Concept 8.4: Enzymes speed up metabolic reactions by lowering energy barriers

Flashcards from Concept 8.4 of Pearson's Campbell Biology, Twelfth Edition.

Spontaneous reaction

A reaction that does not need added energy but can be slow enough to be imperceptible

• Hydrolyzing sucrose to glucose and fructose with water can take years without a catalyst

Catalyst

A chemical agent that speeds up a reaction without being consumed by the reaction

Enzyme

A macromolecule (typically protein) that acts as a catalyst to speed up a specific reaction

• These lower the activation energy barrier enough for the reaction to occur at moderate temperatures while being reusable

• Names typically end in -ase for a specific reaction and substrate

Chemical bonds

These break and form in a chemical reaction

• Breakage requires excessive contortion before absorbing energy from its surroundings

Activation energy (E_A)

The initial energy needed to break the bonds of the reactants, often seen as heat from the surroundings

• Instability occurs when enough energy is absorbed to start the reaction

• Provides a barrier that determines the rate of spontaneous reactions

• Only changes speed and not overall effect of the reaction

Exergonic reaction

A reaction that releases more energy than was initially invested through the formation of new bonds

• These new bonds increase stability

Catalysis

The process by which a catalyst selectively speeds up a reaction without itself being consumed

• Avoids the excessive use of heat to speed up reactions which can cause denaturation

Substrate

The reactant that an enzyme acts on through catalytic activity for a conversion to products

• Typically held in the enzyme’s active site by weak bonds, such as hydrogen bonds

• Can be reoriented, stretched, or placed in a microenvironment to favor the reaction

• Rates of reaction can be increased with higher concentrations until complete enzyme saturation

Active site

The region on the enzyme that binds to the substrate, fitting the specific shape of the substrate

Induced fit

The slight change in shape of the enzyme like a handshake that results from chemical interactions on the substrate and active site

• This ultimately enhances catalysis of the reaction

Optimal conditions

Conditions that are most conducive to an enzyme’s function, mainly defined within a range of temperature, pH, and chemicals that influence the enzyme

Optimal temperature

The temperature at which an enzyme catalyzes its reaction at the maximum possible rate, increasing with increasing temperature until it is met and begins to drop and denature

• Typically defined by the environment in which it functions — human enzymes adapt to human body temperature, or around 37 degrees C

Optimal pH

The pH at which an enzyme is typically active, usually dependent on the environment at which it functions

• Pepsin in the stomach has an optimal pH of 2

Cofactors

Nonprotein helpers that bind to the enzyme permanently or reversibly with the substrate

• Inorganic includes metal atoms such as zinc, iron, and copper in ionic form

• Organics are called coenzymes, found in vitamins or made from their raw materials

Coenzymes

Organic cofactors that include vitamins or their raw materials as helpers to enzymes

Enzyme inhibitor

A chemical that inibits the action of specific enzymes

• Inhibition with covalent bonds to the enzyme are usually irreversible

• Weak interactions can be reversible and are used in most cases

Competitive inhibitors

Inhibitors that closely resemble the substrate of an enzyme and can bind to the enzyme’s active site, thus reducing enzyme productivity due to blockage

Noncompetitive inhibitors

Inhibitors that bind to another part of the enzyme away from the active site

• These cause the enzyme to change shape and makes the active site less effecive at catalyzation

Genes

The encoding method for enzymes; mutations in these can lead to a positive or negative change in the enzyme’s amino acid composition and thus new activity or substrate specificity