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Abstract

The adenylate kinase isoenzyme located in the intermembrane space of mitochondria, AK2, is a monomeric protein of Mr 30000 which catalyzes the reaction ATP + AMP 〙 2ADP.

1

The protein was reduced and S‐carboxymethylated with iodo[14C2]acetate. Using a Laursen sequenator, the N‐terminal sequence of S‐carboxymethylated AK2 was determined as Ala‐Pro‐Asn‐; in some batches of the isolated protein the N‐terminal dipeptide portion was missing. The C‐terminus of AK2 was found to be Met.
2

Cleavage with CNBr yielded eight fragments which could be isolated in one step using high‐performance size‐exclusion chromatography. They ranged in size over 4–88 amino acid residues, the total being approximately 270 residues. All CNBr fragments were overlapped with Met‐containing tryptic peptides of AK2.
3

The N‐terminal 111 residues of AK2 were sequenced. Except for an N‐terminal extension of nine residues, this segment of AK2 could be aligned with the sequence 1–104 of cytosolic AK1. Allowing for two deletions in AK2, 43 of the 102 aligned residues are identical. Since this section contains the catalytic residues such as His‐36 and Asp‐93, we conclude that AK1 can serve as a three‐dimensional model of AK2 in mechanistic and drug‐designing studies.
4

Preliminary sequence results on AK2 beyond position 104 show that AK2 here contains a wing of approximately 50 residues which has no counterpart in AK1. The chain folds of the adenylate kinase isoenzymes are similar again from a position corresponding to residue 115 of AK1 onwards. The additional structural motifs of AK2 are probably related to the location of this isoenzyme in the mitochondrion.