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“The central role of Enzymes as Biological Catalysts” National Library of Medicine, 2000 https://www.ncbi.nlm.nih.gov/books/NBK9921/#:~:text=The%20Catalytic%20Activity%20of%20Enzymes&text=First%2C%20they%20increase%20the%20rate,equilibrium%20between%20reactants%20and%20products.
A fundamental task of enzymes is to act as biological catalysts that increase the rate of all the chemical reactions within cells. Although RNAs (a type of nucleic acid) are capable of catalysing some reactions, most biological reactions are catalysed by proteins like enzymes. Without the presence of these enzymes, most biochemical reactions will take years to complete due to the mild conditions of temperature and pressure in our body and our normal surroundings.
Enzymes increase the rate of chemical reactions without themselves taking part in the reaction, remaining unchanged at the end of the reaction. Additionally, they increase the rates of reaction without altering the chemical equilibrium (the concentration of the reactant and the concentration of the products do not change with time, they don't display any further change in properties). The molecule acted upon by an enzyme is called the substrate [S] and is converted to a product (P) as the result of the reaction.
The equilibrium of the reaction is determined by the final energy states of the substrates and the products. For the reaction to proceed the substrate must first be converted to a higher energy state, called the transition state. The energy required to reach the transition state is called the activation energy. Enzymes act by reducing the activation energy, thereby increasing the rate of reaction.
The catalytic activity of enzymes involves binding their substrates to form an enzyme-substrate complex (ES). The substrate binds to a specific region of the enzyme, called the active site. Each enzyme is specific to only one substrate. Thus the active site of one enzyme is exactly complementary to one particular substrate. In other words, all different substrates won’t fix the same enzyme.
The binding of a substrate to the active site of an enzyme is a very specific interaction. Once a substrate is bound to the active site of an enzyme, multiple mechanisms can accelerate its conversion to the product of the reaction.
In addition to binding their substrates, the active sites of many enzymes bind other small molecules that participate in catalysis. In many cases metal ions (such as zinc or iron) are bound to enzymes and play central roles in the catalytic process. These molecules are called coenzymes because they work with enzymes to increase reaction rates. Unlike substrates, coenzymes are not permanently altered by the reactions, rather, they are recycled and can participate in multiple enzymatic reactions. Many coenzymes are closely related to vitamins, which contribute part or all of the structure of the coenzyme. Vitamins are not required by bacteria such as Escherichia coli. Still, they are necessary components of the diets of humans and other complex animals, which have lost the ability to synthesise vitamins themselves.
The activities of enzymes can also be regulated by their interactions with other proteins. For example, muscle cells respond to adrenaline (a hormone that prepares your body for stressful/dangerous situations) by breaking down glycogen (a complex form of glucose) into glucose, thereby providing a source of energy for increased muscular activity. The breakdown of glycogen is catalysed by the enzyme ‘glycogen phosphorylase’, which is activated by phosphorylation (the process in which a phosphate group is added to a molecule) in response to the binding of adrenaline to a receptor (specialised protein structures found in the cell membrane) on the surface of the muscle cell.
Summarised by Saisha Sikka
Work Cited:
“Adrenaline” Cleveland Clinic, May 15. 2022 https://my.clevelandclinic.org/health/body/23038-adrenaline
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