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In catalytic triads, the basic nitrogen of histidine abstracts a proton from serine, threonine, or cysteine to activate it as a nucleophile. In a histidine proton shuttle, histidine is used to quickly shuttle protons. It can do this by abstracting a proton with its basic nitrogen to make a positively charged intermediate and then use another ...
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]
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.
Based on the crystal structure of the enzyme, the mechanism is proposed to be dependent on proton transfer by a histidine residue in the active site. [1] 6PGL selectively catalyzes the hydrolysis of δ-6-phosphogluconolactone, and has no activity on the γ isomer. [3]
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.
Shuttle catalysis is used to describe catalytic reactions where a chemical entity of a donor molecule is transferred to an acceptor molecule. [1] In these reactions, while the number of chemical bonds of each reactant changes, the types and total number of chemical bonds remain constant over the course of the reaction.
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]