Phosphate group anchoring

Figure 6.6 indicates the various reactions typical of vitamin Av Retinoic acid, oxidized trans-retinal, is apparently involved in epithelial cell physiology. Retinol phosphate, trans-retinol esterified with a phosphate residue, associates with various membrane structures through its hydrophobic isoprenoid residue, leaving its hydrophilic phosphate group in contact with the aqueous environment. It serves as an anchor for growing oligosaccharide chains in the same manner as dolichol phosphate does (see Chapter 18). 

Phosphorylated myo-inositol is an important moiety in biochemistry as the basis for signal transduction pathways in eukaryotes. Inositol also occurs in bacteria and archaea, not as part of a signaling pathway, but with diverse and unique uses. wvo-Inositol occurs as part of mycothiol, a molecule comparable to glutathione in mycobacteria, as part of an unusual osmolyte in hyperther-mophiles [e.g., di-myo-inositol-1,1 -phosphate (DIP)], and as the lipid head-group anchor for a series of glycosylated lipids in mycobacteria that are critical in the interaction of pathogenic mycobacteria with mammalian cells. 

Site 3, built of Arg232 and Lys235, is positively charged to anchor GGPP s phosphate groups. 

The sugar bisphosphate molecule is thus anchored at its two ends on opposite sides of the funnel by the phosphate binding sites. In the activated quaternary complex, CABP is oriented in the middle region by coordination to the active site metal (11). The inhibitor is completely buried in the active site of the enzyme. Only some atoms of the P2 phosphate group and the 2- carboxy group are somewhat accessible to solvent. 

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