Histidines protonation

A histidine deprotonates the acetyl-CoA enol, which adds to the ketone carbonyl group of oxaloacetate in an aldol-like reaction. Simultaneously, an acid N-H proton of another histidine protonates the carbonyl oxygen, producing (S)-citryl CoA. 

In water-suppressed muscle spectra, contributions of lipids, methyl and methylene groups of creatine (Crs, Cr2), trimethylammonium-containing compounds (TMA), including signals from carnitine (Ct), choline (Cho), and taurine (Tau) are well observable as demonstrated in Fig. 15. Furthermore, small signals of histidine protons of carnosine (Cs) can be sometimes identified... 

Roberts et al. (523) emphasized that the in-line mechanism almost requires that two different groups deprotonate the 2 0H and protonate 05 since they are on opposite sides of the basal plane. They are physically removed from each other with a negative P-0- between them to trap any shuttle. Since they observed 3 -CMP interactions with His 12 and 119 by NMR with the greater effect on His 119 and both histidines protonated and since the X-ray structure shows the phosphate between the two histidines, they believed that both histidines are directly involved. Furthermore, the interaction of His 119 is more sensitive to the specific... 

The structure and enzyme kinetics of bovine erythrocyte superoxide dismutase are reviewed. The protein has a novel imidazolate-bridged copper(II)-zinc(II) catalytic center in each of two identical subunits. Since a C /Cu1 redox couple is responsible for the dismutase activity of the enzyme, the role of zinc is of interest. Both 220-MHz NMR measurements of the exchangeable histidine protons and chemical modifications using diethylpyrocarbonate demonstrate that zinc alone can fold the protein chain in the region of the active site into a conformation resembling that of the native enzyme. Other possible roles for zinc are discussed. Synthetic, magnetic, and structural studies of soluble, imidazolate-bridged copper complexes of relevance to the 4 Cu(II) form of the enzyme have been made. 

Figure 9.30. Histidine Proton Shuttle. (1) Histidine 64 abstracts a proton from the zinc bound water molecule, generating a nucleophilic hydroxide ion and a protonated histidine. (2) The buffer (B) removes a proton from the histidine, regenerating the unprotonated form.

Signorini, G.F., Chelli, R., Procacci, P., Schettino, V. Energetic fitness of histidine protonation states in PDB structures. J. Phys. Chem. B 2004,108,12252-7. 

Proton resonances are also observed from histidine-26 and -33 in horse cytochrome 160). A comparison of spectra with tuna, which has no histidine at position 33, permits the assignment of pA, = 6.54 to histidine-33 and 3.5 to histidine-26. Thus, histidine-33 has the pAa that would be expected in a polar environment, and the pA values of histidine-26 and -18 correlate well with their increasingly shielded hydrophobic surroundings as revealed by the X-ray analysis. The three histidine proton resonances do not become equivalent, indicating complete denaturation and opening up of the cytochrome molecule, until the pH falls to around... 

Isotropic Coupling Constants of the Histidine Proton Signals in the ENDOR Spectra of Bovine SOD ... 

Bradbury, J.H., Chapman, B.E. (1972) Assignment of C-2 histidine proton magnetic resonances of ribonuclease-a. Biochemical and Biophysical Research Communications, 49 (4), 891-897. 

Markley, J.L., Kato, 1. (1975) Assignment of histidine proton magnetic-resonance peaks of soybean trypsin-inhibitor (kunitz) by a differential deuterium-exchange technique. Biochemistry, 14 (14), 3234-3237. 

Wimalaseiia, D.S., Janowiak, B.E., Lovell, S., et al. (2010) Evidence that histidine protonation of receptor-bound anthrax protective antigen is a trigger for pore formation. Biochemistry, 49 (33), 6973-6983. Sawaya, M.R., Kraut, J. (1997) Loop and subdomain movements in the mechanism of Escherichia coli dihydrofolate reductase Crysttillographic evidence. Biochemistry, 36 (3), 586-603. 

Kim MO, Nichols SE, Wang Y, McCammon JA (2013) Effects of histidine protonation and rotameric states on virtual screening of M-tuberculosis RmlC. J Comput-Aided Mol Des 27(3) 235-246.doi 10.1007/sl0822-013-9643-9... 

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