Enzymes PDF - MLT 212 Chapter 1

MLT 212 Taibah University Chapter 1 College of Applied Medical Sciences Medical Laboratories Technology Enzymes Dr: Saber Eweda 1446 (2025 Obj...

MLT 212 Taibah University Chapter 1 College of Applied Medical Sciences Medical Laboratories Technology Enzymes Dr: Saber Eweda 1446 (2025 Objectives After studying this chapter, the students should be able to: 1. Expalin the the differences between organic and inorganic catalysts 2. Understand how the enzymes accelerate the biochemical reactions. 3. Identify the types of enzymes specificity 4. Undrastand the differences between the simple and conjugated enzymes 5. Identify the IUB classification of enzymes 6. Explain the role of active site in enzyme specificity  Catalyst ❖ A catalyst does not change the chemical reaction but it accelerates the reaction. ❖ They are not consumed in overall reaction. But they undergo chemical or physical change during reaction and returns to original state at the end of reaction. Catalysts may be:- ❖ Inorganic: Cl- and Mg++. They are non-specific as they catalyse many reactions and not affected by heat. ❖ Organic: enzymes are highly specific and affected by heat. 3 ❖ En= in zyme= yeast ❖ Enzymes are protein catalyst that accelerate rate of chemical reaction. It is not changed by entering the reaction. It catalyse only one type of chemical reaction (They are specific). ❖ All chemical reactions for metabolism occur in living cells have an energy barrier ( = activation energy) that separate reactant from products. These barriers are overcome by enzymes. ❖ The enzyme lowers the energy of activation of a reaction ( Ea), thereby increasing the rate of that reaction without affecting the position of equilibrium (forward and reverse reactions are affected to the same extent) 4 ❖ The substance upon which an enzyme acts is called as substrate. By the action of enzyme it is converted to product. Substrate Enzyme Product Transition state theory was proposed to explain action of catalyst. ❖ For a chemical reaction A → B to occur, energy is required. ❖ When enough energy is supplied. A undergoes to transition state which is an unstable state. So, it gets converted to product B which is more stable. ❖ The amount of energy needed to convert a substance from ground state to transition state is called activation energy. 5 ❖In presence of catalyst, A undergoes to transition state very fast and requires less energy (Fig. 4.1). ❖Hence, a catalyst accelerate the rate of reaction by decreasing the energy of activation. ❖Likewise enzymes also speed up reaction by lowering energy of activation. ❖Further, the activation energy is very much less for a reaction in presence of enzyme than non-enzyme catalyst (Fig. 4.1). Therefore enzymes are more efficient than non- enzyme catalyst. ENZYME SPECIFICITY ❖ Enzymes are highly specific compared to other catalyst. An enzyme catalyzes only specific reaction. Type of Specificity 1. Substrate Specificity ❖ One enzyme acts only on one substrate. For example, glucokinase catalyzes the transfer of phosphate from ATP to glucose. Glucose + ATP glucokinase Glucose- 6-phosphate + ADP 2. Reaction Specificity ❖ A given enzyme catalyze only one specific reaction. For example, lipases only hydrolyze lipids. They do not catalyze any other type of reaction. Lipids lipases Glycerol + Fatty acids 3. Group Specificity ❖ Some lytic (hydrolases) enzymes act on specific groups. Esterase are specific for ester groups. Ester +H2O Esterase Acid + Alcohol 4. Optical Specificity ❖ It means enzymes are able to recognize optical isomers of the substrate. ❖ enzymes of amino acid metabolism act only on L-isomers (L-amino acid) but not D-isomers (D-amino acids). ❖ Likewise enzymes of carbohydrate metabolism act only on D-sugars but not on L-sugars. Enzyme composition, Enzymes are either simple or conjugated proteins. 1. Simple protein: its hydrolysis gives only amino acids. Only native conformation of protein is required for the activity of this type. 2. Conjugated protein: (Also, called holoenzyme) ❖ This type composed of protein part (called Apoenzyme) and non- protein part (called Cofactor) and its activity require these two components. ❖ These cofactors may be prosthetic group or coenzyme ❖ The prosthetic group is a cofactor tightly bound to enzyme protein as in case of FAD or some metals. So, it is not easy to separated from 10 the enzyme without its destruction. ❖ Co-enzymes. It is cofactor loosely (weakly) bound to the enzyme apoprotein. These co-enzyme may be: 1. Hydrogen carrier: as NAD, NADP, FAD, FMN, lipoic acid and coenzyme Q. 2. Other group carrier: ❖ Co-enzyme A................................. acid carrier. ❖ Biotin + TPP (Thiamine pyrophosphate)....decarboxylation. ❖ Pyridoxal phosphate....................NH2 group carrier. ❖ Folic acid.................................... one carbon carrier. ❖ Cobolamine................................methyl group carrier. 11  Enzyme Nomenclature 1-Trivial name: for example; Trypsin, Pepsin. 2- Adding suffix (ase ) The name of an enzyme has two parts. The first part indicates name of its substrate and second part ending in ‘ase’. e.g. Maltase, Lactase. 3- IUB made a systemic name to each enzyme indicating: substrate, coenzyme, type of reaction catalysed e.g. lactate dehydrogenase = lactate-NAD+-oxidoreductase. 4- In addition, IUB classifies the enzymes by giving each enzyme number. This number is called (EC). It contains 4 digits:- First digit = class number. Second digit= functional group (subclass). 12 Third digit= co-enzyme. Fourth digit= substrate.  Classification of Enzymes The IUB system classify the enzyme into 6 different and ordered classes: 13 Class 1- Oxido-reductase This group of enzymes catalyse oxidation-reduction reaction between two substrates. The mechanism of oxidation is either addition of oxygen (Oxygenase) or removal of hydrogen (dehydrogenase or oxidase). This class include some suclasses such as: 1. Dehydrogenases: catalize removal of hydrogen from substrate and donate it to a co-enzyme as NAD+. 2. Oxidases: catalyse transfer of electron or hydrogen from substrate and donate it to oxygen as glucose oxidase that converts glucose to gluconate and H2O2. 3. Oxygenases: catalyses incorporation of oxygen into substrate. It may be Monooxygenase (catalyze incorporation of one oxygen atom into substrate) or Dioxygenase (incorporate two O atoms) 14 Class 2: Transferase: Catalyses the transfer of a group other than hydrogen between two substrates. Groups, which are transferred, are acyl, amino and phosphate. Acetyl-CoA + Choline Transferase Acetyl choline + CoA Class 3: Hydrolase: Catalyses hydrolysis of substrate i.e. breakdown of the chemical bond by addition of water e.g. digestive enzymes; peptidase. 15 Class 4: Lyases: Catalysis removal of a group from substrate by mechanism other than hydrolysis (without H2O) e.g. Fructose 1,6 diphosphate is converted by aid of aldolase into glyceraldehydes-3- phosphate and dihydroxy acetone phosphate. Class 5: Isomerase: Catalysis the interconversion of one isomer into the other e.g. Alanine Racemase L-Alanine D- Alanine Calss 6: Ligase: Catalysis the joining of 2 substrates using high energy released by hydrolysis of high energy bond of ATP e.g. pyruvic acid is joined to CO2 and converted to oxaloacetic acid by aid of carboxylase enzyme. ❖ During enzyme action there is a temporary combination between the enzyme and its substrate. The substrate bounded to specific site on the enzyme called “Active site” which is characterized by the presence of R-groups that comes from side chains of amino acids and the specificity of binding depends on arrangement of these groups.  The substrate binds the enzyme, forming an enzyme–substrate (ES) complex. Binding is thought to cause a conformational change in the enzyme that allows catalysis. ES is converted to an enzyme–product (EP) complex that subsequently dissociates to enzyme and product. ❖ Active site of enzyme consists of few amino acid residues only. ❖ Active site of enzyme takes three-dimensional conformation. ❖ The active site may compose of amino acid residues that are far apart in the enzyme molecule. During catalysis, they are brought together. ❖ The amino acids at the active site are arranged in a very precise manner so that only specific substrate can bind at the active site. ❖ Usually serine, histidine, cysteine, aspartate or glutamate residues make up active site.  Illustrative Questions What are the meaning for the following scientific terms (define): 1. Enzyme with reaction specificity 2. Enzymes with substrate specificity 3. Conjugated protein or enzyme 4. Transferase enzymes What are the differences between the following: 1. Dehydrogenases and oxidases enzymes 2. Prosthetic group and coenzyme Give reason for each of the following (or Explain how): 1. Enzymes are specific molecules. 2. The enzymes accelerate the rate of biochemical reactions What are the roles of the following enzymes classes: 1. Isomerases 2. Ligases 22