Hydrogen bonding phosphate transport

Amino acids involved in binding M6P are shown in O Fig. 8. The phosphate moiety is coordinated by His 105, Asn 104, Asp 103 and divalent cation. Each hydroxyl of Man is hydrogen-bonded to binding site residues, while no interactions between the hydrophobic carbohydrate backbone and aromatic side chains are observed [148]. The multiple contacts between lectin and M6P, in contrast to that observed for many lectins which mediate cell surface interactions, are reflected in pM binding constants, reminiscent of bacterial arabinose-binding protein which functions as a sugar transport protein [149]. 

In nature the selective complexation of anions takes place by hydrogen bonds the selective recognition of phosphate and sulfate in biological systems by transport receptor proteins has recently been described [ret 26]. 

A full equivalent of diadenosine phosphate was extracted into organic solvents per equivalent of 59a, indicating that the receptor has a very high affinity for the dinucleotide. NMR experiments established that the carbazole group tt-stacks with the heterocyclic base of the nucleotide, and the Kemp triacid does indeed hydrogen-bond with the receptor, which is therefore an efficient nucleotide transporter, as determined by U-tube experiments. In these transport measurements, 59c, which contains a 2-naphthoyl moiety, gave increased transport rates for adenosine monophosphate guests. 

This discussion will be limited to aerobic hydroxylation reactions. As already mentioned, it is a reaction which appears to be restricted to the metabolism of rather inert molecules. The reason for this is not apparent, but it may be because the reaction is energetically expensive for the cell. If, in the hydration type of hydroxylation reaction, a pyridine nucleotide functions as the hydrogen acceptor, the subsequent reoxidation of the DPNH or TPNH over the flavin-cytochrome hydrogen transport system could be coupled to the synthesis of 3 moles of ATP per mole of DPNH oxidized. In the aerobic type of hydroxylation reaction, utilizing TPNH as the electron donor, the oxidation of the TPNH is apparently not coupled to high-energy phosphate-bond synthesis and the cell therefore loses the equivalent of three ATP s. 

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