A protein (or protein-based molecule) that speeds up a chemical reaction in a living organism. An enzyme acts as catalyst for specific chemical reactions, convertingt a specific set of reactants (called substrates) into specific products. Without enzymes, life as we know it would not exist. Enzymes are nonetheless subject to error. In 1902 Sir Archibald Garrod was the first to attribute a disease to an enzyme defect, to what Garrod called an "inborn error of metabolism." Today, newborns are routinely screened for certain enzyme defects such as PKU (phenylketonuria) and galactosemia, an error in the handling (metabolism) of the sugar galactose.
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A protein that acts as a catalyst to induce chemical changes in other substances, itself remaining apparently unchanged by the process. Enzymes, with the exception of those discovered long ago ( e.g., pepsin, emulsin), are generally named by adding -ase to the name of the substrate on which the e. acts ( e.g., glucosidase), the substance activated ( e.g., hydrogenase), and/or the type of reaction ( e.g., oxidoreductase, transferase, hydrolase, lyase, isomerase, ligase or synthetase—these being the six main groups in the E. Nomenclature Recommendations of the International Union of Biochemistry). For individual enzymes not listed below, see the specific name. SYN: organic catalyst (1). [G. + L. en, in + zyme, leaven]
- acetyl-activating e. SYN: acetyl-CoA ligase.
- acyl-activating e. 1. SYN: long-chain fatty acid -CoA ligase. 2. SYN: butyrate-CoA ligase.
- adaptive e. SYN: induced e..
- allosteric e. an e. that exhibits the property of allosterism.
- amino acid activating e. SYN: aminoacyl-tRNA synthetases.
- angiotensin-converting e. (ACE) SYN: peptidyl dipeptidase A.
- antitumor e. an e. that stimulates the degradation of a particular metabolite that cannot be synthesized by tumor cells, inhibits the synthesis of a metabolite needed by tumor cells, or inhibits tumor-specific DNA utilization; E.G., asparaginase.
- autolytic e. an e. capable of causing lysis of the cell forming it.
- branching e. SYN: 1,4-α-d-glucan-branching e..
- β-carotene-cleavage e. SYN: β-carotene 15,15′-dioxygenase.
- citrate-cleavage e. SYN: ATP citrate (pro-3S)-lyase.
- cold-sensitive e. an e. that loses its stability as the temperature is lowered.
- condensing e. SYN: citrate synthase.
- constitutive e. an e. that is constantly produced by the cell, regardless of the growth conditions. Cf.:induced e..
- cooperative e. an e. that exhibits the property of cooperativity.
- D e. SYN: 4-α-d-glucanotransferase.
- deamidizing enzymes SYN: amidohydrolases.
- deaminating enzymes SYN: deaminases.
- debranching enzymes enzymes that bring about destruction of branches in glycogen; formerly considered to be one e., now known to be a mixture of transferases (4-α-d-glucanotransferase) and hydrolases (amylo-1,6-glucosidase). SYN: debranching factors.
- digestive enzymes 1. enzymes that are utilized in the digestive system; 2. enzymes that are hydrolases of macromolecules ( E.G., amylases, proteinases).
- disproportionating e. SYN: 4-α-d-glucanotransferase.
- extracellular e. an e. performing its functions outside a cell; e.g., the various digestive enzymes. SYN: exoenzyme.
- heat-stable e. SYN: thermostable e..
- hydrolyzing enzymes SYN: hydrolases.
- immobilized e. an e. that has been bound, usually covalently, to an insoluble organic or inorganic matrix or has been encapsulated.
- induced e., inducible e. 1. an e. that can be detected in a growing culture of a microorganism, after the addition of a particular substance (inducer) to the culture medium, but was not detectable prior to the addition and can act on the inducer. A prototype is the β-galactosidase of Escherichia coli, synthesized upon the addition of various galactosides, whether or not these are good substrates. Cf.:constitutive e.. 2. any e. that has its rate of biosynthesis increased due to the presence of the substrate or some other molecular entity. SYN: adaptive e..
- intracellular e. an e. that performs its functions within the cell that produces it; most enzymes are intracellular enzymes. SYN: endoenzyme (1).
- Kornberg e. dNA polymerase I from Escherichia coli.
- malate-condensing e. SYN: malate synthase.
- malic e. SYN: malate dehydrogenase.
- marker e. an e. that is used to identify a specific cell type, cell organelle, or cell component.
- membrane e. an e. present or embedded in a biomembrane.
- methionine-activating e. SYN: methionine adenosyltransferase.
- new yellow e. a former name for the d-amino-acid oxidase found in yeast, a flavoenzyme; so-called to distinguish it from Warburg old yellow e.. Cf.:amino acid oxidases.
- old yellow e. SYN: NADPH dehydrogenase.
- P e. SYN: phosphorylase.
- pantoate-activating e. SYN: pantothenate synthetase.
- phosphorylase-rupturing e. (PR e.) SYN: phosphorylase phosphatase.
- photoreactivating e. (PR e.) SYN: deoxyribodipyrimidine photolyase.
- PR e. abbreviation for phosphorylase-rupturing e.; photoreactivating e..
- Q e. 1,4-α-glucan branching e. in plants.
- R e. SYN: α-dextrin endo-1,6-α-glucosidase.
- reducing e. SYN: reductase.
- repair e. an e. that can catalyze the repair of damaged DNA; E.G., DNA ligase.
- repressible e. an e. that is produced continuously unless production is repressed by excess of an inhibitor (corepressor). SEE ALSO: inactive repressor.
- respiratory e. a tissue e. that is part of an oxidation-reduction system accomplishing the conversion of substrates to CO2 and H2O and the transfer of the electrons removed to O2.
- restriction e. SYN: restriction endonuclease.
- RNA e. SYN: ribozyme.
- splitting enzymes enzymes that, like aldolases, catalyze the conversion of a molecule into two smaller molecules without the addition or subtraction of any atoms.
- T e. 1,4-α-d-glucan 6-α-d-glucosyltransferase.
- terminal addition e. SYN: DNA nucleotidylexotransferase.
- thermostable e. an e. that is not readily subject to destruction or alteration by heat. SYN: heat-stable e..
- thiol e. an e. whose activity depends on a free thiol group.
- transferring enzymes SYN: transferases.
- Warburg old yellow e. SYN: NADPH dehydrogenase. SEE ALSO: new yellow e., yellow e..
- yellow e. SYN: flavoenzyme. SEE ALSO: Warburg old yellow e., new yellow e..

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en·zyme 'en-.zīm n any of numerous complex proteins that are produced by living cells and catalyze specific biochemical reactions at body temperatures

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a protein that, in small amounts, speeds up the rate of a biological reaction without itself being used up in the reaction (i.e. it acts as a catalyst). An enzyme acts by binding with the substance involved in the reaction (the substrate) and converting it into another substance (the product of the reaction). An enzyme is relatively specific in the type of reaction it catalyses; hence there are many different enzymes for the various biochemical reactions. Each enzyme requires certain conditions for optimum activity, particularly correct temperature and pH, the presence of coenzyme, and the absence of specific inhibitors. Enzymes are unstable and are easily inactivated by heat or certain chemicals. They are produced within living cells and may act either within the cell (as in cellular respiration) or outside it (as in digestion). The names of enzymes usually end in '-ase'; enzymes are named according to the substrate upon which they act (as in lactase), or the type of reaction they catalyse (as in hydrolase).
Enzymes are essential for the normal functioning and development of the body. Failure in the production or activity of a single enzyme may result in metabolic disorders; such disorders are often inherited and some have serious effects.
enzymatic adj.

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en·zyme (enґzīm) [en- + Gr. zymē leaven] a protein molecule that catalyzes chemical reactions of other substances without itself being destroyed or altered upon completion of the reactions. Symbol E. Enzymes are classified according to the recommendations of the Nomenclature Committee of the International Union of Biochemistry. Each enzyme is assigned a recommended name and an Enzyme Commission (EC) number. They are divided into six main groups: oxidoreductases, transferases, hydrolases, lyases, isomerases, and ligases. For individual enzymes, see under the specific name, e.g., glucose-6-phosphate dehydrogenase.

Medical dictionary. 2011.

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