Chapter 7 Biochemistry

7.1 Enzymes

Enzymes allow many chemical reactions tooccur within the homeostasis constraints of a living system. Enzymes functionas organic catalysts. A catalyst is a chemical involved in,but not changed by, a chemical reaction. Many enzymes function by lowering the activation energy of reactions. By bringing the reactants closer together, chemical bonds may be weakened and reactions willproceed faster than without the catalyst.

•Enzymes can act rapidly, as in the caseof carbonic anhydrase (enzymes typically end in the -asesuffix), which causes the chemicals to react 107 times faster than without theenzyme present. Carbonic anhydrase speeds up the transfer of carbon dioxidefrom cells to the blood. There are over 2000 known enzymes, each of which isinvolved with one specific chemical reaction. Enzymes are substrate specific. The enzyme peptidase (which breaks peptide bonds in proteins)will not work on starch (which is broken down by human-produced amylase in the mouth).

•Enzymesare proteins. The functioning of the enzyme is determined by the shape of theprotein. The arrangement of molecules on the enzyme produces an area known asthe active site within which the specific substrate(s)will "fit". It recognizes, confines and orients the substrate in aparticular direction.•The induced fit hypothesis suggests that the binding of thesubstrate to the enzyme alters the structure of the enzyme, placing some strainon the substrate and further facilitating the reaction. Cofactors are nonproteins essential for enzyme activity. Ions suchas K+ and Ca2+are cofactors. Coenzymes are nonprotein organic molecules bound to enzymes nearthe active site.

catalyst ['kætəlist] n. 催化剂

activation energy 活化能

reactant [ri'æktənt] n. 反应物

chemical bond 化学键

carbonic [kɑ:'bɔnik] adj. 碳的

anhydrase [æn'haidreis] n. 脱水酶

substrate ['sʌbstreit]n. 底物

peptidase ['peptideis]n. 肽酶

amylase ['æməleis] n. 淀粉酶

active site 活性部位

induced fit hypothesis 诱导契合假说

cofactor [kəu'fæktə] n. 辅因子

coenzyme [kəu'enzaim] n. 辅酶

intermediate [intə'mi:diət] n. 中间产物

end product 终产物