The biological catalysts which accelerate the rate of chemical reactions in living things are a type of protein called enzymes. Enzymes control the manufacture of complex substances, such as skin, blood, as well as the breaking down of chemicals to provide energy.
They allow the reaction to proceed by providing a pathway with a lower activation energy. A biological reaction often has a large number of stages, each one controlled by its own enzyme.
Compared to inorganic catalysts, enzymes:
• Produce much faster reaction rates.
•Operate under much milder conditions.
•Are more sensitive.
•Are very selective.
•The catalytic activity of an enzyme depends on its tertiary structure. A slight change in its three-dimensional shape can render an enzyme inoperative. The active site of an enzyme is usually a flexible hollow or cavity within the molecule. A reactant molecule, known as the substrate, is maneuvered into this site and it is there at the surface of the enzyme that a reaction takes place.
•Some enzymes have small, non-protein parts called cofactors, such as vitamins or metal atoms, associated with the active site.
Bonding between the enzyme and substrate
The type of bonds that can form between enzyme and substrate:
•ion-dipole interactions (between Zn2+ and O C)
•Hydrogen bonds (between N-H and O C)
•Ionic interactions (between O2- and NH3+)
•Dispersion forces and dipole-dipole interactions may also be involved as an enzyme-substrate complex form.
Enzymes and their substrates are chiral, so one combination will fit like a hand in a glove while the other isomer will not fit.
Denaturation of enzymes
•Slight changes in conditions can disrupt the attractions between parts of the chain, causing the chain to unfold and rendering the protein ineffective. Any change that destroys the biological activity of a protein is called denaturation, this could occur if there is a change in pH, increased temperature, or addition of various chemicals.
•The unfolded chains tend to form randomly looped structures which come into close contact, forming clumps of protein molecules. This is called coagulation.
Lock and key model By Gal m - Own work, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=2924145
Induced fit model By Created by TimVickers, vectorized by Fvasconcellos (Provided by TimVickers) [Public domain], via Wikimedia Commons
Steps of enzyme action.
The three steps of action of an enzyme:
1.The reactant (substrate) enters the active site.
2.Bonds formed between the enzyme and substrate weaken bonds within the substrate, lowering the reaction’s activation energy.
3.The substrate breaks or rearranges into new products and these products are released.
Enzyme and substrate bind together because:
•they are non-polar and so dispersion forces are significant
•the substrate is held in place by attraction between positive and negative charges, eg dipole or hydrogen bonding.
The selectivity of enzymes arises because the shape and functional groups in the active site of the enzyme allow it to bind only with certain substrates. The enzyme and substrate are often described as fitting together ‘like a lock and key’.
Enzyme action and temperature By Gal m - Own work, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=2924145
Denaturation diagram By Scurran15 - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=40659358