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Join date: May 18, 2022

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The importance of active site symmetry in understanding the origin of protein catalytic mechanisms. It is well established that a catalytic reaction usually occurs in a stepwise manner where the substrate is converted to a reactive intermediate which then undergoes a chemical transformation to the product. Although there are a great number of mechanisms that have been proposed to explain how this stepwise mechanism occurs, the underlying reasons are not always clear. This is particularly true in the case of enzymes with active sites that display mirror symmetry, as is the case with many proteins of the TIM-barrel superfamily. An example of a TIM-barrel enzyme is triosephosphate isomerase, where a study of its catalytic mechanisms has led to the discovery of a torsionally strained mechanism of attack of the nucleophile by the substrate. This illustrates the fact that symmetry can sometimes obscure the underlying mechanistic steps. This review seeks to show that the reason for the occurrence of mechanisms with mirror symmetry is simply that the symmetry of the protein active site is such that the substrate must be rotated 180 degrees to form the reactive species and then rotate back in a second step to form the product. A general scheme for the mechanism of a reaction with a mirror symmetry active site is discussed and illustrated. It is shown that, at the heart of any proposed mechanism, there is the crucial requirement of an appropriate symmetry to allow the appropriate chemistry to occur. a serial bit-line charge couple device, in which the capacitor of the unselected bit-line has an initial value of VDD and a final value of ground voltage or 0 V. However, the bit-line precharge is performed by turning on a bit-line precharge transistor, thereby supplying an initial value of 0 V to the unselected bit-line. The precharging of the unselected bit-line is finished before the word-line selection, and the word-line voltage is subsequently supplied to the selected word-line. Therefore, no electric charge is left at the unselected memory cells due to the precharging operation of the bit-line. The charge that is left in the memory cells is a charge corresponding to the leak currents that are normally present in the cell transistors, the bit-line capacitances, and the junction capacitances. The electric charge that is left in the unselected memory cells may be considered to be a fixed leakage current. This is because the stored electric charge in the memory cells can never be recovered even when the word-line is subsequently driven high and




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