Chapter 4: Enzyme
(1)To which class of enzymes does each of the following belong?
a. Pyruvate kinase
Transferases are enzymes that catalyze the transfer of functional groups from one molecule to another. For example, a transaminase catalyzes the transfer of an amino functional group, and a kinase catalyzes the transfer of a phosphate group. Kinases play a major role in energy-harvesting processes involving ATP.
=> Pyruvate kinase is the transferases.
b. RNA ligase
Ligasesare enzymes that catalyze a reaction in which a C—C, C—S, C—O, or C—Nbond is made or broken. This is accompanied by an ATP-ADP interconversion.
=> RNA ligase is the ligases.
c. Triose isomerase
Isomerasesrearrange the functional groups within a molecule and catalyze theconversion of one isomer into another.
=> Triose isomerase is the isomerases.
d. Pyruvate dehydrogenase
Oxidoreductasesare enzymes that catalyze oxidation–reduction (redox) reactions.Lactate dehydrogenaseis an oxidoreductase that removes hydrogen from a moleculeof lactate.
=> Pyruvate dehydrogenase is the oxidoreductases.
e. Phosphoglucoisomerase
Isomerasesrearrange the functional groups within a molecule and catalyze theconversion of one isomer into another.
=>Phosphoglucoisomerase is the isomerases.
(2) What is the substrate for each of the following enzymes?
a. Sucrase
Sucrase – ase + ose = Sucrose
=> So the substrate is sucrose.
b. Pyruvate decarboxylase
Pyruvate decarboxylase removes the carboxyl group from pyruvate.
=> So the substrate is pyruvate.
c. Succinate dehydrogenase
Succinate dehydrogenase removes hydrogen atoms from succinate ions.
=> So the substrate is succinate ions.
(3) Name three major properties of enzyme active sites.
The part of the enzyme that binds with the substrate is called theactive site. There are majorproperties:
- Enzyme active sites are pockets or clefts in the surface of the enzyme. The Rgroups in the active site that are involved in catalysis are calledcatalyticgroups.
- The shape of the active site is complementary to the shape of the substrate.That is, the substrate fits neatly into the active site of the enzyme.
- An enzyme attracts and holds its substrate by weak, non-covalentinteractions. The R groups involved in substrate binding, and not necessarilycatalysis, make up the binding site.
- The conformation of the active site determines the specificity of the enzymebecause only the substrate that fits into the active site will be used in a reaction.
(4) Outline the four general stages in an enzyme-catalyzed reaction. Describe the transition state, mechanism and kinetics, and Inhibition of Enzyme Activity.
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In this state, the transition state,the shape ofthe substrate is altered, because of its interaction with the enzyme, into an intermediate form having features of both the substrate and the final product. Thistransition state, in turn, favors the conversion of the substrate into product (stepIII). The product remains bound to the enzyme for a very brief time, then in stepIV the product and enzyme dissociate from one another, leaving the enzyme completely unchanged.
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Bond breakage is facilitated by theenzyme as a result of stress on a bond.(a, b) The enzyme-substrate complex isformed. (c) In the transition state, theenzyme changes shape and thereby putstress on the O-glycosidic linkage holdingthe two monosaccharides together. Thislowers the energy of activation of thisreaction. (d, e) The bond is broken, andthe products are released.
Inhibition of Enzyme Activity:
Enzyme activity can be destroyed by a variety of inhibitors.Irreversible inhibitors, or poisons, bind tightly to enzymesand destroy their activity permanently. Competitive inhibitorsare generally structural analogsof the natural substrate forthe enzyme. They compete with the normal substrate forbinding to the active site. When the competitive inhibitor isbound by the active site, the reaction cannot occur, and noproduct is produced.
Irreversible enzyme inhibitors,such as arsenic, usually bind very tightly, sometimes even covalently, to the enzyme. This generally involves binding of the inhibitor to one of the R groups of an amino acid in the active site. Inhibitor bindingmay block the active site binding groups so that the enzyme-substrate complex cannot form. Alternatively, an inhibitor may interfere with the catalytic groups of theactive site, thereby effectively eliminating catalysis. Irreversible inhibitors generallyinhibit many different enzymes.
Reversible, competitive enzyme inhibitorsare often referred to as structuralanalogs,that is, they are molecules that resemble the structure and charge distribution of the natural substrate for a particular enzyme. Because of this resemblance,the inhibitor can occupy the enzyme active site. However, no reaction can occur,and enzyme activity is inhibited. This inhibition is said to be competitive becausethe inhibitor and the substrate compete for binding to the enzyme active site. Thus,the degree of inhibition depends on their relative concentrations. If the inhibitor isin excess or binds more strongly to the active site, it will occupy the active site morefrequently, and enzyme activity will be greatly decreased. On the other hand, if thenatural substrate is present in excess, it will more frequently occupy the active site,and there will be little inhibition.
The sulfa drugs,