Carnosine Synthetase

This enzyme [EC 6.3.2.11] catalyzes the reaction of histi-dine with 0-alanine and ATP to produce carnosine, AMP, and pyrophosphate.

G. D. Kalyankar & A. Meister (1971) Meth. Enzymol. 17B, 102.

CARNOT CYCLE

A hypothetical cycle for achieving reversible work, typically consisting of a sequence of operations: (1) isothermal expansion of an ideal gas at a temperature T2; (2) adiabatic expansion from T2 to T1; (3) isothermal compression at temperature T1; and (4) adiabatic compression from T1 to T2 . This cycle represents the action of an ideal heat engine, one exhibiting maximum thermal efficiency. Inferences drawn from thermodynamic consideration of Carnot cycles have advanced our understanding about the thermodynamics of chemical systems. See Carnot's Theorem; Efficiency; Thermodynamics

CARNOT'S THEOREM

A principle stating that for any engine working between the same two temperatures, maximum efficiency will occur by a engine working reversibly between those same two temperatures. Thus, all reversible engines have the same efficiency between the same temperatures and that efficiency is dependent only on those temperatures and not on the nature of the substance being acted upon. See Efficiency; Thermodynamics, Laws of; Carnot Cycle

^-CAROTENE (or, 0-CAROTENOID) 15,15'-DIOXYGENASE

This enzyme [EC 1.13.11.21] catalyzes the reaction of fi-

carotene with dioxygen to produce two retinal molecules.

Both iron ions and bile salts are required cofactors.

L. Villard-Mackintosh & C. J. Bates (1993) Meth. Enzymol. 214, 168. M. R. Lakshman & C. Okoh (1993) Meth. Enzymol. 214, 256. J. A. Olson & M. R. Lakshman (1990) Meth. Enzymol. 189, 425. O. Hayaishi, M. Nozaki & M. T. Abbott (1975) The Enzymes, 3rd ed., 12, 119.

CASBENE SYNTHASE

This enzyme [EC 4.6.1.7] catalyzes the conversion of geranylgeranyl diphosphate to the antifungal diterpene casbene and pyrophosphate (or, diphosphate). The enzyme was first isolated from the castor bean.

R. A. Gibbs (1998) Comprehensive Biological Catalysis: A Mechanistic Reference 1 , 31.

CASPASES (or, CYSTEINYL ASPARTATE-SPECIFIC PROTEINASES)

Caspase-1 [EC 3.4.22.36] (also known as interleukin-1fi converting enzyme and interleukin-1fi convertase precursor) is a member of the peptidase family C14. It catalyzes the hydrolysis of the Asp116-Ala117 and Asp27-Gly28 in the precursor protein, resulting in the release of interleukin-1fi. The enzyme will also hydrolyze the small peptide, Ac-TyrValAlaAsp—NHMEC.

D. W. Nicholson & N. A. Thornberry (1997) Trends Biochem. Sci. 22, 299.

CATALASE

This enzyme [EC 1.11.1.6] catalyzes the conversion of hydrogen peroxide to dioxygen and two water molecules. Both heme and manganese ions are used as cofactors. Several organic substances, e.g. ethanol, can act as the hydrogen donor. A manganese protein containing Mn(III) in the resting state is often called pseudocat-alase.

H. B. Dunford (1998) Comprehensive Biological Catalysis: A Mechanistic Reference 3, 195. J. E. Penner-Hahn (1998) Comprehensive Biological Catalysis: A

Mechanistic Reference 3, 439. D. C. Rees & D. Farrelly (1990) The Enzymes, 3rd ed., 19, 37. M. Ueda, S. Mozaffar & A. Tanaka (1990) Meth. Enzymol. 188, 463. G. R. Schonbaum & B. Chance (1976) The Enzymes, 3rd ed., 13, 363.

CATALYST

A substance that accelerates a chemical reaction but does not become consumed, generated, or permanently changed by such reaction. Thus, a catalyst does not alter the overall stoichiometric expression for the reaction or the overall equilibrium constant. The enhanced reactivity produced by a catalyst is referred to as catalysis.

An important characteristic of a catalyst's action is that the mechanism of the reaction is altered in a manner that allows for a lower activation energy. In a number of nonenzymatic examples, a reactant or product can act as a catalyst as well, and the definition must be altered to include substances that appear in the overall rate expression with a power higher than the corresponding stoichiometry number. One may also describe certain substances as catalysts even when, upon accelerating the reaction, they are destroyed by the process. In fact, nearly every catalyst for hydrogenation of organic molecules becomes progressively more and more poisoned with successive rounds of catalysis. In biology, we recognize that bacterial luciferases catalyze only a single turnover. Likewise, actin and tubulin respectively accelerate ATP and GTP hydrolysis in a single catalytic round associated with polymerization.

At very high concentrations, the enzyme can alter the equilibrium constant. If Keq is calculated by determining the equilibrium concentrations of all free products and reactants, and if the products and reactants have different affinities for the free enzyme, then high [Etot] favors formation of significant amounts of EA and EP, and this may cause an apparent shift in Keq. In such instances, the enzyme is now a stoichiometric participant in the reaction, and the true equilibrium constant has to take this into account.

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