Histidyl groups

Peptides containing histidyl groups were prepared and their catalytic activities discussed121 ... 

Fig. 5. Preparation of polymer (PEI) ghosts in a three-step process (a) adsorption (b) stabilization (c) removal of core. These structures were substituted with lauroyl groups and histidyl groups to yield the solid-phase analog of (26).

Figures 5.10 and 5.11 show a histidyl F8 residue interacting with the heme iron on deoxy- and oxyhemoglobin. Would you expect the imidazole nitrogen of the histidyl group to be protonated or unproton-ated during this interaction Why ... 

The second proton transfer mechanism involves protonation of carboxyl or histidyl groups associated with electron carriers in the membrane and release of protons from these sites through proposed channels when the electron carrier is oxidized. This is essentially a proton channel system with movement through the channel gated by the oxidation-reduction state of the prosthetic group on the electron transport protein. The classical example of this is seen in cytochrome c oxidase (Figure 3). 

In a normal oxygen-binding process, the valence of Fe does not change. Oxygen is bound up with iron and the distal histidyl group. 

The free carbonyl residue can further condense with a histidyl group of another polypeptide chain to give a more complex cross-link. Schiff base formation, illustrated in Figure 8.4, involves an allysine residue at either the C- or N-terminal telopeptide and an uncharged lysine or hydroxylysine residue of another properly juxtaposed tropocollagen molecule. Such bonds are therefore intermolecular. Several other types of covalent cross-links are possible in collagens. They almost always involve lysine, allysine, hydroxylysine, hydroxyal-lysine, or histidine. 

Cmc denotes S-carboxymethylcysteine. Histidyl groups encircled are bound to copper, those in boxes bind to zinc, the shaded one is buried, and His 19 is accessible to solvent. 

FIGURE 17.12. Binding of carboxylate and histidyl groups and water molecules to the two metal ions in the active site of the enzyme D-xylose isomerase (Ref. 45). Note the carboxylate-water-metal ion chelate at the left, and the direct metal-carboxylate bonds to the right. The number of atoms in each closed cycle is shown. 

Determination of Histidyl Groups. Horinishi et al. (20) introduced the reagent diazonium-l-H-tetrazole (DHT) for spectrophotometric determination of histidyl residues in proteins. The method has later been modified by Sokolovsky and Vallee (45). When the P. notatum cellulase (37) reacted with DHT a complete loss of activity was observed at a very low excess of reagent. The DHT is, however, a rather unspecific reagent and it is difficult to draw any definite conclusions about the reasons for the inhibition. 

Fig. 2. Three-dimensional structural representations for zinc metall-oproteins. Comparison of the zinc ion-protein bonding interactions for zinc requiring enzymes (A—C) with the zinc-insulin hexamer (D, E). (A) Human carbonic anhydrase C, redrawn from Ref. (47) with permission. (B) Bovine carboxypeptidase Ay, redrawn from Ref. 30) with permission. (C) Bacillus thermoprotedyticus thermolysin, redrawn from Ref. 45) with permission. (D) and (E) Porcine Zn-insulin hexamer, taken from Ref. 48) with permission. The composite electron density maps in (D) and (E) show that each of the two zinc atoms present in the hexamer is within inner sphere bonding distance of three solvent molecules and three histidyl imidazolyl groups in an octahedral array about the metal ion. The position of one of the three equivalently positioned solvent molecules is indicated in (D). The electron density map in (E) shows the relative orientations of the three histidyl residues (His-BlO). (The atomic positions of one of the three equivalent histidyl groups are shown)...

Figure 1. A drawing of the main polypeptide chain and of the heme arrangement in cytochrome C3 from Desulfovibrio desulfuhcans Hydrogenase is the normai physiological electron donor and/or electron acceptor for cytochrome Cj. The protein of D. d. also has a sulphur reductase activity. The four hemes (in black the coordinated histidyl groups have been removed for clarity) lie near the surface of the protein. They are connected to the apoprotein by two thioether bonds involving well-resolyed cysteins. The short iron-to-iron distances (from 10.9 A to 1 7.3 A) reflect the close packing of the four nonparallel porphyrin rings.

Ohkubo, K Urata, Y. Hirota, S. Funakoshi, Y. Sagawa, T. Usui, S. Yoshinaga, K. Catalytic properties of novel L-histidyl group-introduced polymers imprinted by a transition state analogue in the hydrolysis of amino acid esters. J. Mol. Catal. A... 

If we focus attention on a specific histidyl group in the yth pair, say the left-hand one, then the observed ionization constant is... 

Ohkubo K, Urata Y> Honda Y et al. Preparation and catalytic property of L-histidyl group-introduced, crosslinked poly(ethylene imine)s imprinted by a transition-state analogue of an esterolysis reaction. Polymer 1994 35(24) 5372-5374. 

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