Hein * Best * Pattison * Arena Enzymes Chapter 30 Hein * Best * Pattison * Arena Version 1.0 Colleen Kelley Chemistry Department Pima Community College © John Wiley and Sons, Inc.

Chapter Outline 30.1 Molecular Accelerators 30.2 Rates of Chemical Reactions 30.3 Enzyme Kinetics 30. 4 Industrial Strength Enzymes 30.5 Enzyme Active Site 30.6 Temperature and pH Effects on Enzyme Catalysis 30.7 Enzyme Regulation

Molecular Accelerators

Enzymes are the catalysts of biochemical reactions. Enzymes catalyze nearly all the myriad reactions that occur in living cells. Uncatalyzed reactions that require hours of boiling in the presence of a strong acid or strong base can occur in a fraction of a second in the presence of the proper enzyme. The catalytic functions of enzymes are directly dependent on their three-dimensional structures.

Figure 30.1 A typical reaction-energy profile: The lower activation energy in the cell is due to the catalytic effect of enzymes.

Each organism contains thousands of enzymes: Some are simple proteins consisting of only amino acid units. Others are conjugated and consist of a protein part, or apoenzyme, and a nonprotein part, or coenzyme.

A functioning enzyme that consists of both the protein and nonprotein parts is called a holoenzyme. Apoenzyme + Coenzyme = Holoenzyme Often the coenzyme is derived from a vitamin, and one coenzyme may be associated with different enzymes.

For some enzymes, an inorganic component such as a metal ion (e. g For some enzymes, an inorganic component such as a metal ion (e.g. Ca2+, Mg2+, or Zn2+) is required. This inorganic component is an activator. The activator is analogous to a coenzyme.

Another remarkable property of enzymes is their specificity of reaction – that is, a certain enzyme catalyzes the reaction of a specific type of substance. e.g. lactase

The substance acted on by an enzyme is called the substrate. e.g. Sucrose is the substrate of the enzyme sucrase.

Classes of Enzymes Oxidoreductases: Enzymes that catalyze the oxidation-reduction between two substrates. Transferases: Enzymes that catalyze the transfer of a functional group between two substrates. Hydrolases: Enzymes that catalyze the hydrolysis of esters, carbohydrates, and proteins (polypeptides).

Classes of Enzymes 4. Lyases: Enzymes that catalyze the removal of groups from substrates by mechanisms other than hydrolysis. 5. Isomerases: Enzymes that catalyze the interconversion of stereoisomers and structural isomers. 6. Ligases: Enzymes that catalyze the linking of two compounds by breaking a phosphate anhydride bond in ATP.

Rates of Chemical Reactions

Figure 30.2 The change in product concentration [B] as a function of time. The reaction rate is determined by measuring the slope of this line.

Figure 30.3 An energy profile for the reaction between water and carbon dioxide.

There are three common ways to increase a reaction rate: Increasing the reactant concentration Increasing the reaction temperature Adding a catalyst

Enzyme Kinetics

Figure 30.4 A Michaelis-Menten plot showing the rate of enzyme-catalyzed reaction as a function of substrate concentration. The lower left portion of the graph marks the approximate area where an enzyme responds best to concentration changes.

Figure 30.5 Michaelis-Menten plots for two glucose metabolic enzymes.

Turnover Number An enzyme’s catalytic speed is also matched to an organism’s metabolic needs. This catalytic speed is commonly referred to as turnover number – the number of molecules an enzyme can react or “turn-over” in a given time span.

Industrial Strength Enzymes

Enzymes offer two major advantages to manufacturing processes and in commercial products: Enzymes cause very large increases in reaction rates even at room temperature. Enzymes are relatively specific and can be used to target selected reactants.

Proteases (proteolytic enzymes) break down proteins. Lipases digest lipids. Cellulases, amylases, lactases, and pectinases break down carbohydrates, cellulose, amylose, lactose, and pectin, respectively.

Enzyme Active Site

Catalysis takes place on a small portion of the enzyme structure called the enzyme active site. Often this is a crevice or pocket on the enzyme that represents only 1-5% of the total surface area.

Figure 30.6 A spacefilling model of the enzyme hexokinase (a) before and (b) after it binds to the substrate D-glucose. Note the two protein domains for this enzyme, which are colored differently.

Figure 30.7 Enzyme-substrate interaction illustrating both the lock-and-key hypothesis and the induced-fit model. The correct substrate (orange square-blue circle) fits the active site (lock-and-key hypothesis). This substrate also causes an enzyme conformation change that positions a catalytic group (*) to cleave the appropriate bond (induced-fit model).

Figure 30.8 Strain Hypothesis: The substrate is being forced toward the product shape by enzyme binding.

Temperature and pH Effects on Enzyme Catalysis

Essentially, any change that affects protein structure also affects an enzyme’s catalytic function. If an enzyme is denatured, its activity will be lost. Thus, strong acids and bases, organic solvents, mechanical action, and high temperature are examples of treatments that decrease an enzyme-catalyzed rate of reaction.

Figure 30.10 A plot of the temperature dependence of an enzyme-catalyzed reaction Figure 30.9 A plot of the enzyme-catalyzed rate as a function of pH.

The End