Histidine molecules

Identify (a) the functional groups and (b) the chiral carbon atoms in the histidine molecule shown here. 

This reaction is essential in maintaining a constant pH in blood by the bicarbonate buffer system. Carbonic anhydrase, which contains a single zinc atom in its structure, has a molecular weight of about 30,000. In this structure, zinc is surrounded tetrahedrally by three histidine molecules and one water molecule. The exact role of the catalyst is not known, but it is believed to involve hydrolysis that can be represented as... 

Fig. 2. A section through the surface representation of the Hnl crystal structure. A histidine molecule is shown in the active site...

In this manner, the DNA strand shown above would direct the synthesis of a strand of RNA that would, in turn, direct the synthesis of a protein molecule. The protein molecule in the example given would consist of an isoleucine molecule (-A-T-T-) attached to an arginine molecule (-C-G-G-), attached to a histidine molecule (-G-A-G-), and so on Ile-Arg-His-. The molecule formed in this way might be the enzyme needed hy cells to assist in the breakdown of glucose molecules, to form red blood cells, to build cell walls, or to carry out some other essential function in cells. 

Figure A-1. Histidine molecule and corresponding molecular graph. The number associated to each bond is the probability corresponding to the incident vertex, calculated...

Fig. 13. Schematic structures of two polymorphs of the glycine and L-histidine molecules (A) o-glycine (B) - -glycine (C) A-histidine and (D) B-histidine structures (from ref. 25). Reproduced with permission from Elsevier Science.

To avoid bonding with other, possibly harmful molecules, several histidine molecules — an amino acid — are placed around the oxygen-binding location, that effectively guards the location. There are two histidine molecules attached to each heme group, or subunit, in a hemoglobin complex. These cause a distortion in the protein shape so that only a molecule with a nonlinear geometry can bind to it. Similar to puzzle pieces, only the atom with the correct shape and symmetry can fit comfortably. 

In view of the finding that some ferriheme proteins in insects (37) are able to carry nitric oxide, a NO carrier role for the metmyogiobin in vivo can be considered. As was shown, NO binds reversibly to the six-coordinate, high-spin iron(III) center of metmyogiobin (38) possessing a water and histidine molecule as distal and proximal ligand, respectively. Activation parameters determined for NO interaction with metMb (Table 4) clearly... 

Histidine is one of the 20 amino acids found in proteins. Shown here is a fully protonated histidine molecule where the numbers denote the values of the acidic groups. 

Studies on the biosynthesis of histidine in Salmonella by Ames and Hartman [214] established the simultaneous repression of several enzymes. In the series of reactions (at least 10 different steps) involved in the biosynthesis of the histidine molecule, the 5-carbon chain of phosphoribosyl pyrophosphate is converted to the 5-carbon chain of histidine. In Sal-... 

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