1996

Koivula, Anu

The structure-function relationships of two polysaccharide-degrading enzymes were studied by applying two different protein engineering methods. The whole B. stearothermophilus alfa-amylase gene was subjected to random mutagenesis developed in this study. Nearly 100 diff erent alfa-amylase point mutants were produced. The enzymatic activities and location of the mutation(s) were determined and the data obtained was compared to the constructed structural model of alfa-amylase. A reasonably good overall correlation could be drawn between the mutant data and the model. Two areas of the alfa-amylase structural model were identified as being important for the activity on polymeric substrate: the open active site cleft situated between domains A and B and containing conserved amino acids known to be important for catalysis and multiple binding of glucose units in other alfa-amylases; and an interface between the catalytic domain A and domain C about 30 Å away from the active site groove. The three-dimensional structure of T. reesei cellobiohydrolase II (CBHII) catalytic domain was available. The catalytic domain has an alfa/beta barrel fold similar to a-amylase catalytic domain A. Two stable surface loops generate a 20 Å long tunnel for substrate binding and catalysis. The active site tunnel contains four defined binding sites (A-D) for glucosyl units and a putative binding site F at the entrance of the tunnel. CBHII is an inverting enzyme in which D221, situated between subsites B and C, acts as a proton donor. D175 lying next to D221 either stabilizes hypothetical carbonium ion intermediates or facilitates the protonation of D221, or both. The base has not yet been identified although the role has been attributed to D401. Site-directed mutagenesis was used to study the role of three amino acids in the active site of CBHII. Y169, located at site B close enough to interact with both D175 and the sugar hydroxyl at site B, was mutagenised to phenylalanine. The Y169F mutant showed increased binding but reduced catalytic rate on small soluble cello-oligosaccharides. These data suggest that Y169 helps to distort the glucose ring into more reactive conformation. In addition, the pH-activity profile of Y169F mutant declines at low pH, suggesting that Y169 also affects the protonation state of the active site carboxylates, D175 and D221. The tryptophan residues at subsites A and F were also mutated. In both cases removal of the indole ring affected the catalytic rate. The tight binding of an intact glucose ring in binding site A seems to be essential for efficient catalysis and is partly dictated by the W135. It was also shown that CBHII active site tunnel contains at least one additional binding site (F) at the mouth of the tunnel. It is plausible that site F is important for the breakdown of crystalline cellulose.