Enzymes: Definition, History, Nature, Structure, Classification, Functions: 

Enzyme Definition:

• Enzymes are defined as they are biocatalysts that enhance the rate of reaction by decreasing activation energy.

• Enzymes are protein-rich substances that catalyze or speed up biological reactions.

• The enzyme (E) is a protein that has catalytic properties in the reaction of converting a substrate (S) into a product (P).

What are Enzymes?

• Enzymes are biocatalyst that enhances the rate of reaction by decreasing activation energy.
• Biological enzymes refer to organic substances that have catalytic functions.
• Generally, enzymes are produced by living cells and have a certain catalytic effect. 
• The term enzyme is coined by Kuhne.
• The study of enzymes is known as enzymology.
• All enzymes are proteinous in nature except ribonuclease enzymes.
• Based on what types of body reactions are catalyzed by enzymes, they are divided into digestive And metabolic enzymes.
• Enzymes are classified into six groups oxidoreductases, transferases, hydrolases, lyases, isomerases, and ligases.
• Some common examples of enzymes are lysozyme, protease, pectinase, lipase, amylase, and cellulase.
• Biological enzymes can be used in the textile, petroleum, food, and pharmaceutical industries.
• Enzymes take part in almost all vital processes of the human body: they support the hematopoietic system, reduce thrombosis, normalize blood viscosity, improve microcirculation, as well as the supply of tissues with oxygen and nutrients, normalize lipid metabolism, etc.

History of Enzymes:

• In 1833, Payen and Persoz obtained a substance that hydrolyze starch into sugar and named it diastase, which is now called amylase.
• In 1836, T. Schwann extracted substances for digesting protein from gastric juice and solved the mystery of digestion.
• In 1878, Wilhelm Friedrich Kuhne coined the term “enzyme”.
• In 1913, American scientists Michaelis and Menten proposed the equation of enzyme catalysis based on the intermediate product theory.
• In 1926, for the first time, American scientist Sumner extracted the crystals of urease from the seeds of concanava. (First, crystalize enzyme – Urease)
• In the 1930s, scientists successively extracted protein crystals of various enzymes and pointed out that enzymes are a class of proteins with biological catalysis.
• In 1982, American scientists Cech and Altman found that a few RNAs also have catalytic activity and named it ribozyme. (Nonproteinous enzyme)
• In 1986, Schultz and Lerner successfully developed the antibody-enzyme (abzyme).

Why Enzymes are Important?

• The human body is a complex highly organized biological system, hundreds of biochemical reactions take place in it at the same time. 
• These processes are very complex and cannot take place on their own. 
• This is when the enzymes begin to work. 
• They help a particular reaction to proceed in the desired sequence, which is inherent in us by nature. 
• Enzymes speed up almost all chemical reactions that occur in cells. They have vital importance for humans, facilitate digestion, and speed up metabolism.
• Enzymes are present in the various cells of our body.
• In a healthy person, the enzymatic composition of the blood serum is relatively constant; with a disease, the level of enzymes increases significantly.

Chemical Nature of Enzymes: 

• Almost all enzymes are proteinous in nature with molecular weights ranging from 15,000 to several million Da. 
• Except for some Ribozyme and Ribonuclease enzymes.
• Keep in mind all enzymes are proteins, but not all proteins are enzymes. Like other proteins, enzymes are also made up of amino acids.
• According to the chemical structure enzymes are of two types simple enzymes and conjugate (Complex) enzymes.
• A simple enzyme consists of only a peptide chain consisting of amino acid residues. This protein is part of an enzyme known as apoenzyme.
• Complex enzymes consist of both the protein part and the non-protein parts.
• The non-protein part of an enzyme is a Prosthetic group, Coenzyme or Cofactor.
• The protein part (Apoenzyme) and nonprotein part (Cofactor) together form a holoenzyme. 
• Only the holoenzyme has catalytic activity. and if the two are separated, the enzyme activity disappears. 

Characteristics of Enzymes:

• Specificity: Specific enzymes can only catalyze one or a class of substrates, such as protease can only catalyze the hydrolysis of proteins into polypeptides, Cellulase is only used for cellulose breakdown, Urease is only used to hydrolyze urea to decompose it into carbon dioxide and ammonia

• Enzyme Diversity: There are many types of enzymes, about 4,000 kinds of enzymes have been discovered so far, and these enzymes are categorized into six major types.

• Enzyme Efficiency: The catalytic efficiency of enzymes is higher than that of inorganic or chemical catalysts, making the reaction rate faster;

• Enzyme Activity adjustability: Enzyme activity can regulate by inhibitor and activator regulation, feedback inhibition regulation, covalent modification regulation, allosteric regulation, etc.

• Volatility: Most enzymes are proteins, so they will be destroyed by high temperatures, strong acids, strong alkalis, etc.

• Reaction Speed: Enzymes can speed up the chemical reaction, but the enzyme cannot change the equilibrium point of the chemical reaction, that is to say while promoting the forward reaction, the enzyme also promotes the reverse reaction in the same proportion, so the function of the enzyme is to shorten the time required to reach the equilibrium. time, but the equilibrium constant remains unchanged;

• Activation Energy: Enzymes reduce the activation energy to speed up the chemical reaction rate.

Structure of enzymes: Active Site:

• Enzymes are macromolecules whose molecular weight is generally more than 10,000 and are composed of hundreds of amino acids. 
• The substrate of an enzyme is generally tiny, and only a tiny part of the enzyme molecule directly contacts the substrate and plays a catalytic role. 
• Even though some enzymes have larger substrates, only a small area is in contact with the enzyme. 
• Therefore, we call the site where the enzyme molecule binds to the substrate and catalyzes the reaction as the active site of the enzyme.
• The active site is composed of a few residues in the enzyme molecule. 
• The amino acids in the active site can be divided into substrate-binding sites and catalytic sites according to their functions.
• Generally, monomeric enzymes have only one active site, but some multifunctional enzymes have multiple active sites. 
• Methods such as ultraviolet, fluorescence, and circular dichroism spectroscopy can also be used in the study of active sites.

Classification of Enzymes: 

Enzymes are classified based on their location, their chemical composition, based on their works, and based on the nature of their reactions.

1. According to the Location of Enzymes: 

According to the location of enzymes are two types intracellular enzymes and extracellular enzymes.

a. Intracellular Enzymes: Humans and mammals contain at least 5,000 enzymes. They are either dissolved in the cytoplasm, combined with various membrane structures, or located at specific positions in other structures in the cell, and are activated only when needed. These enzymes are collectively referred to as intracellular enzymes; in addition, they also 

b. Extracellular Enzymes: Some enzymes are synthesized in the cell and then secreted outside the cell such enzymes are called extracellular enzymes

2. According to chemical composition: 

a. Simple Enzymes: Enzymes that only consist of protein part protein, and do not contain other substances are called simple enzymes, such as urease, protease, amylase, lipase, ribonuclease, etc.

b. Conjugated Enzymes: Conjugated enzymes consist of protein parts and some non-protein parts. The protein part is called an apoenzyme, and the nonprotein is called a cofactor. The complex formed is called holoenzyme, i.e. apoenzyme + cofactor = holoenzyme.

3. According to enzymes works in the body:

Enzymes are divided into three types: digestive enzymes, metabolic enzymes, and food enzymes

a. Digestive enzyme: Digestive enzymes are enzymes that help break down the food you eat so that it can be absorbed. Major enzymes include proteases (proteolytic enzymes) that degrade proteins into amino acids, amylases (carbohydrate-degrading enzymes) that degrade carbohydrates into glucose, and lipases (lipolytic enzymes) that decompose fats into fatty acids. It is further subdivided by the organs it consumes and the nutrients it decomposes.

b. Metabolic enzyme: Metabolic enzymes actually put the nutrients absorbed into the body to work. Various metabolic enzymes are at work in all aspects of human life activities, such as breathing, exercising, healing injuries, cell division, and skin metabolism.

c. Food enzyme: Food enzymes are enzymes found in foods. It aids in digestive enzymes and aids in better digestion. Food enzymes, which are abundant in fresh foods such as raw vegetables and fruits, sashimi, and fermented foods such as miso and natto, help digestion and play a role in preventing the wasteful use of digestive enzymes in the body.

4. According to the nature/type of the reaction: 

In 1961, the International Union of Biochemistry and Molecular Biology (IUBMB) unified all enzymes into six categories according to the type of reaction they catalyze. In August 2018, the classification of translocases was added, so there are now seven major enzymes, namely: oxidase-reductase (EC 1), transferase (EC 2), hydrolase (EC 3), lyase (EC 4), isomerase (EC 5), ligase (EC 6) and translocase (EC 7). Whereas EC stands for Enzyme Commission.

• Oxidoreductases:  are enzymes that promote redox reactions of substrates, and are a class of enzymes that catalyze redox reactions.

• Transferases: are enzymes that catalyze the transfer or exchange of certain groups (such as acetyl, methyl, amino, phosphate, etc.) between substrates. For example, methyltransferases, aminotransferases, acetyltransferases, transsulfases, kinases, and polymerases, among others.

• Hydrolases: are enzymes that catalyze the hydrolysis of substrates. For example, amylases, proteases, lipases, phosphatases, glycosidases, and the like.

• Lyases: are enzymes that catalyze a reaction that removes a group from a substrate (non-hydrolytically) leaving a double bond, or its reverse reaction. For example, dehydratase, decarboxylase, carbonic anhydrase, aldolase, citrate synthase, and the like. Many lyases catalyze a reverse response that forms new chemical bonds between two substrates and eliminates a substrate’s double bond. Synthases fall into this category.

• Isomerases: are enzymes that catalyze the mutual conversion between various isomers, geometric isomers, or optical isomers. For example, isomerase, epimerase, racemase, and the like.

• Ligase: is an enzyme that catalyzes the synthesis of two molecules of substrates into one molecule of the compound, and at the same time, the phosphate bond coupled with ATP is broken to release energy. For example, glutamine synthase, DNA ligase, amino acid: tRNA ligase, and biotin-dependent carboxylase, among others.

The hypothesis of Enzyme Action:

The role of enzymes is to catalyze biochemical reactions, that is, to convert substrate into the corresponding product required by the body. To perform catalysis, it must first contact the substrate and then catalyze the reaction through a series of actions. 

Enzymes have high specificity and they can accelerate only one type of reaction. In 1890, E. Fisher suggested that this specificity is due to the enzyme molecule’s unique shape, which matches the substrate molecule’s shape. This hypothesis is called “key and lock”, where the key is the substrate, and the lock – is with the enzyme. The hypothesis is that the substrate fits the enzyme like a key fits a lock.

Mechanism/Hypothesis of Enzyme Action:

The interaction of an enzyme with a substrate can be divided into three stages:

1. Attachment of a substrate to an enzyme macromolecule.
2. Direct enzymatic reaction.
3. Separation of the products of the transformation of the substrate from the enzyme.

1. Attachment of a substrate to an enzyme:

• The first stage, the fastest, is the limiting stage of the catalytic process as a whole. 
• Its rate depends on the structures of the enzyme and substrate, the nature of the environment in which the enzymatic reaction is carried out, pH and temperature. 
• Enzymes are characterized by specificity concerning substrates and high binding energy with them. 
• This energy is partly used to deform the substrate and reduce the activation energy of the subsequent chemical reaction.

2. Direct enzymatic reaction:

• The interaction of the enzyme with the substrate is preceded by the approach and orientation of the substrate with respect to the active center of the enzyme. 
• Then enzyme-substrate complexes are formed, the real existence of which can be fixed in various ways. 
• The most obvious and effective method is X-ray diffraction analysis. 
• An example is the identification of the enzyme-substrate complex of carboxypeptidase A and its substrate glycyl-L-tyrosine. 
• The method makes it possible not only to establish the very fact of complex formation but also to determine the types of bonds. 
• A simpler but sufficiently effective method is the spectral analysis of the enzyme and the corresponding enzyme-substrate complex. 
• Thus, in particular, enzyme-substrate complexes were identified for a number of flavin enzymes.

3. Separation of the product:

• The interaction of the enzyme with the substrate causes a local conformational change in some sites of the protein macromolecule of the enzyme, 
• As a result of which the complementarity of its active center to the substrate sharply increases and makes it possible to carry out the catalytic process. 
• A change in the conformation of an enzyme under the action of a substrate was first shown by D. Koshland and is called induced conformity.

Examples of Enzymes:

1. Alpha-amylase:

• Alpha-amylase (α-amylase, alpha-amylase) is a digestive enzyme that is involved in the digestion of food and accelerates the breakdown of complex carbohydrates like starch and glycogen into their monomers,  
• Alpha amylase also ensures the maintenance of normal blood sugar levels.
• Most amylase in the human body is found in the pancreas and salivary glands. 
• Accordingly, two types of a-amylase are determined in human blood serum: pancreatic (P-type), which is synthesized by the pancreas, and salivary amylase (S-type) – by the salivary glands.
• An increase in α-amylase activity by 2 or more times can be regarded as a symptom of damage to the pancreatic tissue. 

2. Aspartate aminotransferase (AST): 

• This enzyme is located in almost all cells of the body, however, its favorite place of localization is the heart and liver, less of it in the kidneys and muscles. 
• In healthy people, the level of AST in the blood is negligible. When symptoms of a liver or heart disease appear, it enters the bloodstream, and therefore an increase in the serum of this enzyme is an excellent indicator of the pathology of these organs.
• An increase in the concentration of AST can cause myocardial infarction, infectious viral hepatitis, liver cancer, metastases, cirrhosis, sepsis, etc.
• A decrease in the concentration of aspartate aminotransferase in the blood serum is rare and has no special diagnostic value.

3. Alanine aminotransferase (ALT): 

• This enzyme is found in many cells of our body, but its highest concentration is determined in the cells of the liver, and kidneys, in smaller quantities – in the heart, pancreas, and skeletal muscles.
• The concentration of AST in the blood serum of healthy people is low, in men the level of the enzyme is slightly higher than in women. 
• However, when cells that are rich in this enzyme, such as the liver or kidney, are damaged or die, there is a sharp increase, so to speak, the “release” of ALT into the circulatory system. 

4. Lipase:

• Lipase enzymes are present in gastric juice, pancreatic secretions, as well as in dietary fats and are the most important enzyme in the process of digestion of fats.
• This enzyme is synthesized in the pancreas and released into the intestine, where it breaks down fats from food and hydrolyzes fat molecules. 
• Lipase activity is significantly altered in diseases of the pancreas, cancer, and malnutrition.

5. Metabolic enzymes: 

• Enzymes that catalyze biochemical processes inside cells are known as metabolic enzymes.
• During this energy production and detoxification of the body and the removal of waste decay products occur. 
• Each system, organ, and tissue of the body has its own network of enzymes.

6. Lactate dehydrogenase (LDH):

• Lactate dehydrogenase (LDH) is a zinc-containing enzyme that is involved in the oxidation of lactic acid. 
• This enzyme is quite common in our body, it can be found in the kidney tissue, in the heart, in the skeletal muscles, and, of course, in the liver.
• In the body of a healthy person, there are five different forms – isoenzymes. 
• They differ in chemical structure and location in the body. 
• An increase in the concentration of LDH in the blood serum can also be determined under certain physiological conditions: in newborns, pregnant women, as well as during active sports.

7. Alkaline phosphatase:

• Alkaline phosphatase (AP) is a blood serum enzyme that is widely distributed in human tissues. 
• Its largest amount is found in the intestinal mucosa, osteoblasts (young bone cells that form the intercellular substance or matrix), the walls of the bile ducts, the placenta, and in nursing mothers in the lactating mammary gland.
• The enzyme is localized outside the cell, or rather on its membrane, and is involved in the transport of phosphorus.
• The activity of liver alkaline phosphatase is most often increased due to damage to liver cells (hepatocytes) or due to a violation of the outflow of bile.