Biochemical Aspects of Enzyme Production and Activity

The outer membrane of Gram-negative bacteria is spanned by porins, trimeric proteins that form inlet channels between the outer membrane and the periplasmic space. The geometry of the porins defines the substrate uptake limit, which is on the order of 600 daltons (Weiss et al., 1991), approximately equivalent to a trisaccharide. Substrates larger than this limit (with few exceptions) must be hydrolyzed outside the outer membrane prior to uptake. Extracellular hydrolysis is carried out by means of extracellular en- 

In general, hydrolytic enzymes can be classified based on the type of reaction catalyzed, the nature of the enzyme active site, and/or evolutionary relationships among enzymes, as derived from primary sequence data. Among proteases, gross functional distinctions are made between serine proteases, aspartic proteases, cysteine proteases, and metalloproteases. Each of these groups includes a diverse range of enzymes of distinctive size and structure for example, an aminopeptidase isolated from B. lichenformis was found to have a molecular mass of 34,000, whereas an E. coli aminopeptidase had a mass of 400,000 (Rao et al., 1998). 

Glycosyl hydrolases include a similarly wide range of structures. Traditional classification based on substrate specificity can be ambiguous, as some enzymes can hydrolyze a number of distinct substrates, albeit at 

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