Exocellular enzymes

Enzymes most frequently proposed to fruit juices producers are pectinases coming from Aspergillus. Pectinases are exocellular enzymes and are the main activities produced among numerous side activities type hemicellulases, glycosidases. The Table 1 gives the spectrum of enzymatic activities contained in three commercial preparations A, B and C. 

Environmental Potential of the Trichoderma Exocellular Enzyme System... 

The increased use of tanks for the storage of raw milk on the farm between pickups has introduced the danger of potential off-flavor development caused by lipases that are produced by certain microorganisms (psychrotrophs) at low temperatures. The exocellular lipases of psychrotrophic bacteria are extremely heat resistant, and although the microorganisms are killed, the enzymes survive pasteurization and sterilization temperatures. Rancidity may become noticeable when cell counts exceed 106 or 107/ml. Downey (1975) has summarized the potential contribution of enzymes to the lipolysis of milk (Table 5.1). 

Heat-Resistant Lipases. The heat-resistant lipases and proteinases and their effects on the quality of dairy products have been reviewed (Cogan 1977, 1980). Several reports have linked the lipases from bacteria with the off-flavor development of market milk (Richter 1981 Shipe et al. 1980A Barnard 1979B). The microflora developing in holding tanks at 4°C [and presumably in market milk stored at 40°F (Richter 1981)] may produce exocellular lipases and proteases that may survive ordinary pasteurization and sterilization temperatures. Rancidity of the cheese and gelation of UHT milk appear to be the major defects caused by the heat-resistant enzymes. 

Formation of L-guluronic acid, a component of the alginic acid-like polysaccharide produced by P. aeruginosa and Azotobacter vinelandii, requires special comment. In this case, a polymer built from /3-(l- 4)-linked D-mannosyluronic acid residues serves as an intermediate in the biosynthesis.204,205 Part of the D-mannosyluronic acid residues in the polymer is subjected to an epimerization at C-5 catalyzed by an exocellular enzyme of the micro-organism,205-207 producing a polysaccharide composed of structural blocks that contain only D-mannosyluronic acid or only l-gulosyluronic acid residues, as well %s some having both. The mechanism of the epimerization remains unclear. 

Most of the exocellular polysaccharides produced by bacteria are synthesized inside the bacterial cell, with the use of membrane-bound enzymes. Both types of chain assembly were observed for these polymers. In many cases, the mechanism of the assembly remains unidentified, and the nature of the glycosyl acceptors in the process is not clear. 

At the moment, there is no data concerning the compatibility of exocellular proteolytic, amylolytic enzymes and exopolysaccharides and amylopectin, and between exopolysaccharides of different intestinal bacteria species and exopolysaccharides and mucopolysaccharides covering gut wall villae. 

Wines stored in vats lined with epoxy resins can present unusually high levels of benzoic aldehyde (several mg/L). Benzylic alcohol is both a plasticizer and diluent of these resins. Its conversion into benzaldehyde can be due (Blaise 1986) to the action of an exocellular enzyme of Botrytis cinerea called Alcohol Benzylic Oxidase (ABO, E.C. 1.1.3.7.) responsible for this oxidation process (Blaise and Brun 1986). 

A fundamental question involves the biocidal activity of the biocide when it is chemically attached to a polymer. Will the polymer be an active biocide If the biocidal activity results from an interaction of the biocide at the cell wall or by inactivating an exocellular enzyme, then it is possible that the polymer. Itself, could be an active biocide. However, if the biocide must be incorporated into the organism to function, then it will have to be cleaved from the polymer binder prior to exhibiting activity. For that reason polymers having biocides attached by functional groups of differing hydrolytic suscepta-blllty were made. 

Fig. 2. Electron photomicrograph of surface of HB/HV copolymer left - before degradation right - after exposure for 20 hours to an aqueous solution of a purified exocellular enzyme produced by Pseudomonas lemoignei...

Exodextranases catalyze a stepwise hydrolysis of the dextran molecule to yield D-glucose and a residual dextran they have been extracted from cultures of species of Bacillus," Bacteroides, and Lactobacillus bifidus. These enzymes have not, however, been utilized in structural studies. Endodextranases have been isolated from exocellular fluids or cell extracts (or both) of the fungi PenidUium funiculosutn, P. lilacv-... 

Most streptocci that elaborate dextransucrases appear to secrete the enzyme(s) exocellularly, although some strains of Streptococcus mutans also elaborate structure-bound forms of the enzyme. The structure-bound enzyme activities of S. mutans may be situated at several points on the outer smface of the cell wall." Evidence has also been presented that suggests that the proportions of the structure-bound and secreted forms of the enzyme produced by individual streptococci may drange with repeated subculturing of the bacteria." Similar changes have not been detected in Leuconostoc strains. ... 

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