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Perspectives in Human Physiology: Biochemistry, Enzymes and Metabolism

The Structure of Enzymes and Their Role in Metabolic Functions

Enzymes were discovered by the German chemist Eduard Buchner towards the end of the nineteenth century. The word enzyme was coined from the active ingredient of the yeast juice that promotes fermentation. Enzymes are organic catalysts that speed up chemical reactions in organisms. In the absence of enzymes, reactions in cells would be very slow (Roberts 1974, p.88). Each reaction is catalyzed by a specific enzyme. Enzymes are classified into two main groups: intracellular and extracellular. Intracellular enzymes are found inside cells. Their work is to control metabolism. On the other hand, extracellular enzymes are produced in the cells. They undertake functions elsewhere inside the organism.

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Enzymes are named by attaching the suffix –ase to the name of the substrate on which it acts. For example, Maltase acts on maltose, lipase on lipids, and urease on urea. Enzymes are grouped according to the type of reaction they catalyze. For instance, transferases are the enzymes involved in the transfer of atoms. On the other hand, dehydrogenases catalyze the removal of hydrogen atoms from a substrate, while oxidases catalyze the addition of oxygen to hydrogen atoms (Fox 2011). Others include phosphokinases, isomerases, dehydrases, and deaminases. Enzymes work very rapidly, and they are not destroyed by the reactions that they catalyze. Enzymes can also work in either direction, are inactivated by excessive heat, and are sensitive to PH (Grisham & Garrett 1999). Moreover, enzymes are specific in the reactions that they catalyze. Enzymes have a unique structure and they play an important role in metabolic functions.

Enzymes are protein in nature and as such, they exhibit the characteristics of proteins. Like proteins, enzymes are composed of amino acids. The structures formed are enzymes and they have areas of magnetic and electrical charge that are usually in motion (Fox 2011). These pockets are referred to as active sites and this is where a substrate binds.

Like proteins, enzymes have a particular shape. The active site of an enzyme molecule has a distinctive configuration into which only certain specific substrate molecules fit. This specificity is commonly referred to as the lock and key hypothesis which was proposed by Emil Fischer in the late 19th century (Roberts 1974, p. 91). In this hypothesis, the substrate is represented by a padlock and the enzyme is represented by the key. Just like a specific key opens a specific lock, in the same way, a specific substrate acts on a specific enzyme. When too much heat is applied to an enzyme, it becomes denatured, changes shape and prevents the substrates from fitting into the active site.

Energy in living things is generated in a series of chemical reactions. These reactions are referred to as metabolism or respiration. They may be constructive (anabolism) or destructive (catabolism). Energy exchange centers on the transfer of electrons. When electrons are removed from a substance, it is said to be oxidized and when they are added to atoms, these atoms are said to be reduced (Grisham & Garrett 1999). The cellular work necessary for life depends on such electron exchanges and enzymes are a big player in the rate of that process.

Enzymes play a big role in metabolic functions. In order for two molecules to form a compound, they have to collide, resulting in a reaction. The concentration of the substrate molecules, the temperature and the presence of a catalyst influence the collision (Roberts 1974, p. 87). Catalysts found in living things are called enzymes. During reactions, energy barriers may occur. They may be caused by repulsive forces acting between the interacting molecules. The function of an enzyme is to lower the energy barrier to a chemical reaction. It does not affect the course of the reaction or the products of the reaction or the total energy change involved.

Enzymes associate themselves reversibly with the materials on which they act (substrates). They form a unit known as the enzyme-substrate complex. During this interaction, the substrate is activated, subjected to molecular strain or altered. This leads to a smooth reaction path and the formation of end products in a rapid manner (Fox 2011). When the reaction is complete, the substrate remains unchanged and continues to activate other reactions. They are used over and over until inactivation occurs. Not only do enzymes speed up metabolic reactions, but they also control them. In higher vertebrates, digestive enzymes are produced in glands and also in the wall of the gut. Once absorption is achieved the liver performs the job of storing food materials. The digestive enzymes are known as carbohydrates or glycosidases, lipases and esterases and proteinases.

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Enzymes play a part in the production of energy from food. Carbohydrates in food are utilized in the cell as monosaccharides, proteins as amino acids and fats as fatty acids. The metabolism of these foods happens in many different ways. Through a series of reactions, monosaccharides, fats and amino acids are oxidized to acetic acid. Acetic acid combined with coenzyme A enters an enzymatic pool (Hoar 1973). Enzymes ensure that hydrogen is generated for the reduction of gaseous oxygen and the production of Adenosine Triphosphate (ATP) chemical energy. ATP is a nucleotide consisting of a complex organic molecule, adenosine, to which a chain of three phosphate groups is attached. The importance of ATP is that when the terminal phosphate group is broken off, a large amount of energy is released. This phosphate is broken off in the presence of an enzyme known as ATPase.

ATP is found in all cells and it is important to always have a ready supply of it. This means that a reverse of the reaction mentioned above should take place whenever energy needs to be stored. The energy required for this reaction comes from sugars. One of the processes in obtaining energy involves the removal of hydrogen atoms. Dehydrogenation is important in ATP synthesis and is achieved under the influence of a dehydrogenase enzyme.

In fat metabolism, enzymes are involved in different stages. Neutral fats form the main source of lipid energy. These are often esters of glycerol and are long-chain fatty acids with an even number of carbon atoms. Many cells contain lipolytic enzymes which hydrolyze these triglycerides into fatty acids and glycerol (Hoar 1973). This is the first step in their metabolism.

Enzymes play a major role in amino acid metabolism. Amino acids result from the digestion of proteins. During starvation, the body runs out of reserves of fats and this results in the breakdown of amino acids. Before being used as energy, amino acids must undergo deamination. This is the removal of nitrogen from amino acids by splitting off of amino groups. A family of enzymes known as amino acid oxidases catalyzes these oxidative deamination reactions (Hoar 1973, p. 229). There are other deamination methods that are not oxidative. Enzymes involved in non-oxidative reactions are associated with certain specific amino acids such as the hydroxyl amino acids (dehydrases), the sulfur-containing amino acids (desulfhydrases), histidine (histidase), and tryptophan (tryptophanase) among others (Hoar 1973, p. 229).

Reactions in the metabolic pathway have a limit at which they should be stopped. Enzyme inhibitors exist for purpose of slowing down or stopping enzyme-controlled reactions. Inhibitors may be competitive or non-competitive. Competitive inhibitors compete with substrates by binding in enzyme active sites. Non-competitive inhibitors bind to the enzyme permanently preventing the substrate from binding. The end-product in metabolism also acts as an inhibitor when it is in excess. It does this by combining with the enzyme responsible for its production in a process known as negative feedback (Roberts 1971, p. 92).

Current experimental techniques utilize enzymes. Enzymes are essential in the making of 3c assays. 3c refers to chromosome conformation capture. This is a technology that allows in vivo genomic organization to be explored. It is normally done to understand the folding of the genome at a big scale in mammals (Hagege et al. 2007). Enzymes are applied in one of the essential steps in the 3c and 4c processes. In the first step, chromatin segments are cross-linked by formaldehyde treatment. In the second step, DNA is digested by an appropriate restriction enzyme (Hagege et al 2007).

Enzymes are essential in life. They speed up chemical reactions in organisms. They catalyze the formation of ATP which is the energy that drives many biological processes. ATP is important in muscle contraction, nerve transmission, synthesis of many materials, the luminescence of fireflies and others. They assist in the breakdown of fats and carbohydrates in order to sustain cell function. They also catalyze the breakdown of amino acids in times of starvation. Although enzymes are easily denatured, their speed makes them more efficient than any inorganic catalysts.

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This is an experiment to investigate the effects of the variable, temperature on the enzyme catalase.

Hypothesis: Optimum temperature is essential for enzyme function

Materials and equipment

8 Beakers, 6 small and 2 large and a hot plate

750 mL of Hydrogen peroxide

3 pieces of 35g of animal liver

Water and ice

A thermometer


This will be carried out in three experiments. Before beginning the experiments ensure that there is no observable change in the hydrogen peroxide. This will be the control. In the first experiment, Drop one piece of liver into a beaker containing 250mL of hydrogen peroxide. After 3 minutes, observe any changes and record your findings. In the second experiment, heat some water on the hot plate using the big beaker. When the water reaches 55°C places one piece of liver in a small beaker and put it in the water bath for five minutes. Remove the liver and drop it into a 250mL beaker of hydrogen peroxide. After 3 minutes, observe any changes and record your findings. In the third experiment, place some ice in a large beaker and let it melt to reach 5°c. In a small beaker, place a piece of liver and put the beaker in the larger beaker. After five minutes, remove the small beaker. Drop the piece of liver into a beaker containing 250mL of hydrogen peroxide. After 3 minutes, observe any changes and record your findings.

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At the beginning of the experiment, there was no observable change in the hydrogen peroxide. In the first experiment, once the liver had been added, fizzing occurred rising to the top of the beaker and spilling over. In the second experiment when the liver was added, there was very little fizzing. In the third experiment when the liver was added, there was very little reaction and hardly any fizzing was observed. Observations made are shown below

Temperature Fizzing
25°c A lot of fizzing
55°c Little fizzing
5°c Little fizzing


The liver was used because it plays a central role in metabolic reactions. It contains a lot of enzymes one of which is catalase. “Catalase is one of the fastest-acting enzymes known” (Roberts 1974). Its presence in hydrogen peroxide breaks it down resulting in the release of oxygen. The fizzing is evidence of oxygen being released.

Results of this experiment showed that temperature affects enzymes. It is supported by other experiments that show the action of enzymes in different temperatures. Enzymes are proteins and excessive heat denatures them and renders them useless. “At about 50°C, the rate of an enzyme controlled reaction is high but above this temperature, the rate begins to reduce and at about 60°, it ceases completely” (Roberts 1974). This is why the temperature used in this experiment was 55°C. With low temperatures, the reaction rate is too slow and that is the reason why there was minimal fizzing. The rate of reaction reduces significantly as shown in the third experiment when the catalase was unable to break down hydrogen peroxide normally.


This experiment supports the hypothesis that optimum temperature is the best in enzyme reactions. However, the presence of organisms in extremely cold or hot environments does not strengthen the hypothesis stated. There is the need therefore to carry out experiments to determine if the mechanisms of the enzymes involved are heat resistant or if the organisms are able to regulate their own body temperatures.

Reference List

Bairoch A. (2000). The enzyme database in 2000. Nucleic Acids Res, 28(1), 304-5

Fox, M 2011, Explain the role of enzymes in chemical reactions and metabolic pathways. Web.

Grisham, C M., & Garrett, R. G 1999, Biochemistry, Saunders College, Philadelphia.

Hagege, H., Klous, P., Braem, C., Splinter, C., Dekker, J., Cathala, G., Laat, W., and

Forne, T. 2007, Quantitative analysis of chromosome capture assays (3c-qPCR). Nature Protocols.2(7):1722-1733. Web.

Hoar, WG 1973, General and comparative physiology, Prentice-Hall, London. Intermolecular bonding. Web.

Jaeger K. E., & Eggert, T. (2004). Enantioselective biocatalysis optimized by directed evolution. Curr Opin Biotechnol. 15 (4), 305–13.

Roberts, MBV1974, Biology: a functional approach, Thomas Nelson & Sons Ltd, London.

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