Enzyme shift

Because this enzyme catalyzes the committed step in fatty acid biosynthesis, it is carefully regulated. Palmitoyl-CoA, the final product of fatty acid biosynthesis, shifts the equilibrium toward the inactive protomers, whereas citrate, an important allosteric activator of this enzyme, shifts the equilibrium toward the active polymeric form of the enzyme. Acetyl-CoA carboxylase shows the kinetic behavior of a Monod-Wyman-Changeux V-system allosteric enzyme (Chapter 15). 

Allosteric enzymes shift the target enzyme s saturation curve to the left (see p. 92). In Eadie-Hofstee and Lineweaver-Burk plots (see p. 92), allosteric enzymes are recognizable because they produce curved lines (not shown). 

Carreiro, M. M., R. L. Sinsabaugh, D. A. Repert, and M. Parkhurst. 2000. Microbial enzyme shifts explain litter decay responses to simulated nitrogen deposition. Ecology... 

Increase of the load of enzyme of a highly active enzyme shifts the kinetically controlled enzyme electrode to diffusional limitation. Since diffusion-limiting effects are associated with the immobilization, they may be manipulated by the procedure of coupling an enzyme to the electrode. 

Binding of a substrate molecule at one site facilitates binding of other substrate molecules at other sites. The enzyme shifts from a T form to an R form as substrate is bound. 

How can we explain the enzyme s sigmoidal kinetics in light of the structural observations Like hemoglobin (p. 188), the enzyme exists man equilibrium between the T stale and the R stale. In the absence of sub-.strate, almost all the enzyme molecules are in the T state. The T state has a low affinity for substrate and hence shows a low catalytic activity, The oc casional binding of a substrate molecule to one active site in an enzyme increases the likelihood that the entire enzyme shifts to the R state with its higher binding affinity. The addition of more substrate has two effects. First, it increases the probability that each enzyme molecule will bind at least one substrate molecule. Second, it increases the average number ot substrate molecules bound to each enzyme. The presence of additional substrate will increase the fraction of enzyme molecules in the more active R state because the position of the equilibrium depends on the number of dc -live sites that are occupied by subslrate. We considered this property, called... 

Free ACADs stabilize the neutral flavin semiquinone when the enzyme is artificially reduced by singleelectron donors however, binding of the product enoyl-CoA esters shifts the pAi, of the semiquinone to 7.3 to generate the anionic form. The two-electron redox potential of free acyl-CoA substrates (—40 mV) is not low enough to produce free reduced enzyme (—145 mV for MCAD). However, ligands bind much more tightly to the reduced enzyme, shifting the redox equilibrium to promote flavin reduction. For example, the dissociation constant of oct-2-enoyl-CoA from oxidized enzyme is 200 nmol 1, but with reduced enzyme it is 13 pmoll . Therefore, it is thermodynamically unfavorable for the reduced enzyme to release product. Instead, turnover occurs by sequential one-electron oxidations by ETF while the product enoyl-CoA ester is bound " (Scheme 11). 

A substrate-film technique has been used to examine the enzyme system that hydrolyses hyaluronic acid (and chondroitin sulphate) in isolated macrophages and in developing and healing tuberculosis lesions in rabbits. The pH optimum of the enzyme shifted towards neutrality in intact cells and tissues. The level of hyaluronidase in the kidneys of rats suffering from dehydration has been investigated. ... 

---