Lactose metabolism

The Lac operon is made up of three genes (designated A, Y and Z), which code for enzymes which metabolize lactose and the control element whose function is to activate transcription of the A,Y and Z genes. Normally (i.e. when there is sufficient glucose available), a protein called a repressor blocks the control element and so the A, Y and Z genes are off. 

S. thermophilus metabolizes lactose to l( +) lactic acid but utilizes only the glucose moiety of lactose, leaving the galactose moiety in the cheese (Tinson et al. 1982). In Swiss cheese manufacture, S. thermophilus metabolizes the lactose and L. helveticus metabolizes the galactose to d( —) and l( + ) lactic acid (Turner et al. 1983). The l( + ) lactate isomer is preferentially utilized by propionibacteria to form acetic and propionic acids, which are essential for the development of the characteristic flavor in Swiss cheese (Langsrud and Reinbold 1973). 

These two complementary systems allow the bacterial cell to metabolize lactose in response to two stimuli. Switching on the expression of the lac operon requires both the absence of glucose and the presence of lactose. This series of switches allows complex expression patterns to be built up from simple components. For this reason, the lac system is a model for other, apparently more complex, biological control systems, such as hormone action or embryonic development. 

There is also an aspect of positive control in the lac operon. The catabolite activator protein (CAP), carrying bound cAMP, is required for the binding of RNA polymerase to the promoter i.e., it has a direct, positive effect on transcription. However, relief of repression (i.e., induction) will not occur in the presence of glucose, because glucose lowers the level of cAMP, so that CAP is unable to exert its effect. This reflects the preference of the cell to use glucose rather than lactose as a carbon source. Thus it can be seen that the cell stringently controls expression of the lac genes it expresses them only if it needs to metabolize lactose. 

Lactose, the major sugar found in milk, is a disaccharide reducing sugar, but unlike the other sugars it is not particularly soluble. Some individuals are unable to metabolise lactose and are therefore described as lactose intolerant. This is because they lack the enzyme lactase which is needed for lactose metabolism. Lactose intolerance is common in those parts of the world where humans do not consume any dairy products after weaning. In practice this means Asia, so it is possible that the majority of the world s population is lactose intolerant. 

Fig. 7 Chromogenic enzyme assays (A) pyroGlu-Pro-Arg-pNA is a peptide substrate for activated protein C and tissue factor XIa. Protolysis releases the chromophore pora-nitroaniline (pNA), which is monitored by absorbance measurement at 405 nm. (B) ortho-Nitrophenyl-o-galactopyranoside (oNPG) is an artificial substrate for galactosidase that metabolizes lactose. The cleavage of oNPG releases the yellow ortho-nitrophe-nol with maximum absorbance at 420 nm.

EXAMPLE 9.38 The lactose operon (or lac operon) is a region of 5.3 kb of the E. coli chromosome that contains the genes that encode the three enzymes that catabohze lactose. The cell stringently controls expression of the lac genes it expresses them only if it needs to metabolize lactose. Fnrthermore, when glncose is also present, there is no need for high levels of expression of the lac operon. Therefore, as detailed in Fig. 9-17, the lac operon is nnder both negative... 

If this explanation is correct, the f mutant might regain the ability to metabolize lactose in either of two ways by reversion to wild type (/ - / ), or by mutating to the constitutive condition (/ ->/ ). In fact, lac mutants have been found to be very frequent in populations of mutant and about half of these mutants are constitutives of the i type. 

Lactose intolerance. Disorder characterized by the incapacity of the organism in digesting and metabolizing lactose, a sugar present in milk. It is caused by a lack of lactase, required to break down lactose in the digestive tract. 

Glucose and galactose are the products of lactose hydrolysis by the enzyme lactase. Some individuals are unable to metabolize lactose because of reduced lactase levels, a condition termed lactose intolerance. Preparations of the enzyme lactase are now being marketed for commercial or consumer addition to milk. The resulting milk is sweeter tasting because the majority of the lactose is hydrolyzed to glucose and galactose. 

The organism contains enzymes for glucose metabolism, lacks p galac-tosidase (hence is unable to metabolize lactose), some of the enzymes of the Krebs cycle (no isocitrate dehydrogenase), and most important, the urease gene cluster encoding ureA and ureB (urease) and ureE,F,G,H and I... 

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