MLT 212: Catalysts and Enzymes

• Enzymes are studied as part of Medical Laboratories Technology with course code MLT 212.

Catalysts

• Catalysts speed up chemical reactions without being altered themselves.
• Catalysts undergo chemical or physical changes during a reaction, but revert to their original state at the end.
• Inorganic catalysts like Cl- and Mg++ are non-specific, catalyze many reactions, and are not affected by heat.
• Organic catalysts (enzymes) are highly specific and are affected by heat.

Enzymes

• Enzymes are protein catalysts which accelerate the rate of chemical reactions.
• Enzymes are not changed by entering the reaction and catalyze only one type of chemical reaction, making them specific.
• Enzymes lower the energy of activation (Ea) of a reaction, increasing the reaction rate without affecting the equilibrium position.
• Most chemical reactions for metabolism occurring in living cells have an energy barrier (= activation energy) that separates reactants from products, and these barriers are overcome by enzymes.
• The substance upon which an enzyme acts is called a substrate and is converted to a product through the enzyme's action.
• Transition state theory explains the action of catalysts.
• A chemical reaction from A to B requires energy.
• Supplying enough energy, A undergoes a transition state, which is unstable, and is converted to a more stable product B.
• The amount of energy needed to convert a substance from ground state to transition state is called activation energy.
• In the presence of a catalyst, A undergoes transition state very fast and requires less energy.
• Catalysts accelerate the reaction rate by decreasing the energy of activation.
• Enzymes also speed up reactions by lowering the energy of activation and are more efficient than non-enzyme catalysts.

Enzyme Specificity

• Enzymes are highly specific compared to other catalysts and catalyze only specific reactions.

Types of Enzyme Specificity

• Substrate Specificity: One enzyme acts only on one substrate.
• Reaction Specificity: A given enzyme catalyzes only one specific reaction.
• Group Specificity: Some lytic (hydrolases) enzymes act on specific groups.
• Optical Specificity: Enzymes recognize optical isomers of the substrate .
• Enzymes of amino acid metabolism act only on L-isomers but not D-isomers.
• Enzymes of carbohydrate metabolism act only on D-sugars but not L-sugars.

Enzyme Composition

• Enzymes are either simple or conjugated proteins.
• Simple proteins: hydrolysis yields only amino acids; only native protein conformation is required for activity.
• Conjugated proteins (holoenzymes): composed of a protein part (apoenzyme) and a non-protein part (cofactor), both required for activity.
• Cofactors can be prosthetic groups or coenzymes.
• A prosthetic group is a cofactor tightly bound to the enzyme protein, like FAD or some metals, and cannot be easily separated without destroying the enzyme.
• Co-enzymes are cofactors loosely (weakly) bound to the enzyme apoprotein
• Hydrogen carriers: NAD, NADP, FAD, FMN, lipoic acid, and coenzyme Q.
• Other group carriers include Co-enzyme A (acid carrier).
• Biotin + TPP (Thiamine pyrophosphate) for decarboxylation.
• Pyridoxal phosphate (NH2 group carrier).
• Folic acid (one carbon carrier).
• Cobolamine (methyl group carrier).

Enzyme Nomenclature

• Trivial names include Trypsin and Pepsin
• Adding suffix "-ase"
• The name of an enzyme has two parts, the first indicates the name of its substrate, and the second ends in '-ase'
• Examples of this include maltase and lactase.
• IUB naming gives a systemic name indicating substrate, coenzyme, and reaction type, e.g., lactate dehydrogenase = lactate-NAD+-oxidoreductase.
• IUB classifies enzymes by giving each an EC number with 4 digits.
• The first digit = class number, the second digit = functional group (subclass), the third digit = co-enzyme, and the fourth digit = substrate.

Classification of Enzymes

• The IUB system classifies enzymes into six different ordered classes.
• Class 1: Oxidoreductases catalyze oxidation-reduction reactions between two substrates.
• Oxidation mechanisms involve either the addition of oxygen (oxygenase) or the removal of hydrogen (dehydrogenase or oxidase).
• Dehydrogenases catalyze the removal of hydrogen from a substrate and donate it to a co-enzyme like NAD+.
• Oxidases catalyze the transfer of electrons or hydrogen from a substrate and donate it to oxygen to create compounds, such as glucose oxidase that converts glucose to gluconate and H2O2.
• Oxygenases catalyze the incorporation of oxygen into a substrate
• Monooxygenase catalyzes the incorporation of one oxygen atom into a substrate and dioxygenase incorporate two O atoms.
• Class 2: Transferases catalyze the transfer of a group (other than hydrogen) between two substrates, where transferred groups are acyl, amino, and phosphate.
• Class 3: Hydrolases catalyze the hydrolysis of substrates, breaking chemical bonds by adding water, like digestive enzymes and peptidases.
• Class 4: Lyases catalyze the removal of a group from a substrate by mechanisms other than hydrolysis (without H₂O), Fructose 1,6 diphosphate is converted by aid of aldolase into glyceraldehydes-3-phosphate and dihydroxy acetone phosphate.
• Class 5: Isomerases catalyze the interconversion of one isomer into another.
• Class 6: Ligases catalyze the joining of 2 substrates using high energy released by hydrolysis of high energy bond of ATP, pyruvic acid is joined to CO₂ and converted to oxaloacetic acid by aid of carboxylase enzyme.

Enzyme Active Site

• During enzyme action, there's a temporary combination between the enzyme and its substrate.
• The substrate binds to a specific site on the enzyme called the "active site."
• The active site is characterized by the presence of R-groups that come from side chains of amino acids.
• Specificity of binding depends on the arrangement of these groups.
• The substrate binds to the enzyme, forming an enzyme-substrate (ES) complex.
• Binding is thought to cause a conformational change in the enzyme that allows catalysis.
• The ES complex is converted to an enzyme-product (EP) complex that subsequently dissociates to enzyme and product.
• The active site of an enzyme consists of only a few amino acid residues and takes on a three-dimensional conformation.
• Amino acids at the active site are arranged precisely so only specific substrates can bind there.
• Serine, histidine, cysteine, aspartate, or glutamate residues usually make up the active site.