Sequence Comparison and Stereostructure

A E I MRVED DR DQRGY AQEYY RIP LRQIIEQL SG FSPKQPD FKDIVNMLMHH— 

1 Amino acid sequence comparison of a-glucan phosphorylases. H, the potato type-H isozyme L, the potato type-L isozyme and R, rabbit muscle phosphotylase.791 The type-H isozyme sequence is used as the reference sequence only the amino acid residues that are nonidentical in the type-L isozyme and rabbit muscle enzyme are indicated. (From Biol. Chem., 266 (28), 18453 (1991)). 

Previously, it has been shown that most of the residues directly interacting with AMP as well as the phosphorylatable Ser14 and its surroundings in the rabbit muscle enzyme are far less conserved in the potato type-L isozyme sequence.63 Likewise, the amino-terminal region of the potato type-H isozyme is completely different from that of the rabbit muscle enzyme over the first 80 amino acid residues, in which the sites of covalent phosphorylation and of allosteric regulation by AMP are all included. These variances in sequence are compatible with the lack of regulation in the plant phosphorylase isozymes. 

Willnecker, Jahnke and Buehner96 reported recently the preliminary results of X-ray crystallographic analysis of the potato type-L isozyme at 2.6 A resolution, and showed that the overall folding of the potato phosphorylase is essentially the same as that of the rabbit muscle enzyme (more similar to the 7 -form than to the T-form). Only small differences were found in the core of the subunit, although local structures on the surface including the loops deviated significantly, being consistent with the sequence differences with some minor insertions and deletions as compared with the rabbit muscle enzyme (see above). As 

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