Pasteur effect enzymes

Much has been published on the controversial subject of the control of glycolysis. The following brief summary of some of the controls responsible for the Pasteur effect in yeasts is based mainly on a review by Sols and coworkers144 (see also, Fig. 7). (i) Isocitrate dehydrogenase (NAD ) (EC 1.1.1.41), one of the controlling enzymes of the tricarboxylic acid cycle (see Fig. 5), catalyzes the reaction... 

Phosphofructokinase (EC 2.7.1.30) catalyses the phosphorylation of fructose-6-phosphate to fructose-1,6-biphosphate as the key regulatory enzyme of glycolysis. Inhibition of phosphofructokinase by adenosine triphosphate and its activation by adenosine monophosphate and inorganic phosphate is held responsible for the induction of the Pasteur effect (for review see Ramaiah 1974). 

In propionic acid bacteria the Pasteur effect, although less prominent than in the yeast, was found for the first time in P. pentosaceum in 1939 (Chaix and Fromageot). It is known that in yeasts growing under anaerobic conditions the effect is manifested by consuming more substrate to synthesize a unit of biomass than under aerobic conditions. In yeasts, the mechanism of the Pasteur effect may be connected with changes in the enzyme activities of the... 

Because only two molecules of ATP are produced per glucose metabolized under anaerobic conditions, the cell must utilize additional glucose at a faster rate in order to maintain the pool of intracellular ATP. This step is accomplished through activation of the enzyme phosphofructokinase (Fig. 1.10), which, in turn, increases carbon flow through glycolysis. The increase in rate of glucose breakdown under anaerobic conditions is known as the Pasteur effect. This phenomenon is only observable when glucose concentrations are low, approximately 0.9 g/L (Walker, 1998). 

Some have attempted to explain the Pasteur effect on the basis of concentration changes of inorganic phosphate and ATP others have postulated a direct influence on the enzymes. So far, the question remains unsolved. 

This is the reverse Pasteur or Crabtree effect and is also known as glucose inhibition or cataboHte repression. In the presence of higher sugar concentrations, synthesis of respiratory enzymes such as cytochromes is inhibited. 

After the pioneering work of Louis Pasteur and Emil Fischer in the middle and at the end of the nineteenth century, respectively, it still took more than fifty years before chemists started to discuss transition state models together with polar and steric effects to gain more insight into the phenomenon of asymmetric induction. Even first observations in organic synthesis of enantioselectivities comparable to those of enzymes in the late fifties and sixties of the 20 century did not convince the chemical community and the term asymmetric synthesis was regarded a mechanistic curiosity rather than a practical way to synthesize compounds of high enantiomeric purity. 

Acid phosphomonoesterase (EC 3.1.3.2). Milk contains an acid phosphatase which has a pH optimum at 4.0 and is very heat stable (LTLT pasteurization causes only 10-20% inactivation and 30 min at 88°C is required for full inactivation). Denaturation of acid phosphatase under UHT conditions follows first-order kinetics. When heated in milk at pH 6.7, the enzyme retains significant activity following HTST pasteurization but does not survive in-bottle sterilization or UHT treatment. The enzyme is not activated by Mg2+ (as is alkaline phosphatase), but it is slightly activated by Mn2+ and is very effectively inhibited by fluoride. The level of acid phosphatase activity in milk is only about 2% that of alkaline phosphatase activity reaches a sharp maximum 5-6 days post-partum, then decreases and remains at a low level to the end of lactation. 

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. 

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