20-1 Principles and Applications of Inorganic, Organic, and Biological Chemistry Denniston, Topping, and Caret 4 th ed Chapter 20 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Power Point to Accompany

20-2 Introduction The proteins which serve as enzymes, Mother Nature’s catalysts, are globular in nature. Because of their complex molecular structures, they often have exquisite specificity for their substrate molecule and can speed up a reaction by a factor of millions relative to an uncatalyzed reaction. This presentation will describe how enzymes function.

Nomenclature and Classification Oxidoreductases catalyze redox reactions. Eg. Reductases or oxidases Transferases transfer a group from one molecule to another. Eg. Transaminases catalyze transfer of an amino group, kinases a phosphate group

20-4 Enzyme Classes, cont. Hydrolases cleave bonds by adding water. Eg. Phosphatases, peptidases, lipases Lyases catalyze removal of groups to form double bonds or the reverse. Eg. decarboxylasaes or synthases

20-5 Enzyme Classes, cont. Isomerases catalyze intramolecular rearrange- ments. Eg. epimerases or mutases Ligases catalyze a reaction in which a C-C, C-S, C-O, or C-N bond is made or broken.

20-6 Nomenclature With some historical exceptions, the name for an enzyme ends in –ase. The common name for a hydrolase is derived from the substrate. E. g. Urea: -a + ase = urease Lactose: -ose + ase = lactase Other enzymes may be named for the reaction they catalyze. E. g. Lactate dehydrogenase, pyruvate decarboxylase But: catalase, pepsin, chymotrypsin, tripsin

Enzymes and Activation Energy An enzyme speeds a reaction by lowering the activation energy. It does this by changing the reaction pathway. Energy change (  H) for the reaction Activation energy (E a ) for the reaction Transition state

Substrate Concentration Rates of uncatalyzed reactions increase as the concentration increases. Rates of enzyme catalyzed reactions behave as shown below. The first stage is the formation of an enzyme-substrate complex followed by slow conversion to product. Rate is limited by enzyme availability.

Enzyme-Substrate Complex The following reversible reactions represent the steps in an enzyme catalyzed reaction. The first step involves formation of an enzyme-substrate complex, E-S. E-S* is the transition and E-P is the enzyme- product complex.

20-10 Enzyme-Substrate Complex, cont. The part of the enzyme combining with the substrate is the active site. Active sites are: Pockets or clefts in the surface of the enzyme. R groups at active site are called catalytic groups. Shape of active site is complimentary to the shape of the substrate. The enzyme attracts and holds the enzyme using weak noncovalent interactions. Conformation of the active site determines the specificity of the enzyme.

20-11 Enzymes Models In the lock-and-key model, the enzyme is assumed to be the lock and the substrate the key. The two are made to fit exactly. This model fails to take into account the fact that proteins can and do change their conformations to accommodate a substrate molecule. The induced-fit model of enzyme action assumes that the enzyme conformation changes to accommodate the substrate molecule.

20-12 Enzymes Models, cont. Insert Fig 20.3

Specificity of the E-S Complex Absolute: enzyme reacts with only one substrate. Group: enzyme catalyzes reaction involving molecules with the same functional group. Linkage: enzyme catalyzes the formation or break up of only certain bonds. Stereochemical: enzyme recognizes only one of two enantiomers.

Transition State and Product As the substrate interacts with the enzyme, its shape changes and this new shape is less energetically stable. This transition state has features of both substrate and product and falls apart to yield product which dissociates from the enzyme. 1.The enzyme might put “stress” on a bond. 2.The enzyme might bring two reactants into close proximity and proper orientation. 3.The enzyme might modify the pH of the microenvironment, donating or accepting a H +.

Cofactors and Coenzymes Polypeptide portion of enzyme (apoenzyme) and nonprotein prosthetic group (cofactor) make up the active enzyme (holoenzyme). Cofactors may be metal ions, organic compounds, or organometallic compounds. A coenzyme, an organic molecule temporarily bound to the enzyme, is required by some enzymes. Most coenzymes carry electrons or small groups.

20-16 VitaminCoenzymeProcess Thiamine(B 1 )TPPdecarboxylation Riboflavin(B 2 )FMN, FADcarry H atoms Niacin(B 3 )NAD(P) + hydride carrier Pyradoxine(B 6 )Pyridoxal Pamino group transfer Folic acidtetrahydrofolateone-carbon transfer Vit Aretinalvision, growth BiotinbiocytinCO 2 fixing

20-17 Coenzymes, cont Nicotinic acid (niacin) is involved in redox reactions. nicotinamide

20-18 Coenzymes, cont. The nicotinamide part of NAD + accepts a hydride (H plus two electrons) from the alcohol to be oxidized. The alcohol loses a proton to the solvent. + H + Ox formRed form

20-19 Coenzymes, cont. Flavin coenzymes also serve in redox reactions FMN Flavin mononucleotide FAD Flavin adenine dinucleotide

20-20 Coenzymes, cont, The flavin coenzymes accept electrons in the flavin ring system. FAD FADH 2

Environmental Effects An enzyme has an optimum temperature that is usually close to the temperature at which it normally works, ie. 37 o C for humans. Excessive heat can denature a protein.

20-22 Environmental Effects, cont. Enzymes work best at the correct physiological pH. Extreme pH changes will denature the enzyme. Pepsin (stomach) and chymotrypsin (small intestine) have different optimum pHs. pepsin Chymo- trypsin

Regulating Enzyme Activity Some methods that organisms use to regulate enzyme activity are: 1.Produce the enzyme only when the substrate is present. 2.Allosteric enzymes 3.Feedback inhibition 4.Zymogens 5.Protein modification

20-24 Allosteric Enzymes Effector molecules change the activity of an enzyme by binding at a second site. Some effectors speed up enzyme action (positive allosterism) Some effectors slow enzyme action (negative allosterism) E. g. The third reaction of glycolysis places a second phosphate on fructose-6-phosphate. ATP is a negative effector and AMP is a positive effector of the enzyme phosphofructokinase.

20-25 Allosteric Enzymes Insert Fig 20.11

20-26 Regulation, cont. With feedback inhibition, a product in a series of enzyme-catalyzed reactions serves as an inhibitor for a previous allosteric enzyme in the series. A zymogen is a preenzyme. It is coinverted to its active form, usually by hydrolysis, at the active site in the cell. E. g. Pepsinogen is synthesized and transported to the stomach where it is converted to pepsin. The most common form of protein modification is addition or removal of a phosphate group.

Inhibition of Enzyme Activity Irreversible inhibitors bind tightly, sometimes even covalently, to the enzyme and thereby prevent formation of the E-S complex. Reversible competitive inhibitors often structurally resemble the substrate and bind at the normal active site Reversible noncompetitive inhibitors usually bind at someplace other than the active site. Binding is weak and thus inhibition is reversible.

Proteolytic Enzymes Proteolytic enzymes cleave the peptide bond in proteins. They depend on a hydrophobic pocket. Chymotrypsin cleaves the peptide bond at the carboxylic end of methionine, tyrosine, tryptophan, and phenylalanine.

20-29 Proteolytic Enzymes, cont. Chymostypsin: just seen Trypsin cleaves on the carboxyl side of basic amino acids. Elastase cleaves on the carboxyl side of glycine and alanine. These enzymes have different pockets but the catalytic sites remain unchanged during evolution and the mechanism of proteolytic action is the same for all these serine proteases.

Enzymes in Medicine Diagnostic Heart attack: uses levels of lactate dehydrogenase, creatine phosphate, and serum glutamate-oxaloacetate transaminase Pancreatitis: elevated amylase and lipase Analytical Reagents Urea converted to ammonia via urease and then blood urea nitrogen (BUN) measured. Replacement Therapy Administer genetically engineered  - glucocerebrosidase for Gaucher’s disease.

20-31 THE END Enzymes