Tryptophan synthetase, folding

Folding to native-like structure has been demonstrated with fragments of jS-galactosidase, lysozyme, serum albumin, penicillinase, and tryptophan synthetase. The capability of protein fragments for independent formation of structure therefore has substantial experimental basis. This generalization also makes plausible the idea that, in general, protein folding occurs by parts, that is, in a modular fashion. 

Most of the proteins on which fragment folding studies have been carried out are extracellular. Of the nine discussed in this article, only tryptophan synthetase and /3-galactosidase are not secreted. Many secreted proteins are synthesized with 20 or so additional amino acid residues at the N-terminus of the peptide chain (Blobel... 

Many of the enzymes that catalyze these reactions, such as serine hy- droxymethyltransferase, which converts serine into glycine, have the same fold as that of aspartate aminotransferase and are clearly related by divergent evolution. Others, such as tryptophan synthetase, have quite different overall structures. Nonetheless, the active sites of these enzymes are remarkably similar to that of aspartate aminotransferase, revealing the effects of convergent evolution. 

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