Abstract
Adenylate kinase has been isolated from four related methanogenic members of the Archaea. For each the optimum temperature for enzyme activity was similar to the temperature for optimal microbial growth and was approximately 30 C for Methanococcus voltage, 70 C for Methanococcus thermolithotrophicus, 80 C for Methanococcus igneus and 80--90 C for Methanococcus jannaschii. The enzymes were sensitive to the adenylate kinase inhibitor, Ap{sub 5}A [P{sup 1}, P{sup 5}-di(adenosine-5{prime}) pentaphosphate], a property that was exploited to purify the enzymes by CIBACRON Blue affinity chromatography. The enzymes had an estimated molecular weight (approximately 23--25 kDa) in the range common for adenylate kinases. Each of the enzymes had a region of amino acid sequence close to its N-terminus that was similar to the canonical P-loop sequence reported for all adenylate kinases. However, the methanogen sequences lacked a lysine residue that has previously been found to be invariant in adenylate kinases including an enzyme isolated from the Archeon, Sulfolobus acidocaldarius. If verified as a nucleotide binding domain, the methanogen sequence would represent a novel nucleotide binding motif. There was no correlation between amino acid abundance and the optimal temperature for enzyme activity.
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