase and Danio creatine kinase

Stichopus arginine kinase (AK) is a unique enzyme in that it evolved not from the AK gene but from the creatine kinase (CK) gene: the entire amino acid sequence is homologous with other CKs apart from the guanidine specificity region (GS region), which is identical in structure to that of AK. Ten independent mutations were introduced around the GS region in Stichopus AK. When an insertion or deletion was introduced near the GS region, the V-max of the mutant enzyme was dramatically decreased to less than 0.1% of the wild type, suggesting that the length of the GS region is crucial for the recognition of the guanidine substrate. Replacement of Phe(63) and Leu(65) to Gly in the Stichopus enzyme caused a remarkable increase in the K-m(arg). This indicates that Phe(63) and Leu(65) are associated with the arginine substrate-binding affinity. The hydrogen bond formed between the Asp(62) and Arg(93) residues is thought to play a key role in stabilizing the closed substrate-bound structure of AK. Mutants that eliminated this hydrogen bond had a considerably decreased V-max accompanied by a threefold increase in K-m(arg). It is noted that the value of the K-m(arg) of the mutants became very close to the K-d(arg) value of the wild type. Six independent mutations were introduced in the GS region of Danio M-CK. Almost equivalent values of K-m(cr) and K-d(cr) in all of the mutants indicated that a typical synergism was completely lost. The results suggested that the Ile(69) to Gly mutant, displaying a high Kmcr and a low V-max, plays an important role in creatine-binding. This is consistent with the observation that in the structure of Torpedo CK, Ile(69) provides a hydrophobic pocket to optimize creatine-binding.