DAIRY CHEMISTRY AND BIOCHEMISTRY One of the most interesting characteristics of a-lactalbumin is its role actos synthesis UDP-D-Galactose +D-glucose lactose UDP Lactose synthetase, the enzyme which catalyses the final step in the biosynthesis of lactose, consists of two dissimilar protein subunits, A and B: the A protein is UDP-galactosyl transferase while the B protein is a-la. In the absence of B protein, the a protein acts as a non-specific galactosyl transferase, i.e. it transfers galactose from UDP-galactose to a range of acceptors, but in the presence of B protein it becomes highly specific and transfers galactose only to glucose to form lactose(Km for glucose is reduced approximately 1000-fold ) a-Lactalbumin is, therefore, a'specifier protein and its action represents a unique form of molecular control in biological reactions. a-La from the milks of many species are effective modifier proteins for the UDP-galactosyl transferase of bovine lactose synthetase. How it exercises its control is not understood but it is suggested that the synthesis of lactose is controlled directly by z lactalbumin which, in turn, is under hormonal control(Brew and Grobler, 1992). The concentration of lactose in milk is directly related to the concentration of a-la; milks of marine mammals, which contain no x-la, contain no lactose. Since lactose is the principal constituent in milk affecting osmotic pressure, its synthesis must be controlled rigorously and this is the presumed physiological role of a-la Perhaps each molecule of a-la regulates lactose synthesis for a short period and is then discarded and replaced; while this is an expensive and wasteful se of an enzyme component, the rapid turnover affords a faster response should lactose synthesis need to be altered, as in mastitic infection, when the osmotic pressure of milk increases due to an influx of NaCl from the blood hapter 2) 4.8.8 Metal binding and heat stability a-La is a metallo- protein; it binds one Ca2+ per mole in a pocket containing four Asp residues( Figure 4.26); these residues are highly conserved in all az-la's and in lysozyme. The Ca-containing protein is quite heat stable (it is the most heat stable whey protein) or more correctly, the protein renatur llowing heat denaturation(denaturation does occur at relatively low temperatures, as indicated by differential scanning calorimetry). when the pH is reduced to below about 5, the Asp residues become protonated and lose their ability to bind Ca2+. The metal-free protein is denatured at quite low temperatures and does not renature on cooling; this characteristic has been exploited to isolate a-la from whey194 DAIRY CHEMISTRY AND BIOCHEMISTRY 4.8.7 Biological function One of the most interesting characteristics of cc-lactalbumin is its role in lactose synthesis: UDP-D-Galactose + D-glucose ---+ lactose + UDP lactose synthetase Lactose synthetase, the enzyme which catalyses the final step in the biosynthesis of lactose, consists of two dissimilar protein subunits, A and B; the A protein is UDP-galactosyl transferase while the B protein is a-la. In the absence of B protein, the A protein acts as a non-specific galactosyl transferase, i.e. it transfers galactose from UDP-galactose to a range of acceptors, but in the presence of B protein it becomes highly specific and transfers galactose only to glucose to form lactose (K, for glucose is reduced approximately 1000-fold). cc-Lactalbumin is, therefore, a ‘specifier protein’ and its action represents a unique form of molecular control in biological reactions. cc-La from the milks of many species are effective modifier proteins for the UDP-galactosyl transferase of bovine lactose synthetase. How it exercises its control is not understood, but it is suggested that the synthesis of lactose is controlled directly by a-lactalbumin which, in turn, is under hormonal control (Brew and Grobler, 1992). The concentration of lactose in milk is directly related to the concentration of a-la; milks of marine mammals, which contain no x-la, contain no lactose. Since lactose is the principal constituent in milk affecting osmotic pressure, its synthesis must be controlled rigorously and this is the presumed physiological role of a-la. Perhaps each molecule of x-la regulates lactose synthesis for a short period and is then discarded and replaced; while this is an expensive and wasteful use of an enzyme component, the rapid turnover affords a faster response should lactose synthesis need to be altered, as in mastitic infection, when the osmotic pressure of milk increases due to an influx of NaCl from the blood (Chapter 2). 4.8.8 Metal binding and heat stability a-La is a metallo-protein; it binds one Ca2+ per mole in a pocket containing four Asp residues (Figure 4.26); these residues are highly conserved in all a-la’s and in lysozyme. The Ca-containing protein is quite heat stable (it is the most heat stable whey protein) or more correctly, the protein renatures following heat denaturation (denaturation does occur at relatively low temperatures, as indicated by differential scanning calorimetry). When the pH is reduced to below about 5, the Asp residues become protonated and lose their ability to bind Ca2+. The metal-free protein is denatured at quite low temperatures and does not renature on cooling; this characteristic has been exploited to isolate x-la from whey