Definition of an Enzyme
An enzyme is an organic macromolecule produced by living cells that acts as a catalyst for a biochemical reaction. Most enzymes are composed of protein. Enzymes change the rate of chemical reactions without needing an external energy source and without being changed themselves. One enzyme may be capable of catalyzing a reaction numerous times. Different enzymes act on certain substances and produce specific reactions. Examples of enzymes are digestive, glycolytic, activating, and inhibitory.
Features of Enzymes
- Enzymes do not make anything happen that could not happen on its own. This underscores the catalytic nature of enzymes. They do not make reactions occur that are impossible without them, but they greatly speed up these reactions, perhaps by a million times, so that the rate at which reactions reach equilibrium is greatly enhanced.
- Enzymes are not permanently altered or used up in the reactions they catalyze. An enzyme can be used over and over again.
- One enzyme can work both forward and in reverse. In other words, the same enzyme that mediates a reaction forward can also mediate the reverse of this reaction.
- All enzymes are highly selective and will only work with certain substrates, which are molecules that a particular enzyme can chemically recognize and bind to in order to modify the substrate in a specific way. See “Enzyme Specificity” below.
An enzyme binds to a substrate by means of an active site on its surface. An active site is like a crevice that the substrate can fit into. However, according to the induced-fit model, the substrate does not quite fit perfectly so that its contact with the active site allows it to bind but also strains some of the bonds in the substrate. This strain makes it easier for bonds to break and for new bonds to be made, so that new products are formed. The site may also have charged or polar groups which helps to prime the substrate for conversion to an active form.
Enzymes help substrates reach an activated transition state by greatly lowering the required activation energy (Ea). This greatly increases the amount of molecules which complete a reaction.
For example, the binding of a substrate with an enzyme helps to orient it so that when it collides with a specific compound, it collides in a specifically oriented direction that causes mutually attractive chemical groups to make contact, facilitating reaction between the compounds. Without the enzyme, the compounds would collide from a more or less random direction, so that the meeting of mutually attractive sites would be rare and reactions would proceed very, very slowly. There are five basic classes of reactions catalyzed by enzymes:
- Functional-group transfer: a functional group is removed from one molecule and accepted by another.
- Electron transfer: One or more electrons are stripped from a molecule and donated to another.
- Rearrangement: The internal bonds of an compound are rearranged to convert one type of compound to another.
- Condensation: Two molecules are combined to form a larger molecule with covalent bonds.
- Cleavage: A molecule is split into two smaller ones.
Enzymes exhibit different degrees or specificity, or selectivity. The level of this specificity is determined by the active site, and whether is can accommodate only one compound or a family of compounds that are closely related. There are four basic types of enzyme specificity:
- Absolute specificity: The enzyme will work with only one particular substrate. This is uncommon, but an example is urease, catalyzes the hydrolysis of urea.
- Stereochemical specificity: The enzyme can distinguish between stereo-isomers. Amino-acids are chiral, meaning that they can exist in two different configurations that are mirror-images of one another, called L isomers and D isomers. An enzyme exhibiting stereo-isomer, like L amino acid oxidase, will only catalyze L-amino acids.
- Group specificity: The enzyme will work with structurally similar compounds that have the same functional groups. An example is carboxypeptidase, which cleaves amino acids from the carboxyl end of a peptide chain.
- Linkage specificity: The enzyme catalyzes a particular type of bond, regardless of the actual structure in the vicinity of the bond. An example is the ability of phosphatase to hydrolyze phosphate-ester bonds in all types of phosphate esters. This type of enzyme specificity is the most general of all listed.