Vocabulary for Module 11 Enzymes

enzyme

A biological macromolecule that serves as a catalyst in biochemical reactions.

Describe what a catalyst does.

A catalyst is a substance that speeds up a chemical reaction without being consumed by the reaction. Catalysts achieve this by lowering the activation energy of the reaction. By definition, enzymes are macromolecule-based catalysts that speed up biological reactions. Enzymes are generally reusable but only bind to specific substrates.

active site

A location on an enzyme at which a substrate binds.

induced fit

A model of enzyme action in which substrate binding to the active site causes a temporary conformational change in the enzyme's shape, inducing further interactions between the substrate and the active site.

cofactor

A non-protein substance bound to and essential to the activity of a protein, particularly an enzyme.

feedback inhibition

A process in which excess products work to inhibit the biochemical pathways that make those products by binding to and inhibiting enzymes early in the pathway, thus shutting down further product formation.

substrate

A reactant in a biochemical process; usually something that is acted upon by an enzyme.

catalyst

A substance that speeds up a chemical reaction without itself being consumed in the reaction.

ribozyme

An RNA molecule that catalyzes a biochemical reaction.

isozyme

An enzyme that catalyzes the same reaction as another enzyme but at a different optimum temperature.

coenzyme

An organic molecule that acts as a cofactor for an enzyme.

non-competitive inhibition

Binding of a molecule to a site other than the active site in a way that inhibits the binding of a substrate to the enzyme's active site; also called allosteric inhibition.

allosteric inhibition

Binding of a molecule to a site other than the active site in a way that inhibits the binding of a substrate to the enzyme's active site; also called non-competitive inhibition.

allosteric activation

Binding of a molecule to a site other than the active site in a way that promotes the binding of a substrate to the enzyme's active site.

competitive inhibition

Binding of a non-substrate molecule to an enzyme's active site, thus inhibiting the binding of the substrate to the active site.

cooperativity

Describes the phenomenon in which one substrate molecule binds to an active site of a multi-subunit enzyme and thereby increases the affinity of other active sites for additional substrate molecules; a positive regulation of enzyme activity.

Explain how cells regulate enzyme activity.

Enzyme activity can be regulated by binding of inhibitors to the active site (competitive inhibition) or an allosteric site (non-competitive inhibition and allosteric inhibition/activation). In feedback inhibition, the end product of a multi-step enzymatic pathway inhibits enzymes early in the pathway. Covalent modification of enzymes, such as phosphorylation, is a way to rapidly switch an enzyme on or off. Finally, in cooperativity, the binding of substrate to one subunit of a multi-subunit enzyme complex promotes the binding of additional substrate molecules to the other subunits.

Explain the mechanisms by which enzymes lower the activation energy of reactions.

Enzymes lower activation energy through various means, including positioning substrates together in the proper orientation, applying torque on the substrates, providing the proper charge or pH microenvironment, and adding or removing functional groups on the substrates. In each of these methods, the enzyme functions to stabilize the transition state between the substrates as they are transformed into products.

How do enzymes increase the rates of chemical reactions?

Enzymes provide a lower-energy pathway to form the transition state.

Describe environmental factors that affect enzyme activity.

Most enzymes have a narrow range of temperature and pH at which they function optimally. At temperatures below the optimal temperature, there is insufficient kinetic energy for the substrate molecules and the enzyme to collide into each other. At temperatures above the optimal temperature, the bonds holding the enzyme together can be disrupted, resulting in denaturation, or permanent inactivation of the enzyme. A pH outside of the optimal range of the enzyme can result in excessive protonation or deprotonation of the enzyme, changing its conformation and its ability to bind substrates. In addition, extreme pH can denature enzymes.

enzyme-substrate complex

The combination of an enzyme with its substrate(s) bound to the active site.

activation energy

The energy required to initiate a chemical reaction; also defined as the energy required to overcome the energy barrier to a chemical reaction.

Why do enzymes work poorly at suboptimal pH levels?

The pH of the environment can alter the chemistry of the active site, affecting the non-covalent bonds that stabilize the enzyme-substrate complex. This answer is correct because the substrate might not be able to bind to the active site when the pH is lower than the optimum.

product

The result(s) of a chemical reaction.

conformation

The spatial arrangement or shape of a macromolecule, such as a protein or nucleic acid.

optimum temperature

The temperature at which an enzyme functions at maximum efficiency.

transition state

The unstable intermediate condition of a substrate in a biochemical reaction after which the reaction will proceed to forming products.

How can kinases regulate enzyme activity?

They can add phosphate groups to enzymes, typically activating them. Correct This answer is correct because kinases can add phosphate groups to enzymes, typically activating them.

Which of the following types of bonds are NOT commonly used to stabilize an enzyme-substrate complex?

covalent bonds Correct This answer is correct because the enzyme-substrate complex is typically stabilized by non-covalent bonds.

Which of the following is NOT a mechanism by which enzymes can lower activation energy barriers? -increasing the kinetic energy of the substrate. -providing a microenvironment that makes it easier for the transition state to be formed -bringing substrates in close proximity and appropriate orientation -putting stress on the structure of the substrate adding or removing units to the substrate

increasing the kinetic energy of the substrate Correct This answer is correct because enzymes cannot lower activation energy barriers by increasing the kinetic energy of the substrate