PRIMARY STRUCTURE OF MAIZE CHLOROPLAST ADENYLATE KINASE

This paper describes the sequence of-adenylate kinase (Mg-ATP + AMP reversible arrow Mg-ADP + ADP) from maize chloroplasts. This light-inducible enzyme is important for efficient CO, fixation in the C-4 cycle, by removing and recycling AMP produced in the reversible pyruvate phosphate dikinase reaction. The complete sequence was determined by analyzing peptides from cleavages with trypsin, Asp-N protease and CNBr and subcleavage of a major CNBr peptide with chymotrypsin. N-terminal Edman degradation and carboxypeptidase digestion established the terminal residues. Electrospray mass spectrometry confirmed the final sequence of 222 residues (M(r) = 24867) including one cysteine and one tryptophan. The sequence shows this enzyme to be a long-variant-type adenylate kinase, the nearest relatives being adenylate kinases from Enterobacteriaceae. Alignment of the sequence with the adenylate kinase from Escherichia coli reveals 44% identical residues. Since the E. coli structure has been published recently at 0.19-nm resolution with the inhibitor adeno sine(5 ')pentaphospho(5 ')adenosine (Ap(5)A) [Muller, C. W. and Schulz, G. E. (1992) J. Mel. Biol. 224, 159-177], catalytically essential residues could be compared and were found to be mostly conserved. Surprisingly, in the nucleotide-binding Gly-rich loop Gly-Xaa-Pro-Gly-Xaa- Gly-Lys the middle Gly is replaced by Ala. This is, however, compensated by an Ile --> Val exchange in the nearest spatial neighborhood. A Thr --> Ala exchange explains the unusual tolerance of the enzyme for pyrimidine nucleotides in the acceptor site.