Enzymes Enzymes What are enzymes How are enzymes

Enzymes

Enzymes • • What are enzymes? How are enzymes named and classified? What is the terminology used with enzymes? What factors influence enzyme activity? What are the mechanisms of enzyme action? How are enzymes regulated? How are enzymes used in medicine?

Enzyme catalysis • Enzyme: a biological catalyst. – With the exception of some RNAs that catalyze their own selfcleavage, all enzymes are proteins. – Enzymes can increase the rate of a reaction by a factor of 109 to 1020 over an uncatalyzed reaction. – Some catalyze the reaction of only one compound. – Others are stereospecific; for example, enzymes that catalyze the reactions of only L-amino acids. – Others catalyze reactions of specific types of compounds or bonds; for example, trypsin that catalyzes hydrolysis of peptide bonds formed by the carboxyl groups of Lys and Arg

Location of enzymes • Many enzymes are localized according to the body’s need to catalyze specific reactions.

Classification of enzymes • Enzymes are commonly named after the reactions they catalyze and/or the compound. – example: lactate dehydrogenase, acid phosphatase. • Enzymes are classified into six major groups. – Oxidoreductases: oxidation-reduction reactions. – Transferases: group transfer reactions. – Hydrolases: hydrolysis reactions. – Lyases: addition of groups to a double bond, or removal of groups to create a double bond. – Isomerases: isomerization reactions. – Ligases: the joining to two molecules.

Classification of enzymes • Oxidoreductase: • Transferase:

Classification of enzymes • Hydrolase: • Lyase:

Classification of enzymes • Isomerase: • Ligase

Terms in enzyme chem • Apoenzyme: the protein part of an enzyme. • Cofactor: a nonprotein portion of an enzyme that is necessary for catalytic function; examples are metallic ions such as Zn 2+ and Mg 2+. • Coenzyme: a nonprotein organic molecule, frequently a B vitamin, that acts as a cofactor. • Substrate: the compound or compounds whose reaction an enzyme catalyzes • Active site: the specific portion of the enzyme to which a substrate binds during reaction.

Schematic of an active site • Schematic diagram of the active site of an enzyme and the participating components.

Terms in enzyme chem • Activation: any process that initiates or increases the activity of an enzyme. • Inhibition: any process that makes an active enzyme less active or inactive. • Competitive inhibitor: any substance that binds to the active site of an enzyme thereby preventing binding of substrate. • Noncompetitive inhibitor: any substance that binds to a portion of the enzyme other than the active site and thereby inhibits the activity of the enzyme.

Enzyme activity • Enzyme activity: a measure of how much a reaction rate is increased. • We examine how the rate of an enzymecatalyzed reaction is effected by: – enzyme concentration – substrate concentration – temperature – p. H

Enzyme activity • The effect of enzyme concentration on the rate of an enzymecatalyzed reaction. • Substrate concentration, temperature, and p. H are constant.

Enzyme activity • The effect of substrate concentration on the rate of an enzymecatalyzed reaction. • Enzyme concentration, temperature, and p. H are constant.

Enzyme activity • The effect of temperature on the rate of an enzyme-catalyzed reaction. • Substrate and enzyme concentrations and p. H are constant.

Enzyme activity • The effect of p. H on the rate of an enzymecatalyzed reaction. • Substrate and enzyme concentrations and temperature are constant.

Mechanism of enzyme action • Lock-and-key model of enzyme mechanism. – The enzyme is a rigid three-dimensional body. – The enzyme surface contains the active site.

Mechanism of enzyme action • Induced-fit model – The active site becomes modified to accommodate the substrate.

Mechanism of enzyme action • The mechanism of competitive inhibition. – the inhibitor fits into the active site, thereby preventing the substrate from entering.

Mechanism of enzyme action • Mechanism of noncompetitive inhibition. – the inhibitor binds to a site other than the active site, thereby changing the conformation of the active site. The substrate no longer fits.

Mechanism of enzyme action • Enzyme kinetics in the presence and absence of inhibitors.

Catalytic power of enzymes – Both the lock-and-key model and the induced-fit model emphasize the shape of the active site. – However, the chemistry of the active site is the most important. – Just five amino acids participate in the active sites in more than 65% of the enzymes studies to date. – These five are His > Cys > Asp > Arg > Glu. – Four these amino acids have either acidic or basic side chains; the fifth has a sulfhydryl group (-SH).

Catalytic power of enzymes • Enzymes provide an alternative pathway for reaction, one with a significantly lower activation energy and, therefore, a faster rate.

Enzyme regulation • Feedback control: an enzyme-regulation process where the product of a series of enzyme-catalyzed reactions inhibits an earlier reaction in the sequence. • The inhibition may be • competitive • noncompetitive.

Regulation of enzyme • Proenzyme (zymogen): an inactive form of an enzyme that must have part of its polypeptide chain hydrolyzed and removed before it becomes active. – An example is trypsin, a digestive enzyme. – It is synthesized and stored as trypsinogen, which has no enzyme activity. – It becomes active only after a six-amino acid fragment is hydrolyzed and removed from the N-terminal end of its chain. – Removal of this small fragment changes in not only the primary structure but also the tertiary structure, allowing the molecule to achieve its active form.

Regulation of enzyme • Allosterism: enzyme regulation based on an event occurring at a place other than the active site but that creates a change in the active site. – An enzyme regulated by this mechanism is called an allosteric enzyme. – Allosteric enzymes often have multiple polypeptide chains. – Negative modulation: inhibition of an allosteric enzyme. – Positive modulation: stimulation of an allosteric enzyme. – Regulator: a substance that binds to an allosteric enzyme.

The allosteric effect • The allosteric effect. Binding of the regulator to a site other than the active site changes the shape of the active site.

Regulation of enzyme • Protein modification: the process of affecting enzyme activity by covalently modifying it. • The best known examples of protein modification involve phosphorylation/dephosphorylation. • Example: pyruvate kinase (PK) is the active form of the enzyme; it is inactivated by phosphorylation to pyruvate kinase phosphate (PKP).

Regulation of enzyme • Isoenzyme: an enzyme that occurs in multiple forms; each catalyzes the same reaction. – Example: lactate dehydrogenase (LDH) catalyzes the oxidation of lactate to pyruvate. – The enzyme is a tetramer of H and M chains. – H 4 is present predominately in heart muscle. – M 4 is present predominantly in the liver and in skeletal muscle. – H 3 M, H 2 M 2, and HM 3 also exist. – H 4 is allosterically inhibited by high levels of pyruvate while M 4 is not. – H 4 in serum correlates with the severity of heart attack.

Enzymes in Medicine • Enzyme assays useful in medical diagnosis.