Histidine polypeptide

The 12 residues between the second cysteine zinc ligand and the first histidine ligand of the classic zinc finger motif form the "finger region". Structurally, this region comprises the second p strand, the N-terminal half of the helix and the two residues that form the turn between the p strand and the helix. This is the region of the polypeptide chain that forms the main interaction area with DNA and these interactions are both sequence specific. 

The light-harvesting complex LHl is directly associated with the reaction center in purple bacteria and is therefore referred to as the core or inner antenna, whereas LH2 is known as the peripheral antenna. Both are huilt up from hydrophohic a and p polypeptides of similar size and with low hut significant sequence similarity. The two histidines that hind to chlorophyll with absorption maxima at 850 nm in the periplasmic ring of LH2 are also present in LHl, but the sequence involved in binding the third chlorophyll in LH2 is quite different in LHl. Not surprisingly, the chlorophyll molecules of the periplasmic ring are present in LHl but the chlorophyll molecules with the 800 nm absorption maximum are absent. 

Kentolysin Compared to Heliantholysin. Stoichactis helianthus occurs in the Caribbean region whereas another species, Stoichactis kenti is distributed in the Indo-Pacific area. The latter produces a toxin, kentolysin, that is similar to, but not identical with heliantholysin (6). The amino acid compositions of the two polypeptides show a distinct resemblance but appear to differ significantly in number of residues of lysine, methionine, tyrosine and histidine. IgG from a rabbit immunized against heliantholysin neutralizes both heliantholysin and kentolysin, but neutralization of the homologous toxin is more efficient (Table III). It can be seen that in the concentrations used, the IgG failed to neutralize the related lytic peptides of Condylactis gigantea and Epiactis prolifera. 

Figure 7. Traces of the a-carbon polypeptide backbone of domains 1 and 6 in the hCP structure. Domain 1 is shown (shaded) on the left hand side of the diagram this domain contributes four histidine residues (not shown) to the trinuclear cluster copper atoms are depicted as black spheres. Domain 6 is on the right hand side of the figure and also contributes four histidine residues to the cluster. The portion of the polypeptide chain colored black is that which is missing in the truncated enzyme. This polypeptide, residues 991 to 1046 inclusive, includes two histidine residues bound to the trinuclear copper center and three residues bound to the mononuclear copper in domain 6 these residues are depicted in black. The absence of the C-terminal polypeptide would also remove over 50% of the interdomain hydrogen-bond and iron-pair interactions observed in the intact enzyme.

In one case, a small peptide with enzyme-like capability has been claimed. On the basis of model building and conformation studies, the peptide Glu-Phe-Ala-Ala-Glu-Glu-Phe-Ala-Ser-Phe was synthesized in the hope that the carboxyl groups in the center of the model would act like the carboxyl groups in lysozyme 17). The kinetic data in this article come from assays of cell wall lysis of M. lysodeikticus, chitin hydrolysis, and dextran hydrolysis. All of these assays are turbidimetric. Although details of the assay procedures were not given, the final equilibrium positions are apparently different for the reaction catalyzed by lysozyme and the reaction catalyzed by the decapeptide. Similar peptide models for proteases were made on the basis of empirical rules for predicting polypeptide conformations. These materials had no amidase activity and esterase activity only slightly better than that of histidine 59, 60). 

Histidine residues are efficient nucleophiles in aqueous solution at pH 7, much more so than lysines, and this is the basis for the site-selective functionalization of lysine residues in folded polypeptides and proteins [24, 25]. p-Nitrophenyl esters react with His residues in a two-step reaction to form an acyl intermediate under the release of p-nitrophenol followed by the reaction of the intermediate with the most potent nucleophile in solution to form the reaction product. In aqueous solution the reaction product is the carboxylic acid since the hydroxide ion is the most efficient nucleophile at pH 7. If there is an alcohol present the reaction product will be an ester and the overall reaction is a transesterification reaction. 

Miller s biomimetic approach inspired Ishihara [234] to develop a minimal artificial acylase for the KR of mono-protected cw-l,2-diols and A-acylated 1,2-amino alcohols. Derived from (S)-histidine, Ishihara s organocatalyst contains only one stereogenic centre and incorporates a sulfonamide linkage in place of a polypeptide chain to allow the NH group to engage as an H-bond donor with the substrates (Fig. 13) [234]. 

Figure 2-4. Structure of hemoglobin and its oxygen-binding site. An expanded view of the heme ring within the hydrophobic crevice is shown to the right. The polypeptide backbone of the nearby F helix is indicated by the ribbon with the imidazole ring of the F8 histidine residue projecting out as one of the ligands of the heme iron atom.

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