An enzyme (E) combines with its substrate (S) to form an intermediate enzyme-substrate complex (ES) , which then decomposes into reaction products(P) and the free enzyme, as seen in the equation below.
E S ————— > ES ———- > E + P
enzyme-substrate complex
An enzyme causes the substrate upon which it is acting to be much more reactive than when it is free. One postulate accounting for this is that the enzyme holds the substrate in a position which strains and weakens the substrate’s molecular bonds. This weakening of the bonds within the substrate makes them easier to cleave and results in a general lowering of the energy of activation of the reaction. This postulate is extremely simplistic – the actual forces at work are much more numerous and complex.
When the substrate binds to the enzyme, it combines with only a relatively small part of the enzyme molecule the active site. Information about the active site, such as its location and the nature and sequence of amino acids in it, provides an indication of the mechanism of binding and catalysis. The binding of the substrate to the enzyme’s active, site depends on many forces: hydrogen bonding, the interaction of hydrophobic (water-repelling) groups, and the electrostatic interaction between charged groups on the amino acids. Many active sites also contain metal ions which aid in binding the substrate or expediting the catalytic reaction by withdrawing or stabilizing electrons. For example, the enzyme carboxypeptidase, which hydrolyzes polypeptide bonds of proteins in food, contains a zinc atom in its active site. The electrophilic (electron-attracting) zinc atom coordinates electrons from the carbonyl of the peptide bond, weakening the bond for attack by a specific amino acid of the enzyme at the active site. Such a mechanism, however, is beyond the scope of elementary biology and one would require a good course in biochemistry to understand fully.
Some enzymes, the regulatory or allosteric enzymes, have two binding sites: an active site and a regulatory site. Regulatory enzymes are a key controlling factor in metabolic pathways. If the end product of a pathway is in excess, it inhibits the action of the regulatory enzyme by binding to its regulatory site. The end product shuts off the catalytic activity of the active site by altering the arrangement of the enzyme’s polypeptide chains, thus deforming and inactivating the enzyme (see diagram below).
Fig. Schematic diagram showing binding at the active site (a)
and regulatory site (b) of an enzyme. Note the change in
enzyme conformation accompanying binding of product to the
regulatory site.
Allosteric enzymes have, in addition to an active site, another stereo-specific site to which an effector or modulator molecule can bind. When it does, the shape of the active site is altered so that it can or cannot bind substrate (allosteric stimulation or inhibition respectively). In this way the enzyme can be part of a fine control circuit, requiring the presence or absence of a substrate—in addition to substrate presence— before enzyme activity proceeds. Some allosteric enzymes respond to two or more such modulators, permitting still liner control over timing of enzymes activity.
Tags: allosteric enzyme, enzyme
- Cofactors are metal ions.
- These ions commonly provide a needed change within an active site.
- Many enzymes use metal ions to change a non-functioning active site to a functioning one. In these enzymes, the attachment of a cofactor causes a shape change in the protein that allows it to combine with its substrate.
- The cofactors of other enzyme participate in a temporary bonds between the enzyme and its substrate when the enzyme-substrate (ES) complex is formed.
- Coenzymes are non-protein, organic molecules that participate in enzyme-catalytic reactions, often by transporting electrons in the form of hydrogen atoms, from one enzyme to another.
- Many vitamins function as coenzymes or are said to make coenzymes (e.g., Niacin and Riboflavin).
- One of the most important, coenzymes in the cell is the hydrogen acceptor Nicotine Adenine Dinucleotide (NAD+) is made from a B-Vitamin.
- Some enzyme (e.g.. Aspartase) bind just one very specific substrate molecule; others bind a variety of the same kind (o.g., all terminal peptide bonds in the case of exopeptidases).
- The difference arises from the degree of stereospecificity of the enzyme.
- Many need an attached prosthetic group or a diffusible coenzyme for activity. In such enzymes the protein component is termed the apoenzyme and the whole functional enzyme-cofactor complex is termed the holoenzyme.
Tags: apoenzyme, Coenzymes, Cofactors, Holoenzyme, Stereospecificity of the enzyme
- Any condition that affects the three-dimensional shape of an enzyme will effect its activity.
- Two such factors that affect enzyme activity are temperature and pH.
- The shape of a protein is determined by its hydrogen bonds. Hydrogen bonds are easily disrupted by temperature changes. For example, most higher mammals have enzymes that function best within a relatively narrow temperature range between 35°C and 40°C. Below 3SX. the bonds that determine protein shape are not flexible enough to permit the
shape change necessary for substrate to tit into a reactive site. - Above 40°C, the bonds are too weak to hold the protein in proper position and maintain its shape.
- When proper shape is lost, the enzyme is destroyed, this loss of shape is called denaturation.
- The enzyme action is like a ‘lock-and-key’ fit.
- Most enzymes also have a pH optimum, usually between 6 and 8. For example, when the pH is too low, the H+ ions combine with the R-groups of the enzyme’s amino adds, reducing their ability to bind with substrate.
- Acid environments can also denature enzymes. That is why some enzymes function at a low pH. For example, pepsin is the enzyme found in the stomach of mammals and has an optimal pH of approximately 2.
- Conversely trypsin is active in the more basic medium (pH 9) and found in small intestine. Overall the pH optimum of an enzyme reflects the pH of the body fluid in which the enzyme is found.
- All enzymes are proteins.
- An enzyme molecule may contain one or more polypeptide chains.
- The sequence of amino acids within the polypeptide chains is characteristic for each enzyme, and is believed to determine the unique three-dimensional conformation in which the chains are folded.
- This conformation, which is necessary for the activity of the enzyme, is stabilized by interactions of amino acids in different parts of the peptide chains with each other and with the surrounding medium. These interactions are relatively weak and may be
disrupted readily by high temperatures, acid or alkaline conditions or changes in the polarity of the medium. - Such changes lead to an unfolding of the peptide chains (denaturation) and a uncomitant loss of enzymatic activity, solubility and other properties, characteristic of the active enzyme.
- Because enzyme molecules are generally globular proteins, their shape and functions may be affected by pH changes in the aqueous environments.
- Denaturation by extremes of pH Is usually reversible, not so denaturation by heat.
Temperature increase will raise the rate of collision of enzyme and substrate molecules, thus increasing the rate of enzyme-substrate (ES) complex formation and raising the reaction rate. - This is opposed by increased enzyme denaturation as the optimum temperature for the reaction is exceeded.
Tags: Chemical structure of enzyme, enzyme, Function of enzyme
Comments