The Structure of Lactose

In 1812, Vogel identified glucose as a product of acid hydrolysis of lactose and, in 1855, Erdmann detected another component, which was shown by Pasteur to be different from glucose. After its crystallization in 1856, this component was named galactose. Fischer established the con- 

The attachment to the D-glucose residue was assigned to the hydroxyl group at either C-4 or C-5 by the identification of a tetra-O-methyl-D-galactose, later recognized as the 2,3,4,6-isomer (3), and 2,3,6-tri-O-methyl-D-glucose (4) in the hydrolysis products of a crystalline, fully methylated lactose. These results were verified by Irvine and Hirst and by Schlubach and Moog.  

Repetition of this procedure on the galactosyl-arabinose gave O-jS-n-galacto-pyranosyl-D-erythrose (6) which did not give a phenylosazone. Hence, it was suggested that a 3-d-(1 4)-linkage is present in lactose (5). 

Conclusive evidence for this structure was also obtained by the following, 

The isolation of tetra-O-methyl-n-glucono-l,4-lactone (8) from a hydrolyzate of the octa-O-methyllactobionic acid (7 R = Me).  

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Exercise 20-13 Show how the structure of lactose may be deduced from the following ... 

The structure of lactose was, therefore, firmly established as i-O-fi-n-galactopyranosyl-D-glucopyranose (5). 

The structure of lactose permease has been determined (Figure 13.11). As expected from the sequence analysis, this structure consists of two halves, each of which comprises six membrane-spanning a helices. Someot these helices are somewhat irregular. The two halves are well separated and are joined by a single stretch of polypeptide. In this structure, the sugar lies in a pocket in tlie center of the protein and is accessible from a path leads from the interior of the cell. On the basis of these structures and a wide range of other experiments, a mechanism for symporter action has been... 

However, it has to be pointed out that it is only an approximate calculation, because we did not consider intermolecular hydrogen bonds which surely also make the structure of lactose more rigid. In such a case the activation energy of the slower secondary relaxation should be comparable to that for the y-relaxation (Ea=44 kJ/mol). This may imply that the slower secondary relaxation seen in lactose may be undetectable in the case of acethyl derivative of this disaccharide, because maxima of both secondary relaxations can be too close to each other. In fact, the inspection of the dielectric loss spectra obtained for octa-0-acetyI-Iactose below its glass transition temperature (see Fig. 2) showed that there is only one secondary relaxation peak. However, a detailed analysis of the y- loss peak revealed that probably two secondary processes contribute to it. 

Outline the metabolism of lactose in E. coli. Draw the structure of lactose, describe its entry into the cell, and write the equations for the reactions catalyzed by j3-gaIactosidase, providing both the substrates and the products. 

You can find a good conformational picture of (3-D-galactose on page 1146 of the textbook, as the left-hand part of the structure of lactose. Switch stereochemistry at Cl for the a isomer. Notice in the answer below how the name, 3-(a-D-galactopyranos-l-yl)- (J-D-galactopyranose. translates into the structure ... 

Figure 7.10 The structures of lactose and lactitol mono-ester surfactants. The asterisks denote alternative locations for the fatty acid substitution.

Show how the following experimental evidence can be used to deduce the structure of lactose (Section 22.12D) ... 

Draw the structure of lactose. Label the glucose and galactose rings in this disaccharide. 

FIGURE 22.61 The issues that must be addressed to determine the structure of (+)-lactose. 

Draw the structure of lactose, which is broken down by the enzyme lactase to give equimolar amounts of j8-galactose and jS-glucose. Linkage occurs between carbon atom 1 of galactose and 4 of glucose. continued... 

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