Lecture 7 - Isoenzymes 2024/25

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 isoenz...

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. us try to analyze this with an Isoenzyme Pair (Hexokinase and Glucokin 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 high glucose Active at low glucose concentrations (postprandial), Function concentrations, ensuring glucose is promoting glucose storage as utilized even during fasting. glycogen in the liver. Not inhibited by glucose-6- Inhibited by its product, glucose-6- Regulation phosphate, allowing continuous phosphate. (Product) glucose processing. Provides a constant supply of Acts as a glucose sensor in the Physiological Role glucose-6-phosphate for energy pancreas and regulates glycogen production in essential tissues. synthesis in the liver. Ensures survival by maintaining Regulates blood glucose levels and Adaptive Significance energy production under low prevents hyperglycemia after meals. glucose conditions. Hexokinase Deficiency: Causes Glucokinase Mutations: nonspherocytic hemolytic anemia Associated with MODY (Maturity- Clinical Relevance due to insufficient energy in red Onset Diabetes of the Young), blood cells. affecting blood glucose 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).  LDH-3 LDH-4 Feature LDH-1 (H4) LDH-2 (H3M) LDH-5 (M4) (H2M2) (HM3) Tissue Heart, red blood Reticuloendothelia Kidney, Skeletal muscle, Lungs Expression cells l system pancreas liver H2M2 (2 HM3 (1 Subunit H4 (4 heart-type H3M (3 heart, 1 heart, 2 heart, 3 M4 (4 muscle-type Composition subunits) muscle subunit) muscle muscle subunits) subunits) subunits) Optimal Anaerobic Aerobic conditions Aerobic conditions Intermediate Intermediate Conditions conditions High affinity for Low affinity for Kinetic Moderate affinity pyruvate, slower Intermediate Intermediate pyruvate, faster Properties for pyruvate turnover turnover Converts lactate to Supports Facilitates Converts pyruvate Balance between pyruvate in tissues glycolysis in glycolysis in to lactate in Physiologic aerobic and with high oxygen moderately less anaerobic tissues al Role anaerobic availability (e.g., oxygenated oxygenated (e.g., skeletal metabolism. heart). tissues. tissues. muscle). Marker for Associated Seen in some Elevated in Marker for liver Clinical myocardial with kidney hemolytic lung diseases diseases or skeletal Relevance infarction (elevated or pancreatic anemias. or leukemia. muscle damage. levels in serum). 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 Causes "flip" of Flipped of the ratio. 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 Provides energy for muscle contraction during exercise or strenuous activity. Muscle Supports continuous energy demands of the heart, especially under stress or Cardiac Muscle ischemia. Maintains ATP levels for ion gradients, neurotransmission, and cognitive Brain 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 muscle Brain, smooth muscle, GI tract Expression skeletal muscle) Subunit M1B1 (One muscle and one M2 (Two muscle subunits) B2 (Two brain subunits) Composition brain subunit) Energy metabolism in cardiac Physiological Provides energy for muscle Energy metabolism in the brain and muscle, especially during stress Role contraction during physical activity smooth muscle or injury Normal High (dominant isoenzyme in Low (