Lecture 7 - Isoenzymes PDF

Lecture 7: Isoenzymes MEDC-1242 Fall Semester 2024/25 Dr Yaj Objectives for the lecture: Define isoenzymes and explain their structural and functional characteristics. Understand the biochemical and clinical significance of isoenzymes. Explore examples of clinically important isoenzym...

Lecture 7: Isoenzymes MEDC-1242 Fall Semester 2024/25 Dr Yaj Objectives for the lecture: Define isoenzymes and explain their structural and functional characteristics. Understand the biochemical and clinical significance of isoenzymes. Explore examples of clinically important isoenzymes and their diagnostic applications. Relate isoenzyme analysis to medical conditions and therapeutic monitoring. Isoenzymes Isoenzymes (or isozymes) are structurally distinct forms of the same enzyme that catalyse the same chemical reaction but differ in: Amino acid sequence Physicochemical properties (e.g., pH stability, electrophoretic mobility) Key Features: 1) Tissue Specificity: Different isoenzymes are expressed in specific tissues, allowing tailored metabolic regulation. 2) Kinetic Properties: Isoenzymes exhibit variations in substrate affinity (Km​) and reaction velocity (Vmax​). 3) Regulatory Mechanisms: Isoenzymes differ in sensitivity to allosteric effectors or inhibitor. Let us try to analyze this with an Isoenzyme Pair (Hexokinase and Glucokinase) Feature Hexokinase Glucokinase Ubiquitous: Found in most tissues, Tissue Expression Liver and pancreatic beta cells. including muscle and brain. Low Km​ (~0.1 mM): High affinity for High Km​ (~10 mM): Low affinity for Affinity for Glucose (Km​) glucose. glucose. Active at low glucose concentrations, Active at high glucose concentrations Function ensuring glucose is utilized even during (postprandial), promoting glucose storage fasting. as glycogen in the liver. Inhibited by its product, glucose-6- Not inhibited by glucose-6-phosphate, Regulation phosphate. (Product) allowing continuous glucose processing. Provides a constant supply of glucose-6- Acts as a glucose sensor in the pancreas Physiological Role phosphate for energy production in and regulates glycogen synthesis in the essential tissues. liver. Ensures survival by maintaining energy Regulates blood glucose levels and Adaptive Significance production under low glucose conditions. prevents hyperglycemia after meals. Glucokinase Mutations: Associated Hexokinase Deficiency: Causes with MODY (Maturity-Onset Diabetes of Clinical Relevance nonspherocytic hemolytic anemia due to the Young), affecting blood glucose insufficient energy in red blood cells. regulation. LDH Structure and the Isoenzymes Lactate Dehydrogenase (LDH) is a tetrameric enzyme, meaning it is composed of four subunits. These subunits are of two types: Subunit A (Red): Often referred to as the M (muscle) subunit. Subunit B (Green): Often referred to as the H (heart) subunit. The combination of these subunits determines the isoenzyme type: LDH-1 (H4): Composed of four H subunits (green). LDH-2 (H3M): Three H subunits and one M subunit. LDH-3 (H2M2): Two H subunits and two M subunits. LDH-4 (HM3): One H subunit and three M subunits. LDH-5 (M4): Composed of four M subunits (red). Feature LDH-1 (H4) LDH-2 (H3M) LDH-3 (H2M2) LDH-4 (HM3) LDH-5 (M4) Tissue Reticuloendothelial Kidney, Heart, red blood cells Lungs Skeletal muscle, liver Expression system pancreas H2M2 (2 heart, HM3 (1 heart, Subunit H4 (4 heart-type H3M (3 heart, 1 M4 (4 muscle-type 2 muscle 3 muscle Composition subunits) muscle subunit) subunits) subunits) subunits) Optimal Aerobic conditions Aerobic conditions Intermediate Intermediate Anaerobic conditions Conditions High affinity for Low affinity for Kinetic Moderate affinity for pyruvate, slower Intermediate Intermediate pyruvate, faster Properties pyruvate turnover turnover Converts lactate to Supports Facilitates Balance between Converts pyruvate to pyruvate in tissues glycolysis in glycolysis in Physiological aerobic and lactate in anaerobic with high oxygen moderately less Role anaerobic tissues (e.g., skeletal availability (e.g., oxygenated oxygenated metabolism. muscle). heart). tissues. tissues. Associated Marker for myocardial Elevated in lung Marker for liver Clinical Seen in some with kidney or infarction (elevated diseases or diseases or skeletal Relevance hemolytic anemias. pancreatic levels in serum). leukemia. muscle damage. injury. Flipped LDH Profile Normal LDH Profile LDH-2 > LDH-1 > LDH-3 > LDH-4 > LDH-5 LDH-2 (reticuloendothelial system and red blood cells) is typically the most abundant isoenzyme in healthy individuals. LDH-1 (predominantly in the heart) is present in lower amounts under normal conditions. Flipped LDH Profile A flipped LDH profile refers to a situation where the relative levels of LDH-1 and LDH-2 in the blood are reversed compared to their normal proportions. Normally, LDH-2 is more abundant than LDH-1 in the bloodstream, but in certain pathological conditions, LDH-1 becomes more prominent, resulting in a "flip" of the ratio. Causes of Flipped LDH Profile A flipped LDH profile is most commonly associated with conditions that cause significant heart damage or hemolysis. These include: 1.Acute Myocardial Infarction (Heart Attack): a) LDH-1 levels increase due to damage to cardiac muscle, where LDH-1 is highly expressed. b) This results in LDH-1 > LDH-2, indicating myocardial injury. c) Typically, the flipped profile appears 12–24 hours after a heart attack and persists for several days. Creatine kinase and its Isozymes Creatine kinase (CK) is an enzyme that plays a crucial role in energy metabolism, particularly in tissues with high and fluctuating energy demands such as muscles, brain, and heart. I ts main function is to catalyze the reversible conversion of creatine phosphate and adenosine diphosphate (ADP) into creatine and adenosine triphosphate (ATP). Tissue Function Skeletal Muscle Provides energy for muscle contraction during exercise or strenuous activity. Cardiac Muscle Supports continuous energy demands of the heart, especially under stress or ischemia. Brain Maintains ATP levels for ion gradients, neurotransmission, and cognitive functions. Smooth Muscle Supplies energy for contractile activity in organs like the gastrointestinal tract. Feature CK-MM (Muscle Type) CK-MB (Cardiac Type) CK-BB (Brain Type) Tissue Cardiac muscle (also trace in skeletal Skeletal muscle Brain, smooth muscle, GI tract Expression muscle) Subunit M1B1 (One muscle and one brain M2 (Two muscle subunits) B2 (Two brain subunits) Composition subunit) Energy metabolism in cardiac Physiological Provides energy for muscle contraction Energy metabolism in the brain and smooth muscle, especially during stress or Role during physical activity muscle injury Normal Serum Low (