Subtilisin sequence comparisons

Despite their lack of stabilizing disulfide bridges Potl inhibitors feature a common, stable fold. The N-terminus is coiled, although in some structures a small /3-strand has been identified. After a turn the structure adopts an a-helical structure, followed by a turn and an other /3-strand. The sequence then features an extended turn or loop motif that contains the reactive site of the inhibitor before it proceeds with a /3-strand running almost parallel to the /3-strand after the a-helix. After another turn and coiled motif a short /3-strand antiparallel to the other /3-strands precedes the coiled C-terminus. Usually the N-terminal residue in the reactive site is an acidic residue followed by an aromatic amino acid, that is, tyrosine or phenylalanine. Figure 11 shows the complex of chymotrypsin inhibitor (Cl) 2 with subtilisin, the hexamer of Cl 2 from H. vulgare and a structural comparison with a trypsin inhibitor from Linum usitatissimum ... 

Fig. 12.2 Comparison of the amino acid sequence of aqualysin I with those of other subtilisin-type proteases.

Drenth and his co-woricers have presented a comparison of subtilisin BPN and subtilisin Novo which have the same sequence but have been crystallized under very different conditions ... 

Fig. 3. Comparison of sequences of thermitase, subtilisins and proteinase K. Top numbering - thermitase, bottom numbering - subtilisin BPN . 

Comparison (or alignment) of amino acid sequences, also called homology search, often provides first-hand information on such conserved structural features and enables one to classify enzymes into families and predict the possible function of a new enzyme (86). A family of enzymes usually folds into similar 3-D structures, at least at the active site region. A typical example is the serine protease family whose members—trypsin, chymotrypsin, elastase, and subtilisin—commonly contain three active-site residues, Asp/His/Ser, which are known as the catalytic triad or charge relay system. Another example is the conserved features of catalytic domains of the highly diverse protein kinase family. In this kinase family, the ATP-binding (or phosphate-anchoring) sites present a consensus sequence motif of Gly-X-Gly-X-X-Gly (67,87). 

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