Enzyme - Part I PDF

Enzyme - Part I Foundation – I Module Johar Iqbal Faculty of Medicine 2024-2025 Objectives by the end of the lecture the student should be able to: o Define enzymes, substrate, substrate binding site….etc o Understand en...

Enzyme - Part I Foundation – I Module Johar Iqbal Faculty of Medicine 2024-2025 Objectives by the end of the lecture the student should be able to: o Define enzymes, substrate, substrate binding site….etc o Understand enzymes classification. o Understand the mechanism of enzyme substrate interaction. o Understand the main properties of enzymes. o Enumerate factors affecting enzymes. Definitions 1- Enzymes: Enzymes are protein biocatalysts. They accelerate the rate of chemical reactions by lowering activation energy. The enzymes have no effects on equilibrium. Enzymes are highly specific. 2- Substrate: is a substance upon which the enzyme acts to form the product. The enzyme binds to its substrate to form an enzyme substrate complex. The binding of substrate with enzyme takes place at substrate binding site (OR Active site). 3- Substrate binding site (active site, catalytic site, Isosteric site): It is a small region on the surface of the enzyme. It has specific chemical groups that bind a specific substrate. The substrate binding site is the enzyme active site or catalytic site.  4- Allosteric sites: Some enzymes contain other binding sites beside their active sites, called as allosteric sites. The allosteric sites bind small molecules called allosteric effectors. These effectors modulate (increase or decrease) enzyme activity. Definitions 5- Holoenzymes, apoenzymes, and coenzymes: Enzymes are proteins in nature. Some of them are simple proteins that do not require additional chemical groups to act e.g., pepsin, trypsin etc. Other enzymes are conjugated enzymes. Conjugated enzymes are formed from a protein part called apoenzyme and a non-protein part called coenzyme (cofactor). The whole enzyme is the holoenzyme i.e. Holoenzyme = apoenzyme + coenzyme.  Mechanism of enzyme action Formation of the enzyme substrate complex: The first step in any enzyme catalyzed reaction is binding of the substrate to the active site of the enzyme to form the enzyme substrate complex. This complex is then converted to a product and the enzyme is regenerated again ready for a second reaction. E+S ES complex E+P Enzyme Products + + Enzyme-Substrate Complex Substrate Enzyme Nomenclature and Classification of Enzymes Nomenclature: Most enzymes are named by adding suffix (ase) to a word or phrase describing enzyme activity e.g. hydrolase. Sometimes the name is not ending by (ase) e.g. pepsin Enzymes are classified into six major classes based upon the type of reaction they catalyze. Each class is subdivided into several subclasses, which again are subdivided into sub-subclasses. Oxidoreducatses Transferases Transaminases (aminotransferases) are example of tranferase group Hydrolases Lyases Isomerases Ligases  Factors affecting enzyme activity Substrate concentration Temperature pH To study the effect of any factor , other factors must be kept constant Substrate concentration Increasing the substrate concentration, while all other conditions are kept constant, will increase the initial velocity (V, point A) to a maximum (Vmax, point C). It is a hyperbolic curve. The initial velocity is the velocity measured at the beginning of the reaction. Point B (velocity is half of the Vmax) at substrate concentration Km  Michaelis-Menten Equation Michaelis –Menten equation describes how reaction velocity changes with substrate concentration Km = Michaelis constant V0 = initial reaction velocity Vmax = maximum velocity [S] = substrate concentration Km is the substrate concentration at which rate of reaction of an enzyme is half of its maximum (Vmax ) Km is specific for each enzyme q Low Km = a high affinity of the enzyme for substrate because a low concentration of substrate is needed to reach half Vmax q High Km = low affinity of the enzyme for substrate because a high concentration of substrate is needed to reach half Vmax  Temperature Ø Higher temperatures usually accelerate reactions Ø The relation is bell-shaped Ø The optimum temperature is the temperature at which enzyme velocity is maximum. Ø In humans, most enzymes have an optimum temperature of around 37oC. The too high temperature will denature the enzyme and the enzyme will lose its activity.  pH Ø Enzyme activity depends upon the pH. Ø It shows a bell-shaped curve, with maximum activity at the optimum pH. Ø Every enzyme has its own optimum pH at which activity is the best. Ø The pH of human blood is 7.4. Ø Most of the enzymes have an optimum pH between 6 to 8. However, pepsin, a gastric enzyme, functions at the low pH of gastric juice, it has an optimum pH of 2.  Thank you Faculty of Medicine 2024-2025