All living organism depend on catalytic proteins called enzymes, these compound are responsible for metabolism and biochemical regulation of energy, there are millions of enzymes and hundreds of enzymes have been identified in the human body as well. All living system posses enzymes as vital components of the body, enzymes are found in all organisms. Life is based on the interaction of enzymes within the body of an organism, thus they are critical for life and necessary in all living systems and near living replicating systems like viruses. The identification to date of three thousand different enzymes in the human body by scientists is an indication of the myriad roles that enzymes play in the living body. Enzymes are degraded and renewed almost every second in the human body mostly at an unbelievable rate. They perform numerous chemical functions vital to the existence and survival of the body.
The performance as well as the strength and the total number of the enzymes in the human body directly influence the ability of the body to function, to repair itself when injured, and to resist diseases. Enzymes mediate all biochemical actions in the body of a living person and this is the main reason for the fact that an enzyme deficiency causes such a severe problem.
Life and the chemical processes that underlie it can be said to consist of a chain or complex series of chemical reactions that take place at the cellular level in the body. The general term “metabolism” is used to point to these life sustaining reactions. The process of chemical metabolism is made possible by the catalysis performed by enzymes – enzymes are thus, biological catalysts or mediators. In chemistry, a substance called a catalyst is an agent that mediates a chemical reaction, this enables the reaction of two molecules to proceed under different or less than optimal conditions than would normally be possible – such as low temperature. Thus enzymes catalyze reactions by aiding the speed of the reaction and improving the normal performance of the reactants.
All the chemical reactions that occur at the cellular level in the body will not happen without the presence of enzymes – there is no life without enzymes. The process of respiration and digestion, the process of growth and development, the process of coagulation or clotting of blood, of perception and sensory stimulus, and reproduction are all dependent on enzymatic catalysis – none of these processes can occur in the absence of specific enzymes. It is estimated that the human body contains millions of enzymes, these compounds are involved in the continual renewal and maintenance of life processes, in protection and in secretion. Thus in the absence of enzymes the life processed of a person, a plant, or an animal ceases.
The majority of people have unfortunately not heard of enzymes, as a result, most are unaware of the vital importance that enzymes play in regulating the various life processes in the human body and their primacy in the maintenance of health and vigor. The only reason for the availability of energy in the human body is due to the metabolic role that enzymes play at the cellular level. Amino acids are one type of substrate that some class of enzymes act on, these enzymes are responsible for the duplication, the synthesis, and the joining together of entire chains of amino acids to make proteins. Amino acids themselves are the building blocks or basic of all proteins in the body. In fact, almost all enzymes and the most important enzymes are also proteins; some enzymes are small and consist of short chains of amino acids, while others are larger. Enzymes differ in the order and number of amino acids as well as the type of amino acids that are contained in them.
The effectiveness of all catalysts is influenced by the immediate environment, as enzymes are all catalysts; the cellular environment plays a great part in their efficiency in the catalysis of any reaction within the body. The efficiency of any enzyme will thus be actively affected by an acidic or an alkaline environment, the ambient temperature also effects the rate of enzyme catalysis, as do the concentration of the substrate – which is simply the substance on which catalysis is being carried out, the nature of the coenzymes or cofactors also effects enzyme activity, as do inhibitors and poisons.
Enzymes have a great specificity, which means they are very specific on type of substrate they act on. Usually, every single enzyme in the body will catalyze and promote only one particular type of chemical reaction – this is often strictly so, thus they are very specific on what they act on. Different enzymes carry out different reactions in the body, thus some types of enzymes help in the degradation of large nutrient compounds in food into smaller molecules for digestion and ultimate absorption at the cellular level – these are proteins, carbohydrates, and lipids or fats. Some types of enzymes are solely involved in distinct functions, including the storage and release of biochemical energy. Other enzymes catalyze the cellular processes of respiration, other enzymes are involved in the process of reproduction, some are involved in vision and sensory reactions, etc.
Specific enzymes are involved in almost every activity that occurs at the cellular level within the body. These include cardiac muscle contraction responsible for the heart beat, the restoration and renewal or repair of tissues, as well as in the digestion and absorption of food in the stomach. In fact, all biochemical processes in the body are mediated by enzymes. The body runs on energy produced in metabolic reactions, there is no action without this energy and the biochemical energy cannot be utilized or produced without the presence of enzymes. Therefore, all bodily functions involve enzymes at most stages of their operation. Living cells exists due to a complicated series of chemical reactions which require a constant supply of energy and the involvement of enzyme complexes. The cells suffer illness and death, and lose their organization without the presence of energy. This is the principle reason the requirement for energy is paramount above all other requirements of the body.
The food that is consumed is broken down during digestion into its component parts – proteins, carbohydrates, and fats – cells utilize these biomolecules and oxidize them to gain energy during cellular respiration. Thus, cellular energy generation with the aid of oxygen is conducted in the presence of enzymes that act as catalyst for the reaction. Most protein products of digestion will undergo chemical conversion into amino acids long before they reach a cell, similarly all fats will be converted to fatty acids, and all carbohydrates are transformed into glucose – the principal sugar in the body. These biomolecules are in turn oxidized in the cells in the process called cellular respiration, this results in the production of large amounts of energy as well as carbon dioxide and other metabolites as waste. The most important element for cellular respiration is oxygen, it reacts within cells in chemically productive reactions with other nutrients mediated by the catalytic supervision of certain specific enzymes, these enzymes regulate the rate of the biochemical reactions and also direct the energy thus produced to useful ends.
The same compounds found in food when combined with oxygen and combusted in a fire outside the body will release huge amounts of energy as heat. The energy in the biomolecules is converted directly into unusable heat in this case. The body cannot use heat as an energy source however, and the physiological processes that take place in the body require energy stored and used in the form of chemical bonds and not as heat. Energy produced in the digestion of biomolecules during cellular respiration is stored in the form of high energy chemical bonds, as ATP. The chemical energy bound in these bonds is necessary to bring about mechanical movements in the muscles and for the performance of other bodily functions. In order for this energy to be formed, the biochemical reactions are always “coupled” with the systems that are responsible for these physiological functions. The bonding of this potential energy from the combustion of the different biomolecules is achieved via special energy transfer systems mediated by different cellular enzymes in each cell of the body.
Classification of enzymes
There are many classification schemes for enzymes, one example, is to classify them according to the substrate they act on, thus proteases act on proteins, lipases of lipids and amylases on carbohydrates. In the most common scheme, all enzymes are classified with respect nature of the reaction they catalyze. In this scheme, enzymes are divided into six main groups, each group possessing fundamentally different activities and catalyzing a unique reaction. The enzymes classified into the six groups according to the reactions induced are 1) oxidoreductases 2) hydrolases 3) transferases 4) lyases 5) isomerases, and 6) ligases. A brief discussion on these reactions follows below.
- are an important class of enzymes that have a pivotal role in metabolism inside the cell. As the name suggests, this class of enzyme is responsible for oxidation and reduction processes that are always coupled in the cell. There are many ways to define oxygenation or oxidation, one way would be to speak of the addition of one oxygen molecule or removing two hydrogen atoms from a molecule, the process of reduction is just the opposite and occurs concurrently. During the oxidation process, one substrate is oxidized and another is always reduced, thus these two processes are linked and simultaneous.
- this class of enzymes are involved in the catalysis of chemical reactions in which the chemical bonds are broken off through the addition of a molecule of water. More properly, it can be understood as the addition of hydrogen [H] to a fragment of the molecule while a hydroxide [OH] ion is fixed to the other molecule fragment. This important class of enzymes includes the familiar proteolytic protein digesting enzymes, the lipolytic fat digesting enzymes, and the amylolytic carbohydrate digesting enzymes.
- this class of enzymes is responsible for the catalysis of biochemical reactions during which the ions other than hydrogen are transferred between the reactants. A good example of this type of enzyme is the class known as the transamidases that transfer amide ions across reactants; many medical diagnostic techniques employ these enzymes.
- this class of enzymes is responsible for catalyzing reactions that result in the rearrangement of the molecular structures within the reactants. This action results in the chemical conversion of a molecule into another related molecule called an isomer of the original molecule – the process is called isomerization. The chemical conversion of a D-form of a sugar to an L-form of a sugar would be a good example of an isomerization reaction catalyzed by such enzymes.
- are enzymes that catalyze only isomerization reactions. These enzymes are responsible for the chemical interconversion of aldose to ketose sugars, as an example.
- are enzymes that actively catalyze the formation of a chemical bond between two unrelated molecules. They add molecular groups to a substrate.
Why we require enzymes
All the chemical reactions in the body necessary for life are mediated by enzymes, for example, the digestive system functions because of the presence of specific enzymes. Enzymes are needed at all stages of the digestive process and without these enzymes, the body is unable to adequately digest food. All food can be classed into three main constituents, proteins, fats, or carbohydrates – these three types of molecules make up all the food consumed. During the process of digestion, these high molecular weight compounds of protein, fats and carbohydrates is degraded down into basic nutrient materials that can be used by the human body, the three digestive enzyme groups, namely the proteolytic, the lipolytic, and the amylolytic work in tandem to break down food into its basic constituent for easy assimilation by the cells.
The protein component of food is digested by different proteolytic enzymes or proteases – these degrade proteins into the basic amino acids. All proteins are composed of twenty common amino acids – these twenty amino acids can be said to be the alphabet or code in which protein structure is written. While there are biochemical pathways in the body for the manufacture of most of these amino acids, nine of them called the essential amino acids cannot be synthesized by the body or are produced in insignificant amounts – they are thus classed as being essential in the diet. As mentioned earlier about the specific nature of enzymes, each individual protease will only mediate the reaction for a specific amino acid. As an example, the digestive protease called trypsin, will splits only the amino acids lysine and arginine along a protein molecule, while the enzyme chymotrypsin will split protein molecules only where phenylalanine, tyrosine, and tryptophan are found along the protein.
Lipases or general lipolytic enzymes, aid in the biochemical decomposition of fats or lipids into constituents fatty acids. The broad term, lipid indicates a family of molecular compounds that includes the triglycerides – all fats and oils, as well as the phospholipids – for example, lecithin, as well as the sterols such as cholesterol.
Carbohydrates are digested by the amylolytic enzymes, or the amylases secreted by various glands in the body. Carbohydrates enter the body from several food sources consumed in the normal human diet – in fact, all plant foods are mainly carbohydrates, including rice, cereals, vegetables, and wheat, etc. The basic carbohydrates found in food include the simple sugar sucrose – cane sugar – which is a disaccharide, common in many plant foods; the milk sugar lactose, the disaccharide in milk, the fruit sugar or fructose, which is a monosaccharide found in fruits and vegetables. The most abundant carbohydrates are the
starches, which are large polysaccharides found distributed in all plant foods, being especially rich in grains and cereals. Aside from these primary sources of carbohydrates, the average diet of a person, will also include some alcohol, as well as amylose and glycogen, as well as lactic acid, pyruvic acid, the pectins, and dextrins, etc. The normal human diet also contains very large amounts of the plant based carbohydrate known as cellulose. The large amount of cellulose ingested as plant matter is never digested and is thrown out of the body unused due to the fact that the human digestive tract cannot produce cellulase, an enzyme that breaks down cellulose into simpler sugars – thus cellulose is more important as a fiber in the process of digestion, and is not an energy source in humans. Cud chewing animals like the cow and deer, along with other ruminants make use of cellulose as a main source of carbohydrates by digesting it with the help of intestinal bacteria.
Enzymes contained in gastric juices
The gastric secretions of the stomach contain a lot of different enzymes, one of them is the protein digesting enzyme known as pepsin or gastric protease. Digestive enzymes in the gastric cavity are initially secreted in the form of inactive precursors, for example, the peptic and mucous cells in the stomach secrete many different types of pepsinogen, the inactive precursor enzyme with no digestive activity. The inactive pepsinogen is converted into the active form pepsin as soon as it comes into contact with the hydrochloric acid in the stomach; this chemically converted pepsin now begins to act on the proteins in the food – converting them into simpler amino acids. Evolution has ensured a safety feature in digestion by the secretion of this inactivated form of the enzyme, the fact is that if the pepsin was secreted in the active form all the time, it would quickly digest the proteins present in the cells secreting it in addition to digesting the walls of the stomach – by secreting the enzyme in the precursor form, such a danger is avoided. The proteolytic enzyme pepsin functions best at an optimal pH of 1.8 to 3.5 in the stomach – this pH range is brought about the release of HCI into the lumen of the stomach. The enzyme pepsin functions only inside an acidic medium, and is fully inactivated within a short period of time displaying very little proteolytic activity when the pH crosses 5.4, it thus becomes inactive when it passes into the alkaline medium found in the small intestine. Thus the acidic medium maintained by secreted hydrochloric acid in the stomach is necessary for the functioning of pepsin and is ultimately important for protein digestion in the stomach. Protein digestion continues in the small intestine through the use of enzymes that function in an alkaline medium. Therefore, the proteins consumed in food are sometimes not digested fully when the secretions of hydrochloric acid in the stomach is deficient, in such cases, the pepsinogen that is secreted is not fully converted into pepsin as the proper acidic level of gastric juice is not maintained.
The stomach or gastric milieu is acidic due to the hydrochloric acid that is secreted from the parietal cells lining the walls of the stomach. The acidic environment created by the presence of hydrochloric acid performs the initial digestion of food arriving from the esophagus, the stomach acid denatures proteins to simpler peptides, and it further permits the chemical conversion of the inactive pepsinogen into the active enzyme pepsin. The acidic nature of the gastric secretion also ensures that some important nutrients and minerals are readily absorbable, these include the essential nutrients calcium and iron, and last but not least, the acidity eliminates all bacteria and other pathogens. The acidic environment in the stomach also stimulates the production of hormones, while boosting the absorption of essential minerals and trace elements into the blood – some of these essential nutrients serve as important co-factors in enzyme catalysis.
Fat and lipids in the food is digested by the gastric lipase enzyme. The simple lipids and fatty acids released as a result of fat digestion reduce the effects of allergic conditions and resist the infectious viruses. Another important function of lipids and fats in the body is as a shock absorbing layer in the skin that cushions the body, softening traumatic blows and reducing bruises. The digestion of triglycerides is also initiated by gastric lipases.
Carbohydrates which have been subjected to digestion in the mouth itself, via the action of salivary amylase are further digested by the gastric amylase enzymes secreted in the stomach. Complex carbohydrates like starch are broken down into simpler sugars as a result. The digestion of carbohydrates continues in the small intestine.
Milk is digested by an important enzyme known as rennin. The catalytic action of the rennin enzyme liberates essential minerals from milk – including iron, calcium, phosphorus, and potassium. These important minerals are vital to many biochemical processes in the human body, helping in strengthening the nervous system, maintaining and strengthening teeth and bones, as well as aiding in the stabilization of the water balance, etc. Ingested milk is first coagulated by the rennin enzyme; this chemically converts the main milk protein casein, into a chemical form that can be utilized by the human body.
The digestive actions of the various enzymes in the stomach may well affect the lining of the gastrointestinal tract; this is prevented by the mucus that coats the stomach. The mucus resists the digestive gastric enzymes and prevents the enzymes from digesting the walls of the stomach.
The absorption of digested food is aided by a substance called the “intrinsic factor” that chemically binds with the vitamin B12 and is required for intestinal absorption to occur.
The rate of secretion of various gastric juices is also under the regulatory control of other classes of enzymes, these respond to certain stimuli.
The amount of gastric juice secreted by the stomach on a daily basis to complete break down and digest ingested food, various between one and two liters. The stomach has many distinct functions, which include the following given below.
The stomach not only begins the process of digestion but also functions as a reservoir for the food, it stores the food for some period of time and is involved in the transport of this food to the duodenum, which is the name for the first part of the small intestine – where the process of digestion and absorption of nutrients continues. The stomach has a strong churning action and thoroughly mixes the food with the various gastric secretions in an acidic medium, this action results in the formation of a semi fluid mixture known as chyme – which is what is passed to the duodenum. The stomach stores food and slows down the rate of entry of food into the small intestine to a rate that is ideal for proper digestion and absorption to occur – most of the nutrients are absorbed in the small intestine.