Chapter 4 BioChem.pdf

MODULE BIOCHEMISTRY FOR MEDICAL LABORATORY SCIENCE Chapter 4: Enzymes Enzymes are compound, usually proteins that act as catalyst for a biochemical reaction. In general terms, a catalyst is a substance that increases the rate, or velocity, of achemical rea...

MODULE BIOCHEMISTRY FOR MEDICAL LABORATORY SCIENCE Chapter 4: Enzymes Enzymes are compound, usually proteins that act as catalyst for a biochemical reaction. In general terms, a catalyst is a substance that increases the rate, or velocity, of achemical reaction without itself being changed in the overall process. Enzymes arebiological catalysts, most of which are proteins. Catalysis is essential to make most critical biochemical reactions proceed at useful rates under physiological conditions. Most enzymes are globular proteins. Some are simple proteins, consisting entirely of amino acid chains. Others are conjugated proteins, containing additional chemical components. Enzymes undergo all the reactions of proteins, including denaturation. Slight alterations in pH or temperature affect enzyme activity drastically. Enzyme Structure Simple Enzyme – is an enzyme composed only of protein (amino acid chains). Conjugated Enzyme – is an enzyme that has a non protein part in addition to a protein part. Neither the protein part nor the non protein portion has catalytic properties. Apoenzyme – is the protein part of a conjugated enzyme. Cofactor – is the non protein part of a conjugated enzyme. Holoenzyme – is the biochemically active conjugated enzyme produced from an apoenzyme and a cofactor. Coenzyme – is a small organic molecule that serves as a cofactor in a conjugated enzyme. Conenzymes are synthesized within the human body using building blocks obtained from other nutrients. Most often, one of these building blocks is a B vitamin or B vitamin derivative. Vitamins must be obtained through dietary intake. Nomenclature and Classification of Enzymes Oxidoreductase – is an enzyme that catalyzes an oxidation-reduction reaction. Transferase – is an enzyme that catalyzes the transfer of a functional group from one molecule to another. Transaminase and kinase are both major subtype of transferase. Hydrolase – is an enzyme that catalyzes a hydrolysis reaction in which the addition of a water molecule to a bond causes the bond to break. Hydrolysis reactions are central to the process of digestion. MODULE BIOCHEMISTRY FOR MEDICAL LABORATORY SCIENCE Lyase – is an enzyme that catalyzes the addition of a group to a double bond or the removal of a group to form a double bond in a manner that does not involve hydrolysis or oxidation. Isomerase – is an enzyme that catalyzes the isomerisation (rearrangement of atoms) of a substrate in a reaction, converting it into a molecule isomeric with itself. Ligase – is an enzyme that catalyzes the bonding together of two molecules into one with the participation of ATP. ATP involvement is required because such reactions are generally energetically unfavourable and they require the simultaneous input of energy obtained by a hydrolysis reaction in which ATP is converted to ADP. Enzyme Reaction Rate First-Order Reactions – reaction rate depends on the concentration of the substrate. The reaction rate steadily increases as more substrate is added. The most common example of a first-order reaction is the decay of radioactive elements. Second-Order Reactions – occurs when two molecules come together to form products. Role of Metal Ions Over one third of enzymes characterized to-date contain metal ions in theiractive sites; thus, metalloenzymes are an important class of enzymes and muchcurrent research is devoted to understanding the roles of the metal ions in catalysis.If the metal ion behaves as a Lewis acid, by accepting electron density from anelectron-rich atom (e.g., an atom that develops negative charge in the transitionstate), it is acting as an electrostatic catalyst.Metal ions can also promote the formationof hydroxide ion in the enzyme active site. This species is animportant nucleophile in many hydrolytic reactions, such as the cleavage of peptidebonds in proteins or phosphodiester bonds in DNA and RNA. Factors Affecting Enzyme Activity Temperature – athigher temperatures molecules are moving faster and colliding more frequently. Thisconcept applies to collisions between substrate molecules and enzymes. As the temperatureof an enzymatically catalyzed reaction increases, so does the rate (velocity)of the reaction.However, when the temperature increases beyond a certain point, the increasedenergy begins to cause disruptions in the tertiary structure of the enzyme; denaturationis occurring. MODULE BIOCHEMISTRY FOR MEDICAL LABORATORY SCIENCE pH – The pH of an enzyme’s environment can affect its activity. This is not surprisingbecause the charge on acidic and basic amino acids located at the activesite depends on pH. Small changes in pH (less than one unit) can result in enzyme denaturation and subsequent loss of catalytic activity. Substrate Concentration – When the concentration of an enzyme is kept constant and the concentration ofsubstrate is increased, the enzyme activity pattern shown in is obtained.This activity pattern is called a saturation curve. Enzyme activity increases up to a certainsubstrate concentration and thereafter remains constant. Enzyme Concentration – Because enzymes are not consumed in the reactions they catalyze, the cell usuallykeeps the number of enzymes low compared with the number of substrate molecules.This is efficient; the cell avoids paying the energy costs of synthesizing and maintaininga large work force of enzyme molecules. Thus, in general, the concentration ofsubstrate in a reaction is much higher than that of the enzyme. For more understanding on Enzymes, please visit: https://www.youtube.com/watch?v=yk14dOOvwMk https://www.youtube.com/watch?v=qgVFkRn8f10 https://www.youtube.com/watch?v=ozdO1mLXBQE Bibliography Mathews, C. K., Van Holde, K. E., Appling, D. R., & Anthony-Cahill, S. J. (2013). Biochemistry 4th Edition. Pearson. Nelson, D. L., & Cox, M. M. (2017). Lehninger Principles of Biochemistry 7th Edition. New York. Stoker, H. S. (2014). Organic & Biological Chemistry 7th Edition. Boston: Cengage Learning.