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Histidine ball and stick model spinning. Histidine (symbol His or H) [2] is an essential amino acid that is used in the biosynthesis of proteins.It contains an α-amino group (which is in the protonated –NH 3 + form under biological conditions), a carboxylic acid group (which is in the deprotonated –COO − form under biological conditions), and an imidazole side chain (which is partially ...
Histidine is thus able to act as a powerful general base, activating the serine nucleophile. The histidine base aids the first leaving group by donating a proton, and also activates the hydrolytic water substrate by abstracting a proton as the remaining OH − attacks the acyl-enzyme intermediate.
Additionally, pH levels control specificity of substrate binding by malate dehydrogenase due to proton transfer in the catalytic mechanism. [17] A histidine moiety with a pK value of 7.5 has been suggested to play a role in the pH-dependency of the enzyme.
The catalytic effect of the above example is mainly associated with the reduction of the pKa of the oxyanion and the increase in the pKa of the histidine, while the proton transfer from the serine to the histidine is not catalyzed significantly, since it is not the rate determining barrier. [13]
His207 and Asp82 most likely facilitate this process. Other studies claim that Tyr83 of subunit D is coordinated to a nearby histidine as well as the O1 carbonyl oxygen of ubiquinone. The histidine residue decreases the pKa of tyrosine, making it more suitable to donate its proton to the reduced ubiquinone intermediate.
A highly conserved histidine residue (His183 in B. subtilis, His263 in humans) is essential for determining the type of distortion, as well as acting as the initial proton acceptor from protoporphyrin. [6] [7] Anionic residues form a pathway facilitating proton movement away from the catalytic histidine. [6]
This mechanism both requires the presence of the catalytic histidine and the correct orientation of ATP in the ATP-binding pocket of the FICD protein. Interactions between the secondary arginine (HxFx(D/E)GN(G/K)R 1 xxR 2 ) and the γ-phosphate of ATP orients ATP correctly in the pocket so that the proton transfer between the hydroxyl group of ...
The net effect of the malate–aspartate shuttle is purely redox: NADH in the cytosol is oxidized to NAD +, and NAD + in the matrix is reduced to NADH. The NAD + in the cytosol can then be reduced again by another round of glycolysis, and the NADH in the matrix can be used to pass electrons to the electron transport chain so ATP can be synthesized.