Arabinose complex

Figure 2. Calculated vs. observed free energies of binding for DHFR complexes (circles), the arabinose complex with arabinose binding protein (triangle), the lysine complex with lysine binding protein (diamond), the complexes of stearate and elaidate with muscular fatty acid binding protein (squares) and the complexes of retinol, retinoic acid, n-ethyl-retinamide and fenretinide with bovine plasma RBP (stars).

ABP-L-arabinose complex 240-1,24/ lectin complexes 246, 247,247 MBP-maltose complex 245,245... 

Fig. 14.1 Scheme of the hydrogen bond network in the ARP-L-arabinose complex (from Quiocho 1989) (reproduced witb kind permission from the International Union of Pure and AppUed Chemistry). 

These principles are well illustrated by the 1.1-k analysis of the L-arabinose complex with L-arabinose-binding protein shown in Fig. 2 44). The extensively refined structure is the most detailed example of a hydrogen bond network at a... 

Fio. 2. Schematic representation from the extensively refined structure analysis of L-arabinose complexed with the L-arabinose-binding protein. Two levels of hydrogen bonds stabilizing the structure are shown. The residues in shell I hydrogen bond to L-arabinose and to shell II. Adapted horn Quiocho and Vyas (44) with permission from Nature. 

Regulation - The araC gene encodes a regulatory protein that binds to arabinose, the inducer of the operon. Binding of the AraC - arabinose complex at a site called aral activates transcription of the arabinose operon, but only when the cAMP - CRP complex (see here) is bound at an adjacent site. Thus, whereas the lac operon requires one protein to be bound and one to be dissociated for maximal transcription, the arabinose operon requires two proteins to be bound at adjacent sites. 

Gum ghatti is the calcium and magnesium salt of a complex polysaccharide which contains L-arabinose, D-galactose, D-mannose, and D-xylose and D-glucuronic acid (48) and has a molecular weight of approximately 12,000. On dispersion in water, gum ghatti forms viscous solutions of viscosity intermediate between those of gum arabic and gum karaya. These dispersions have emulsification and adhesive properties equivalent to or superior to those described for gum arabic. 

StmcturaHy, arabinogalactan is a complex, highly branched polymer of arabinose and galactose in a 1 6 ratio (67). It is composed of one fraction with an average molecular weight of 16,000, and one of 100,000 (68). 

The hydrolysis of zeaxanthin esters by a carboxyl ester lipase indeed enhanced both the incorporation of zeaxanthin in the micellar phase and uptake of zeaxanthin by Caco-2 cells. As mentioned earher, carotenoids can also be linked to proteins by specific bindings in nature and these carotenoid-protein complexes may slow the digestion process and thus make their assimilation by the human body more difficult than the assimilation of free carotenoids. Anthocyanins are usually found in a glycosylated form that can be acetylated and the linked sugars are mostly glucose, galactose, rhamnose, and arabinose. 

Photolysis of three 2,4-dinitroanilino-substituted carbohydrates, compounds that differ considerably from each other in photochemical reactivity, has been reported.150,151 l-Deoxy-l-(2,4-dinitroanilino)-D-glucitol (73) is photochemically unreactive in contrast, sodium 2-deoxy-2-(2,4-dinitroanilino)-D-gluconate (74) produces D-arabinose in 52% yield upon irradiation.150 The behavior of compounds 73 and 74 indicates that oxidative loss of the 2,4-dinitroanilino group during photolysis is only possible when it is accompanied by simultaneous decarboxylation. The evidence gathered from the considerable study of this reaction for noncarbohydrate systems suggested that this process is quite complex. Although useful, mechanistic proposals have... 

Gum tragacanth is a mixture consisting of a salt of a complex acidic polysaccharide (tragacanthic acid) and a neutral polysaccharide which contains arabinose units. It is the latter which probably accounts for the isolation of arabinose from the crude gum. ... 

Unlike the gums referred to above, which are essentially homogeneous, this gum is a complex mixture, a fact recognized by O Sullivan who had subjected it to very careful fractionation. Hydrolysis of the crude gum with dilute mineral acid has been shown to afford L-arabinose, D-xylose... 

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