Lactose structure

The ultimate proof of the lactose structure was provided in 1942 by Haskins, Hann, and Hudson, who synthesized lactose and its epimer,... 

Lactose is sometimes referred to as milk sugar because it is the principal carbohydrate in milk. The lactose structure follows. Is this a monosaccharide, disaccharide, or polysaccharide Explain. 

Figure 9.6 Open-chain and lactose structures of the acylsophorose from Candida bombicola. Ri and Rj = -H or -COCH3, R3 = -H or -CH3 /i = 16 if R3 = -H or 15 if R3 = -CH3. Position of lactose ring indicated by broken line.

D-galactose, C HiiOe. Crystallizes in the pyranose form m.p. 1I8-120 C (monohydrate), 165-5" C (anhydrous). An isomer of glucose which is fairly widely distributed in plants. It is a constituent of raffinose and slachyose, of hemicelluloses, of pectin, of gums and mucilages, and of some glycosides. In animals it forms half the lactose molecule and is the sugar found in the brain. Chemically it is very similar to glucose. It has the structure... 

Lewis, M., et al. Crystal structure of the lactose operon repressor and its complexes with DNA and inducer. Science 271 1247-1254, 1996. 

FIGURE 10.12 The structures of several cardiac glycosides. The lactose rings are yellow. 

Ovchinnikov 234 237) has shown that bovine rhodopsin, although quite different in amino acid sequence (348 residues), also forms seven transmembrane helices. This structural similarity between bacterial and mammalian light activated membrane proteins is remarkable. Since the two amino acid sequences have little in common it would appear that the necessary requirement is seven transmembrane helices to form a channel which is specific for proton migration. For example it has been suggested that a similar arrangement and function is performed by the lactose permease of E. coli237). 

Abramson, J., Smirnova, I., Kasho, V., Verner, G., Kaback, H.R. and Iwata, S. (2003). Structure and mechanism of the lactose permease of Escherichia coli. Science, 301, 610-615... 

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