Enzymes: Biological Catalysts

The rate of a chemical reaction is independent of DG, but is determined by the size of the energy barrier. Catalysts speed reactions by lowering the energy barrier. Enzymes are biological catalysts, proteins that are highly specificfor their substrates. Substrates bind to the active site, wherecatalysis takes place, forming an enzyme–substrate complex.

At the active site, a substrate can be oriented correctly, chemically modified, or strained. As a result, the substrate readily forms its transition state, and the reaction proceeds.

Molecular Structure Determines Enzyme Function

The active site where substrate binds determines the specificity of an enzyme. Upon binding to substrate, some enzymes change shape, facilitating catalysis.

Some enzymes require cofactors to carry out catalysis. Prosthetic groups are permanently bound to the enzyme. Coenzymes are not usually bound to the enzyme. They can be considered substrates, as they are changed by the reaction and then released from the enzyme.

Substrate concentration affects the rate of an enzymecatalyzed reaction.


Metabolism and the Regulation of Enzymes

Metabolism is organized into pathways in which the product of one reaction is a reactant for the next reaction. Each reaction in the pathway is catalyzed by an enzyme. Enzyme activity is subject to regulation. Some inhibitors react irreversibly with enzymes and block their catalytic activity. Others react reversibly with enzymes, inhibiting their action only temporarily. A compound closely similar in structure to an enzyme’s normal substrate may competitively inhibit the action of the enzyme. Allosteric regulators bind to a site different from the activesite and stabilize the active or inactive form of an enzyme.Many such enzymes have multiple subunits.

For allosteric enzymes, plots of reaction rate versus substrate concentration are sigmoid, in contrast to plots of the same variables for nonallosteric enzymes. The end product of a metabolic pathway may inhibit the allosteric enzyme that catalyzes the commitment step of that pathway.

Enzymes are sensitive to their environment. Both pH and temperature affect enzyme activity.

Identifying Photosynthetic Reactants and Products

Photosynthesizing plants take in CO2, water, and light energy, producing O2 and carbohydrates. The overall reaction is

6 CO2 + 12 H2O + light →C6H12O6 + 6 O2 + 6 H2O

The oxygen atoms in the O2 produced by photosynthesis come from water, not from CO2.

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