Regulation of Enzyme Activity PDF

24 Regulation of Enzyme Activity: ILOs By the end of this lecture, students will be able to 1. Differentiate between the different regulators of enzyme activity 2. Correlate regulation of enzyme activity with body needs How can Enzyme activity be regulated? There are several means by wh...

24 Regulation of Enzyme Activity: ILOs By the end of this lecture, students will be able to 1. Differentiate between the different regulators of enzyme activity 2. Correlate regulation of enzyme activity with body needs How can Enzyme activity be regulated? There are several means by which the activity of a particular enzyme is specifically regulated: 1- Zymogen activation Some enzymes are secreted in an inactive form that can be activated when they are required these are known as “Proenzymes” or “Zymogens”. At the site of action of these enzymes, specific peptide bonds are hydrolysed. Many of the digestive enzymes and enzymes concerned with blood coagulation are in this group e.g. Pepsinogen to pepsin, Trypsinogen to trypsin, plasminogen to plasmin. After hydrolysis when the enzyme is activated, it cannot be reconverted into proenzyme form. Zymogens are considered a protective mechanism to prevent auto-digestion of tissue producing the digestive enzymes and to prevent intravascular coagulation of blood. 2- Covalent Modification Many enzymes are regulated by covalent modification, most often by the addition or removal of phosphate groups to serine or tyrosine residues on the enzyme chain. Enzymes maybe activated by phosphorylation and inactivated by dephosphorylation or vice versa. This means that the enzyme is present in two inter-convertible forms (phosphorylated and dephosphorylated -only one of them is the active form). Figure (8-1): Covalent modification by phosphorylation and dephosphorylation 3- Allosteric Modulation Some enzymes do not only bind to substrates and inhibitors, but they also bind to “Effectors”. These Enzymes are known as “Allosteric enzymes”. Page 1 of 4  “Effectors” are small, regulatory physiologically important molecules that lead to catalytic modification by binding to the enzyme at distinct “Allosteric sites”. “Allosteric sites “are sites of the enzyme other than the active site that binds the substrate. The Allosteric sites bind to effectors and not to substrates. Allosteric effectors that increase the catalytic activity of enzyme are known as “Positive effectors”, while those that reduce it are “Negative effectors”. 4- Feedback inhibition Allosteric regulation in which end products inhibit the activity of the enzyme is called “ feedback inhibition”. Feedback regulation generally occurs at the “Committed-Step Enzyme” Committed-Step Enzyme: is the first irreversible enzyme unique to the pathway, it is not necessarly the rate limiting step. 5- Enzyme synthesis Cells can also regulate the amount of enzyme present by altering the rate of enzyme synthesis. The increase (induction) or decrease (repression) of enzyme synthesis leads to an alteration in the total number of active sites. This usually occurs under selected physiologic or pathological conditions. For example, elevated levels of insulin as a result of high blood glucose levels causes an increase in the synthesis of key enzymes involved in glucose metabolism. The induction-repression regulation is a slow long term regulation (hours to days), compared with the short term allosterically or covalently regulated changes in enzyme activity, which occur in seconds to minutes. Substances that increase the expression of the gene coding for the enzyme are called ”Inducers”, while those that decrease the gene expression are called “Repressors”. Enzymes that are in constant use are usually not regulated by altering the rate of enzyme synthesis. Inhibitors Inhibtors: Any substance that can decrease the velocity of an enzyme-catalyzed reaction is called an inhibitor A. Competitive inhibition: It is the type of inhibition occuring when the inhibitor binds reversibly to the same site that the substrate would normally occupy and, therefore competes with the substrate for that site. The Inhibitor is structurally similar to the substrate. The inhibitor binds to enzyme & forms an [EI] complex at the active site. No product is formed Page 2 of 4  In this type of inhibition the Vmax of the enzyme is not decreased, but the apparent Km is increased i.e the affinity of the enzyme to the substrate is decreased. This type of inhibition is reversible and the inhibition is removed by increasing the substrate concentration Clinical Implications: Statin drugs competitively inhibit the rate-limiting step in cholesterol biosynthesis as they are structurally similar to the natural substrate for this enzyme B. Non-Competitive inhibition: In non-competitive inhibition the inhibitor binds at different site other than the substrate- binding site leading to conformational changes in the enzyme and reversible inactivation of the catalytic site. Non-competitive inhibitors bind reversibly either to the free-enzyme or the ES complex to form the inactive complexes EI and ESI (Enzyme substrate Inhibition). Inhibitors often have no structural similarity to substrate and the inhibition is not reversed by increasing the amount of substrate. The most important non-competitive inhibitors are naturally occurring metabolic intermediates that can combine reversibly with specific sites on enzymes inhibiting it. It lowers the Vmax of enzyme but does not affect the Km as the inhibitor does not interfere with the binding of substrate to enzyme. Thus, the enzyme shows the same Km in the presence or absence of the noncompetitive inhibitor. Clinical Implications: Lead may non-competitively inhibit important enzymes in the heme synthesis pathway leading to anemia. Reducing agents as vitamin C and vitamin E can regain the activity of the enzyme. Routine supplementation of these vitamins is therefore advised for people living or working in areas of high pollution or near factories. C. Irreversible Inhibition (Enzyme poison) The Inhibitor combines covalently tightly and irreversibly to the functional group of the enzyme destroying it. At any given concentration the inhibitor will destroy the enzyme permanently, thus it’s called “Enzyme Poison”. This type of inhibition decreases the Vmax while the Km stays the same unchanged. No competition between substrate and inhibitor because the inhibitor has no structural resemblance to the substrate. Clinical Implications: Organophosphates found in most pesticides when accidently ingested or inhaled lead to irreversible inhibition of enzymes controlling nerve signals in the body. Also cyanide leads to irreversible inhibition of enzymes responsible for ATP synthesis. Therefore, low doses of cyanide present in cigarettes are responsible for many of smoking side effects. ❖ Enzymes in clinical use: A- Diagnosis of Diseases: Page 3 of 4  Plasma enzymes can be classified into two major groups: 1. Functional Plasma Enzymes: Those, relatively, small group of enzymes secreted into the plasma by certain organs and performing a certain function in the plasma For example: - the liver secretes zymogens involved in blood coagulation cascade. 2. Non-functional plasma enzymes: Those large enzyme species released from cells during normal cell turnover. These enzymes are normally intracellular and have no physiologic function in the plasma. In healthy individuals the levels of these enzymes are fairly constant and represent steady state in which the rate of release from cells into the plasma is balanced by an equal rate or removal from the plasma. Many diseases that cause tissue damage result in an increased release of intracellular enzymes into the plasma which correlates with the extent of tissue damage. Thus, the degree of elevation of a particular enzyme in plasma is often useful in evaluating the diagnosis and prognosis for the patient. For example, the enzyme Alanine aminotransferase (ALT) is abundant in the liver. The appearance of elevated levels of ALT in plasma signals possible damage to hepatic tissue. Therefore it is part of the liver function test panel. Also Alkaline phosphatase enzyme present in bone increases with diseases affecting bone as osteoperosis, rickets and bone tumors. Increases in plasma levels of enzymes with a wide tissue distribution provide a less specific indication of the site of cellular injury and limits their diagnostic value. B- Treatment of diseases: Enzymes are used in treatment of some diseases e.g. 1. Some digestive enzymes are now supplied as treatment for maldigestion. 2. Alteplase in treatment of myocardial infarctions. It enhances the activation of enzymes needed for degradation of thrombus. Page 4 of 4