Table of Contents
| Concept/Term | Description |
|---|---|
| Enzymes | A class of proteins that act as catalysts and increase the rates of biochemical reactions. They are not consumed during the reaction. |
| General properties of enzyme structure | Enzymes are linear chains of amino acids that fold into a three-dimensional structure. The sequence of amino acids determines the catalytic activity. Enzymes have an active site consisting of a catalytic site and binding sites. The structure of the catalytic site is essential for activity. |
| Factors influencing enzyme activity | Concentrations of reactants (substrates, coenzymes, cofactors), products, and enzyme; temperature; pH; presence of activators, inhibitors, and inactivators. |
| Effect of increasing substrate concentration | Increasing substrate concentration increases the reaction rate until a certain point. After reaching saturation, further increase in substrate concentration does not affect the rate as the enzyme molecules become saturated. |
| Effect of increasing enzyme concentration | Increasing enzyme concentration increases the reaction rate until a saturation point where the enzyme is no longer the limiting factor. |
| Influence of temperature on enzyme activity | Increasing temperature initially increases enzyme activity, but beyond a certain point, the enzyme becomes denatured, leading to a decline in activity. |
| Influence of pH on enzyme activity | pH dependency varies among enzymes. There is an optimal pH at which the enzyme activity is highest, and deviations from this pH result in reduced activity. |
| Activators, inhibitors, and inactivators | Activators are molecules that bind to enzymes and increase their activity. Inhibitors bind to enzymes and inhibit their activity. Inactivators irreversibly inhibit enzyme activity. These molecules are involved in the allosteric regulation of enzymes. |
| Classification of enzymes | Enzymes are classified based on the type of reactions they catalyze. The main enzyme classes include oxido-reductases, transferases, hydrolases, lyases, isomerases, and ligases. |
| Examples of coenzymes with different enzyme classes | Examples of coenzymes include NAD+, NADH + H+, NADP+, NADPH + H+, FAD, FADH2, Cu2+, Cu+, ATP, and biotin. These coenzymes are associated with specific enzyme classes. |
| Distribution of enzymes | Enzymes are distributed within cells and tissues depending on the type of reactions required. For example, hydrolases are found in lysosomes, peroxidases in peroxisomes, and DNA polymerases in the nuclei. |
| Interconvertible forms of enzymes | Some enzymes exist in different forms that can be interconverted through phosphorylation or dephosphorylation. Examples include glycogen phosphorylase, glycogen synthase, and others. |
| Isozymes | Multiple forms of enzymes with genetically determined differences in polypeptide amino acid sequences. They have identical functions but different structures. An example is lactate dehydrogenase (LDH). |
| Properties of lactate dehydrogenase (LDH) | LDH is composed of four subunits (tetramer) and catalyzes the conversion of lactate to pyruvic acid and back. It has five isozymes (LDH-1 to LDH-5) with different tissue distributions. |
| Isoenzymes of LDH and their locations | LDH-1 (H4) – heart and RBC; LDH-2 (H3M) – reticuloendothelial system; LDH-3 (H2M2) – brain and kidneys; LDH-4 (HM3) – kidneys, placenta, and pancreas; LDH-5 (M4) – liver and skeletal muscle. |
| Use of LDH as a cardiac marker | LDH-1 isozyme is normally found in the heart muscle. A high LDH-1 level to LDH-2 suggests a myocardial infarction. LDH levels are also high in tissue breakdown or hemolysis. |
| Coenzyme | A nonprotein compound necessary for the functioning of an enzyme, acting as a helper molecule. Example: Nicotinamide adenine dinucleotide (NAD) in lactate dehydrogenase. |
| Apoenzyme | The protein component of an enzyme without a coenzyme, considered an incomplete enzyme. |
| Cofactor | An inorganic substance that regulates and controls enzymatic activities. Examples include Mg2+ required by hexokinase in phosphoryl group transfer. |
| Holoenzyme | The complete form of an enzyme with the required cofactor. Examples include DNA polymerase composed of several protein subunits. |
| Properties of enzymes | Catalytic activity, specificity, reversibility, sensitivity to heat and temperature, sensitivity to pH, degradation, signal transduction, energy generation, ion pump action, clearance of foreign substances. |
| Types of enzyme specificity | Bond specificity, group specificity, substrate specificity, optical specificity, co-factor specificity, geometric specificity. |
| Active site | The part of an enzyme where the substrate binds for the reaction to occur. It is a specific region of the protein formed by folding. Substrates must combine with the active site for the reaction to proceed. |
| Types of enzyme inhibition | Competitive inhibition, non-competitive inhibition, uncompetitive inhibition, suicide inhibition. Different types of inhibitors bind to the enzyme and affect its activity. |
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