Enzyme

An enzyme is a biocatalyst, usually a protein that increases the rate of chemical reaction in living organisms. We rely heavily on enzymes, which are involved in nearly all physiological processes from digestion to DNA replication. They do this by reducing the amount of energy that is needed to catalyse a reaction, thus enabling biological processes to happen at temperatures compatible with life.

Related Terms to Enzymes:

Active Site – The region on an enzyme that is a binding site for the substrate. The special shape and chemical screening in the active site promotes binding of the substrate and catalytic reaction taking place making such enzymatic action specific.

Allosteric Site – The portion of an enzyme other than the active site, where molecules called allosteric regulators can attach. By binding to the allosteric and changing the shape of an enzyme, it can either activate or inhibit its function allowing for regulation of enzyme activity.

Apoenzyme – The form of the enzyme when not united with its cofactor or coenzyme; in this case, an inactive enzyme. The holoenzyme is the complete, active form of the enzyme and requires binding with these other components to be considered an active enzyme with an apoenzyme.

Cofactor – A non-protein chemical that is required for the activity of the protein. Cofactors are either metal ions (like zinc or magnesium) or organic molecules that facilitate the catalytic reaction through stabilization of reactions and/or assistance in substrate binding.

Coenzyme – An organic cofactor that attaches onto the enzyme and assists in performing its function. Coenzymes, in contrast with the other cofactors are mostly loosely bound and may transfer a chemical group or an electron from one enzyme to another.

Denaturation – The loss of the specific three-dimensional structure of an enzyme (or protein) due to heating, changes in pH, or exposure to chemicals. Denaturation is the process in which an enzyme loses its function due to disruption of its active site, which means that they cannot grab the substrates anymore.

Enzyme-Substrate Complex: A transient complex formed when an enzyme binds to its substrate(s) in the active site Stabilization of this catalytic state by the enzyme is key to lowering activation energy and allowing a reaction to proceed (this entire complex is known as the transition state).

Kinetics of Enzymes – Study of reaction rates in enzyme catalysis Enzyme kinetics studies how the rate and efficiency of enzymatic activity is influenced by factors including substrate concentration, temperature, pH, and inhibitors.

Holoenzyme— The total and active form of an enzyme, including the apoenzyme and its associated cofactors or coenzymes. Thus, the holoenzyme does catalyze reactions, because all needed components are present.

Inhibitor — A substance that decreases or stops the activity of an enzyme Enzymes can be inhibited by the binding of inhibitors to either the active site (competitive inhibition) or other sites on the enzyme (non-competitive or allosteric inhibition), preventing substrate from binding, causing a change in enzyme shape.

Isoenzyme-variants of the same enzyme which catalyze the same reaction yet have slightly different structures and characteristics. Isoenzymes can enable regulation of identical metabolic pathway at a tissue or condition-specific level in different regions of the same organism.

Lock-and-Key Model – a model of enzyme specificity in which the active site of an enzyme is complementary in shape to that of the substrate. The induced-fit model, which allows for flexibility in the active site, largely supplemented this rigid-body model.

Metalloenzyme: An enzyme that contains a metal ion cofactor, which is required for its catalytic activity. The interaction of a metal ion within the enzyme stabilizes charged intermediates or aids in electron transfer during the reaction.

Michaelis Constant (Km) — Amount of substrate concentration at which an enzyme reaches half its maximum velocity. Km value gives an idea about the affinity of enzyme towards it substrate, low km= high affinity.

Optimum pH – The ideal range of the pH in which an enzyme works in optimal activity. While enzymes have specific pH optima, which is related to their biological environment, alterations from this range will decrease activity or lead to denaturation.

Protease — enzyme that breaks down proteins by cutting peptide bonds. Proteases perform indispensable functions, including digestion and cellular maintenance as well as recycling of cellular proteins.

Substrate – The molecule that an enzyme works on. The substrate binds in the active site of the enzyme, undergoes a chemical reaction and converts to the product, with the unchanged enzyme coming out after it.

Temperature Optimum — the temperature range over which an enzyme operates at maximum efficiency. The reaction rates can be increased by raising the temperatures, but an extreme temperature can lead to denaturation of the enzyme which in turn will make it inactive.

Turnover Number — also known as k_cat, the turnover number is the number of substrate molecules that one enzyme molecule can convert into product per unit of time when the enzyme is fully saturated with substrate. Turnover number gives us an idea about how efficient the catalytic activity of an enzyme is.

Zymogen — An inactive precursor of an enzyme that requires a biochemical change (e.g. peptide bond cleavage) to become active For example, many enzymes that require activation control digestive activity and reduce the prospect of cellular damage (Loeffler & Schneider 2006), this is done by producing originally inactive enzymatic pre-cursors called zymogens or proenzymes.