enzyme /en·zyme/ (en´zīm) a protein that catalyzes chemical reactions of other substances without itself being destroyed or altered upon completion of the reactions. Enzymes are divided into six main groups: oxidoreductases, transferases, hydrolases, lyases, isomerases, and ligases. Symbol E.
allosteric enzyme one whose catalytic activity is altered by binding of specific ligands at sites other than the substrate binding site.
brancher enzyme , branching enzyme 1,4-α-glucan branching enzyme: an enzyme that catalyzes the creation of branch points in glycogen (in plants, amylopectin); deficiency causes glycogen storage disease type IV.
constitutive enzyme one produced constantly, irrespective of environmental conditions or demand.
debrancher enzyme , debranching enzyme
2. any enzyme removing branches from macromolecules, usually polysaccharides, by cleaving at branch points.
induced enzyme , inducible enzyme one whose production can be stimulated by another compound, often a substrate or a structurally related molecule.
proteolytic enzyme peptidase.
repressible enzyme one whose rate of production is decreased as the concentration of certain metabolites is increased.
respiratory enzyme one that is part of an electron transport (respiratory) chain.
re·press·i·ble enzyme (r -pr s  -bl)n. An enzyme whose production is generally continuous but can be halted if a particular substance is present in concentrations greater than normal. |
enzyme
any protein that acts as a catalyst, increasing the rate at which a chemical reaction occurs. The animal body probably contains about 10,000 different enzymes. At body temperature, very few biochemical reactions proceed at a significant rate without the presence of an enzyme. Like all catalysts, an enzyme does not control the direction of the reaction; it increases the rates of the forward and reverse reactions proportionally.
activating enzyme
one that activates a given amino acid by attaching it to the corresponding transfer ribonucleic acid.
enzyme assays
several enzymes are important in clinical pathology. Enzymes characteristic of a tissue are released into the blood when the tissue is damaged, and enzyme levels in the blood can aid in the diagnosis or monitoring of specific diseases. Lipase and amylase levels are useful in pancreatic diseases; alkaline phosphatase (ALP), aspartate aminotransferase (AST) and alanine aminotransferase (ALT) in liver diseases; and lactate dehydrogenase (LD), AST and creatine kinase (CK) in muscle disease. ALP is also released in bone diseases. Many enzymes have different forms (isoenzymes) in different organs. The isoenzymes can be separated by electrophoresis in order to determine the origin of the enzyme. Isoenzymes of LD, CK and ALP have the most clinical utility.
brancher enzyme, branching enzyme
amylo-(1,4→1,6)-transglycosylase; important in the synthesis of the branched glycogen molecule. Absence of the enzyme causes an increase in the length of the glucose chains and a decrease in the number of branch points in the glycogen molecules.
congenital enzyme deficiency
in humans hundreds of genetic diseases that result from deficiency of a single enzyme are now known. Many of these diseases fall into two large classes. The aminoacidopathies, e.g. phenylketonuria (PKU), result from deficiency of an enzyme in the major pathway for the metabolism of a specific amino acid. The amino acid accumulates in the blood, and it or its metabolites are excreted in the urine. The lysomal storage diseases, e.g. gangliosidosis, mannosidosis, result from deficiency of a lysomal enzyme and the accumulation of the substance degraded by that enzyme in lysosomes of cells throughout the body. The stored material is usually a complex substance, such as glycogen, a sphingolipid or a mucopolysaccharide. Many similar diseases are now identified in animals and are to be found under the specific name of each disease.
constitutive enzyme
one produced by a microorganism regardless of the presence or absence of the specific substrate acted upon.
core enzyme
the smallest aggregate of an enzyme's subunits that has enzymatic activity.
debrancher enzyme, debranching enzyme
dextrin-1,6-glucosidase: an enzyme that acts to move glucose residues of the glycogen molecule, and is important in glycogenolysis.
induced enzyme, inducible enzyme
one whose production requires or is stimulated by a specific small molecule, the inducer, which is the substrate of the enzyme or a compound structurally related to it.
enzyme induction
the effect some compounds such as phenobarbitone and phenytoin have in increasing the activity of microsomal hepatic enzymes. This may cause alterations in the metabolism of concurrently administered drugs.
microsomal e's
those associated with the endoplasmic reticulum of cells, particularly of the liver.
proteolytic enzyme
one that catalyzes the hydrolysis of proteins and various split products of proteins, the final product being small peptides and amino acids.
repressible enzyme
one whose rate of production is decreased as the concentration of certain metabolites is increased.
respiratory e's
enzymes of the mitochondria, e.g. cytochrome oxidase, which serve as catalysts for cellular oxidations.
restriction e's
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