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2 ENZYME They are classified into six classes: 1-Oxidoreductases Oxidoreductases are important in the body that these reactions are responsible for the production of heat and energy. Oxidoreductase (Alcohol dehydrogenase),oxidize of alcohols to an aldehyde by removing two electrons as 2H+ from alcohol to yield an aldehyde. Ethanol + NAD⁺→Alcohol dehydrogenase→ Acetaldehyde + NADH + H⁺ Lactate + NAD⁺ → Lactate dehydrogenase →Pyruvate + NADH + H⁺ 2-Transferases Transfer functional groups between donor and acceptor molecules. Kinases are specialized transferases that regulate metabolism by transferring phosphate from ATP to other molecules. Glucose + ATP → Hexokinase → Glucose-6-phosphate + ADP Fructose + ATP → Fructokinase → Fructose-1-phosphate + ADP 3-Hydrolases Add water across a bond, hydrolyzing it. Lactose + H2O → Lactase → Glucose + Galactose Maltose + H2O → Maltase → Glucose + Glucose 4-Lyases Add water, ammonia or carbon dioxide across double bonds, or remove these elements to produce double bonds. Fructose-1,6-bisphosphate →Aldolase A →Glyceraldehyde-3-phosphate + Dihydroxyacetone phosphate 5-Isomerases Catalyze racemization of optical isomers. Carry out many kinds of isomerization: L to D isomerization, mutase reactions (interconversion of chemical groups) and others. Glucose-6-phosphate → Isomerase → Fructose-6-phosphate Glucose → Epimerase → Galactose 6-Ligases Catalyze formation of bonds between Carbon and Oxygen, Nitrogen and Sulphur. This reactions in which two chemical groups are joined (or ligated) with the use of energy from ATP. The Michaelis-Menten equation What is Michaelis-Menten equation? Definition: The Michaelis-Menten equation describe how reaction initial velocity Vₒ varies with substrate concentration[S],by the following equation: K₁ E+S K₃ [ES] complex K₂ E+P K₄ E is an enzyme while S is a substrate & P is a product. Where K₁,K₃ are forward reaction rate constant. Where K₂,K₄ are reverse reaction rate constant [ V [S] Vₒ=Initial Velocity, Vmax =Maximum Velocity, [S]=Substrate concentration, and Km = Michaelis-Menten Constant is to measure of how efficiently an enzyme converts a substrate into product.. Fig (1):- Plot of reaction velocity on a function of [S] to the enzyme by Michalis – Menten equation. If Km numerically small (low) reflects a high affinity of the Enzyme to bind substrate by diffusion of substrate into the active site , while if Km numerically high reflects a low affinity of the Enzyme to substrate concentration, Fig (1), indicate an increased rate of unbinding this in fact decrease the reaction rate. The Effect Of Activators and Inhibitors on Enzyme activity INHIBITORS:Effects of Inhibitors on Enzyme Activity: Enzyme inhibitors are substances which alter the catalytic action of the enzyme and consequently slow down, or in some cases, stop catalysis. Inhibitors may act combining directly with the enzyme and so effectively remove it from the substrate (like Drugs: Heat, pH changes, strong acids, alcohol & alkaloidal reagents cause protein denaturation, (Captopril). e.g. The optimum temperature for most human enzymes start between 35⁰C and 40⁰C. Human enzymes start to denature above 40⁰C and stop its catalytic activity. There are three types of Inhibitors : 1-Competative Inhibitor: Substances [I] that compete with the substrate[S] for the active site of enzyme molecule and form new enzyme- substrate complex [ES]. 2-Reversbile Inhibitor: Decrease enzyme activity and full activity return when inhibitor [I] is removed. 3-Irreversbile Inhibitor: Those inhibitor [I] binds tightly to the enzyme, and inactivate E or destroy a functional group on the enzyme molecule, that is necessary for its catalytic activity (enzyme inactivation), as in fig-2 below:ACTIVATORS :COFACTORS: A cofactor is a non-protein chemical compound or metallic ion (Ca⁺², Fe⁺², Mg ⁺²,Mn ⁺², Zn ⁺², and K⁺), that must bind to particular enzymes before a reaction occurs, Cofactors can be sub classified as either inorganic ions binding cofactors or complex organic molecules binding cofactors called coenzymes. Cofactors can be considered "helper molecules" that assist in biochemical transformations, fig-3. Fig-2:Types of enzyme inhibitors. Some enzymes containing or requiring metal ions as ───────────────────────── Fe⁺² or Fe⁺ᵌ : Peroxidase Zn⁺² : Alcohol dehydrogenase. ───────────────────────── Some coenzymes in group transferring reactions:Coenzyme Entity transferred Flavin mononucleotide Hydrogen atom (electron( Figure 3:-metal ions as cofactores. ACTVIATORS:ISOENZYMES: Some of the enzymes are present in more than one form having the same molecular weight and differ in conformational structures called isoenzymes, e.g. Trypsinogen isoenzymes are present in three conformational structures :1- cationic Trypsinogen 2- anionic Trypsinogen 3- mesotrypsinogen These conformational structures of isoenzymes are capable of digesting the cell and causing significant damage. But there are mechanisms to prevent these enzymes from potentially digesting the pancreas including: storage and packing in acidic media to inhibit enzyme activity synthesis and storage as inactive precursor forms. some of the enzymes that are stored in the pancreas before secretion as inactive precursor forms, then activated when they enter the duodenum. Activation of these enzymes takes place in the surface of the duodenal lumen, microvilli where Enterokinase, activates Trypsinogen by removing (by hydrolysis) an N-terminal hexa peptide fragment of the molecule (Val–Asp– Asp–Asp–Asp–Lys). The active form, Trypsin, then catalyzes the activation of the other inactive proenzymes. Of note, many key digestive enzymes, such as α-amylase and lipase, are present in the pancreas in their active forms. Presumably, these enzymes would not cause pancreatic cellular damage if released into the pancreatic cell/tissue because there is no starch, glycogen or triglyceride substrate for these enzymes in pancreatic tissue. Coenzyme: Coenzymes are organic cofactors. They are Coenzymes serve as a second substrates for enzymatic reactions, such as nucleotide phosphates and vitamins. When bound tightly to the enzyme, coenzymes are called prosthetic groups. For example, NAD as a cofactor may be reduced to nicotinamide adenine dinucleotide phosphate (NADH) in a reaction in which the primary substrate is oxidized (the equation below). Increasing coenzyme concentration will increase the velocity of an enzymatic reaction. Holoenzyme: When bound tightly to the enzyme, the coenzyme is called a prosthetic group. The enzyme portion (apoenzyme), with its respective coenzyme, forms a complete and active system, a holoenzyme. Zymogen: Some enzymes, mostly digestive enzymes, are originally secreted from the organ of production in a structurally inactive form, called a proenzyme or zymogen. Other enzymes later alter the structure of the zymogen to make active sites available by hydrolyzing specific amino acid residues. This mechanism prevents digestive enzymes from digesting their place of synthesis. Trypsinogen, is a precursor of trypsin, its a storage of an inactive form of trypsin so that it may be kept in the pancreas and released in significant amount when required for protein digestion. Trypsin is formed in the small intestine when its proenzyme Enterokinase produced by pancreas. This figure indicate the activation of Inactive Trypsinogen into Trypsin in small intestine by Enterokinase..