So…I haven’t written a sole biology blog post in such a long time so I thought I would share some of the knowledge covered in class, more specifically on exciting enzymes.
Enzymes are biological catalysts made by living organisms which speed up chemical reactions. Each cell in our bodies is like a factory, constantly using up raw materials to turn them into useful products and also resulting in waste products. These reactions are usually slow if unaided by either heat or enzymes. This can be annoying in everyday life, for example, delayed respiration rates which rely heavily on enzymes. The food we eat in turn become the energy we use (vastly simplified).
So imagine if you ate an apple but it would take a week to release the energy trapped in it, so you can only move using the energy derived from the apple the following week, a weird analogy I know, but it illustrates my point on the importance of enzymes. On a chemical level, an enzyme consists of a folded polypeptide chain held in shape by weak hydrogen bonds. They are used to control many metabolic processes, because of this, most enzymes are globular proteins. This structure allows water solubility, enabling movement of the enzyme across the body to catalyse reactions. Catalysed processes belong to one of two types of pathways, anabolic and catabolic. Anabolic pathways involve the biosynthesis of complex molecules from simpler molecules, requiring energy to do so. Catabolic pathways involve the breakdown of complex molecules to simpler molecules, releasing energy in the process. As enzymes are vital to the functioning of organisms by controlling reactions, they are present all around the body and are even embedded in a membrane of phospholipids. Some may even form a multienzyme complex which ensures a series of chemical reactions happen in a certain order. As enzymes are not used up during reactions they are able to repeatedly catalyse reactions.
In enzymes, a section of the protein chain is exposed which is able to bind to substrates, this is the active site of the enzyme. The active site is flexible and dynamic and substrates show chemical attraction or affinity for it. Substrates, molecules which the enzyme works upon have a high affinity for the active site. Each enzyme is specific to its substrate which operates like a ‘lock and key’. The shape of the enzyme’s active site is complementary to the shape of the substrate. During catalysed reactions, the substrate is bound to the enzyme to form an enzyme-substrate complex. The two are brought even closer together by the induced fit. This is where the enzyme changes shape ever so slightly so that the substrate is tightly bound to it. The enzyme then proceeds to weaken the chemical bonds of the substrate. This reduces the activation energy, the energy required to break the bonds of the reactants to start a chemical reaction. When the energy supplied, usually in the form of heat, is greater or equal to the activation energy the reactants reach the transition state. It is a very unstable phase in which the reactants can move forward to become reactants or fall back to reactants. The end products of the reaction are released from the enzyme with low affinity for the enzyme. Lowering the activation energy increases the rate of reaction as less input energy is required to break the bonds in the reactant. However enzymes also increase the rate of reaction by ensuring the active site is bound to the substrate so that it is in the correct orientation for the reaction to occur. This increases the chance of collisions with other reactants and therefore more successful collisions which increase the rate of reaction.
Sometimes the rate of enzyme controlled reactions requires being regulated so that there is not an excess of end products which are produced. This is crucial in a metabolic reaction which is reversible so that they can be catalysed either way, forward or reverse. As enzymes are proteins which are produced by living organisms, they are affected by an array of different factors. Controlling the factors such as temperature, pH, signal molecules, substrate concentration and use of inhibitors enables regulation of enzyme activity and therefore the rate of reaction. Signal molecules can be hormones which act very slowly to bring about changes in the body. They control the activity of certain enzymes by activating them to trigger a series of events. Low substrate concentrations mean that there are too few substrate molecules to make uses of all the active sites of enzymes, resulting in a slower rate of reaction. On the other hand, a high substrate concentration leads to more active sites occupied and therefore higher reaction rates. However, there is a point in which increasing the substrate concentration will not increase the rate of reaction. When the maximum substrate concentration is reached, it is no longer a limiting factor in the reaction. Inhibitors decrease the rate of enzyme-controlled reactions.
There are two types of inhibitors, competitive and non-competitive which both prevent the substrate from binding with the enzyme. As the name suggests, competitive inhibitors compete with the substrate for the active site. It is able to do this because it has a similar shape to the substrate which also corresponds to the enzyme. As more competitive inhibitors are added, it occupies more and more active sites. It is the blocked sites which result in a lower reaction rate. Non-competitive inhibitors do not combine directly with the enzyme’s active site. Instead, it binds to a different allosteric, non-active site. This changes the enzyme’s tertiary structure and there for the shape of the active site. The now altered active site is unable to bind with the substrates as it is no longer complementary in shape to it, resulting in a lower reaction rate. Another type of regulatory molecule, an activator ensures enzymes adopt the active from which is able to catalyse reactions. Whereas non-competitive inhibitors make enzymes adopt their inactive from.
Examples of very effective non-competitive inhibitors would be heavy metals. These included arsenic, lead and mercury which are involved in heavy metal poisoning. The have a high affinity for sulfhydryl functioning groups (-SH) in enzymes. The metal ion is able to take the place of the hydrogen and bond with the sulphur ion which changes the tertiary structure, inhibiting the reaction. So stay away from the above, as it will cause a painful death.
Author – Jiangmin Hou
Jiangmin is a 5th year high school student currently studying five STEM subjects at Scottish Higher level-Mathematics, Physics, Biology, Computer Science and Chemistry. She is interested in pursuing a degree in Medicine after completion of Secondary Education.