Covalently modified enzymes

Figure 9.2 A schematic presentation of different types of enzyme preparations used in non conventional media, a enzyme powder, b enzyme crystals c enzyme on a porous support d covalently modified enzyme dissolved in the solvent e enzyme solubilised by surfactant ...

Covalently modified enzymes soluble in organic media... 

Correct answer = B. The increased insulin and decreased glucagon levels characteristic of the fed state promote the synthesis of fructose 2,6-bisphosphate. Most covalently modified enzymes are in the dephosphorylated state and are active. Acetyl CoA is not elevated in the fed state. The transport of glucose in the liver is not insulin sensitive. Synthesis of glucokinase is enhanced in the fed state. 

The following type of reaction, in which the enzyme reacts with one substrate to give a covalently modified enzyme and releases one product, and then reacts with the second substrate, gives rise to the characteristic family of Lineweaver-Burk plots illustrated in Figure 3.6 ... 

Kamat, S. Critchley, G. Beckman, E. J. Russell, A. J. Biocatalytic Synthesis of Acrylates in Organic Solvents and Supercritical Fluids HI. Does Carbon Dioxide Covalently Modify Enzymes Biotechnol. Bioeng. 1995, 46, 610-620. Kamihira, M. Taniguchi, M. Kobayashi, T. Synthesis of Aspartame Precursors by Enzymatic Reaction in Supercritical Carbon Dioxide. Agric. Biol. Chem. 1987, 51, 3427-3428. 

In a double-displacement reaction, at first only one substrate is bound the release of one product and covalently modified enzyme (E ) follows. E then combines with the second substrate to form a second product. Reactions catalyzed by aminotransferases are of this type. In this mechanism, no ternary complex EAB is formed. The double displacement reaction sequence is shown below ... 

Kamat S, Critchley G, Beckman EJ et al (1995) Biocatalytic synthesis of acrylates in organic solvents and supercritical fluids. III. Does carbon dioxide covalently modify enzymes Biotechnol Bioeng 46 610-620... 

Three possible kinetic mechanisms for a two-substrate/two-product reaction depicted in Cleland notation. Substrates are designated as A and B , products as C and D while E represents the enzyme and E is a covalently modified enzyme that forms transiently as part of the normal catalytic pathway in the ping-pong mechanism. 

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