Histidine ligands

In nomenclature for amino acids as ligands, the atoms in the R group are labeled with Greek letters starting with (5 for the first atom attached to the a carbon followed by y and 8 and e. The common bioinorganic ligands histidine, cysteine, and aspartic acid have their atoms labeled in the manner shown in Figure 2.5. The... 

Finally, there is the issue of which, if any, surface(s) of the protein(s) is used in electron transfer to specific electron transfer partners. Adman (1985) summarized the knowledge about the sites of interaction at that time. The hydrophobic surface surrounding the more exposed ligand histidine was one such surface (the point of view in Fig. 1), and the new... 

It is noteworthy that the proximity of the copper sites in ceruloplasmin, and, indeed, the involvement of most of the correct ligand histidines, were predicted some time ago by Ryden (1982, 1984) strictly on the basis of sequence homologies to plastocyanin. A similar prediction was made for laccase based on sequence similarities around the cysteine regions (Briving et al, 1980). Proximity of the type II site to the type III site (e.g., a trinuclear site) was also predicted by Solomon and co-workers (Allen-dorf et al., 1985 Spira-Solomon et al, 1986) on the basis of spectroscopic analysis of azide binding to laccase. What could not have been foreseen... 

Scatchard plots. Stability constants for the binding of Ou by a model ligand (histidine) and by natural organic ligands in river water were computed using Scatchard plot diagrams as described previously by Mantoura and Riley (1 ). The general equation for this analysis was ... 

The third, and, we feel most reasonable explanation is the presence in the active site of a bound buffer species. At equilibrium, this proton transfer group would alternately protonate and deprotonate, as the enzyme correspondingly dehydrated and hydrated CO2 species, free of proton diffusion limitations. The most likely candidate for this internal buffer is the imidazole of the conserved non-ligand histidine in the active site of carbonic anhydrase (His -64 in HCAC), as proposed earlier(3,24,26,29). 

Figure 11. ESEEM spectra of the oxidized form of the Type I copper piotein msticyanin, which has the classic His2CysMet Cu(II) binding motif. These are exact cancellation-like spectra that are obtained at spectrometer operating frequencies from 7.0 to 13 GHz. The peaks are mobile, as predicted by the nuclear Zeeman dependence. The upper spectrum is derived from an engineered form of the protein in which one of the two ligand histidines is removed the spectrum is characteristic of a single hyperfine spectrum. The lower spectrum corresponds to the modulation spectrum of two nearly equivalent N interactions and features the sum/harmonic lines in the 2-3 MHz region.

The " N ENDOR spectroseopy of mixed-valent MMOH has identified two sets of nitrogen resonances (superhyperfine coupling constants of 13.6 and 2-5 MHz) that were assigned to the two N-eontaining ligands (histidines) directly coordinated to the Fe " and Fe " ions, respectively, in agreement with the crystal structure shown in Figure 2B [85]. 

---