Chapter 6- Energy and Enzymes- Study Questions

Label the diagram using the following list: Active site Enzymes Substrates Products Enzyme-substrate complex

A = enzyme B = active site C = substrates D = Enzyme-substrate complex E = Products

What is a competitive inhibitor? Add a curve to the graph above to show what affect a competitive inhibitor would have. Make sure you consider how Vmax would be affected.

A competitive inhibitor binds to the enzyme active site and prevent substrate from binding. Vmax is not altered because we can still reach maximum velocity, but more substrate is needed to reach Vmax because it must compete with the inhibitor for the enzyme active site. The more substrate that is present, the more likely it will be that substrate will bind to the active site rather than inhibitor.

What is a non-competitive inhibitor? Add a curve to the graph above to show what affect a non-competitive inhibitor would have. Make sure you consider how Vmax would be affected.

A non-competitive inhibitor binds to an allosteric site on the enzyme (a site that is not the active site) and prevents the enzyme from functioning, therefore decreasing the number of available enzymes. A non-competitive inhibitor decreases the maximum velocity (Vmax) because there are fewer working enzymes, so less product can be made.

What affect do lower temperatures have on the rate of an enzyme catalyzed reaction and why? What affect do higher temperatures have on the rate of an enzyme catalyzed reaction and why?

At lower temperatures, molecules are moving slower, so there are fewer interactions between substrate and enzyme, resulting in a lower reaction rate. At higher temperatures, enzymes can become denatured (this means that the chemical bonds that hold the enzyme protein in its 3D shape get broken, which harms the shape and therefore the function of the enzyme), which decreases the reaction rate.

What type of macromolecule are enzymes? Although enzymes catalyze most cellular reactions, which other type of macromolecule is known to have catalytic activity?

Enzymes are proteins. Enzymes catalyze most of the chemical reactions inside cells, but some RNA molecules are also known to have catalytic activity, and those RNA molecules are sometimes referred to as ribozymes.

What is activation energy?

It is the initial input of energy that is needed to start a chemical reaction. Specifically, it is the energy required to reach the transition state, which is an intermediate state between reactants and products where the molecules are optimally positioned for old bonds to be broken and new bonds to be formed.

What is Km a measure of? What is the relationship between the Km value and the affinity (binding strength) of an enzyme for its substrate?

Km measures the affinity of an enzyme to a substrate (how well it will bind with the substrate). If Km is higher, then the enzyme has lower affinity for. Remember that Km is a determined by the substrate concentration needed to reach ½ Vmax. So more substrate concentration (and thereby higher Km) means your enzyme is LESS affinitive towards the substrate.

What are prosthetic groups, coenzymes, and cofactors?

Many enzymes require non-protein helper molecules to catalyze the chemical reaction. For example, the helper molecule may stabilize the transition state, or it may participate in the chemical reaction, e.g., NAD and FADH2 accept electro ns from a substrate as it is converted into product. A prosthetic group is a non-protein helper molecule, which is permanently attached to the enzyme. A coenzyme is an organic, non-protein helper molecule that is non-permanently attached to the enzyme A cofactor is an inorganic non-protein helper molecule that is not permanently attached to the enzyme.

What is metabolism? What is the difference between anabolic reactions and catabolic reactions?

Metabolism refers to the chemical reactions that take place in cells. Anabolic reactions build larger molecules from smaller building blocks and are endergonic reactions. Catabolic reactions break larger molecules down into smaller building blocks and are exergonic.

Does the following reaction diagram represent an exergonic or endergonic reaction? Which arrow correctly indicates the activation energy? Which arrow correctly indicates ΔG? What affect would an enzyme have on the activation energy?

The figure represents an endergonic reaction because the products have more energy than the reactants, so there has been a net input of energy. Line A represents the activation energy, which is the initial input of energy needed to get from the reactants to the transition state at the top of the hump. Line B represents ΔG, which is the Gibbs free energy value (i.e., difference in energy between the reactants and products). An enzyme would lower the activation energy.

Does the following reaction diagram represent an exergonic or endergonic reaction? Which arrow correctly indicates the activation energy? Which arrow correctly indicates ΔG? What effect would an enzyme have on the activation energy?

The figure shows an exergonic reaction because the products have less energy than the reactants, so there has been a net release of energy. Line A represents the activation energy. It is the energy needed to get from the reactants to the transition state at the top of the hump. Line B represents ΔG, which is the Gibbs free energy value (i.e., difference in energy between the reactants and products). An enzyme would lower the activation energy.

Look at the reaction below. Is it exergonic or endergonic? Does it occur spontaneously? Do the products or reactants have more energy? 6CO_2 + 6H2O → C6 H12 O6 + 6O2 ΔG = 685 kcal/mol

The reaction is endergonic, which means it does not occur spontaneously, and the products have more energy than the reactants because a net input of energy was supplied to make the products.

Look at the reaction below. Is it exergonic or endergonic? Does it occur spontaneously? Do the products or reactants have more energy? C6 H12 O6 + 6O2 → 6CO_2 + 6H2O ΔG = -686 kcal/mol

The reaction is exergonic, which means that it does occur spontaneously, and the products have less energy than the reactants because energy is released during the reaction.

In two or three sentences, describe how an enzyme catalyzes a chemical reaction referring to the induced fit model.

The substrate(s) binds to the enzyme active site, and the enzyme undergoes a change in conformation (shape) to fit the substrate more snugly. The confirmation change helps to position substrates optimally to stretch and strain existing chemical bonds to promote breaking of old bonds and formation of new bonds. This model, which describes the enzyme undergoing a conformational change after substrate binding is called the induced fit model.

What is Vmax? When do reactions reach Vmax? Indicate Vmax on the graph below. What affect does adding more substrate have on the rate of the reaction between points A and B and why? What affect does adding more substrate have on the rate of reaction between points and X and Y and why?

Vmax is the maximum velocity (speed) of an enzyme catalyzed reaction. Reactions reach Vmax when all of the enzyme active sites are saturated with substrate, so there are no more free enzymes. Adding more substrate between A and B increases the rate of reaction because more enzymes will be occupied with substrate so more enzymes are converting substrate to product. Adding more substrate between points X and Y will have no effect on the rate of reaction because the reaction is at maximum velocity (Vmax). Saturation has been reached where all enzymes are in use, so there are no free enzymes to bind to additional substrate.

Why do enzymes function optimally within a narrow pH range (i.e., why do changes in pH affect enzyme activity)?

pH affects charges. The function of an enzyme is determined by its 3D structure. It is held in its 3D structure by various chemical bonds, including ionic bonds between charged amino acids. If pH changes, the charge of a charged amino acid can change (refer to the example in the class video), which can subsequently affect ionic bonds, affecting the folded structure and function of the enzyme. Also, ionic bonds may be involved in the binding of substrate to the enzyme active site, so a change in pH may affect the ability of substrate to bind.