Enzymes: Biochemistry Lecture Notes PDF

Biochemistry Department Enzymes By Dr/ Marwa Sedira Lecturer of Medical Biochemistry & Molecular Biology 2025 ILOs: isoenzymes 1- Define enzymes, substrate, catalytic site and(isozymes) 2- Nomenclature of e...

Biochemistry Department Enzymes By Dr/ Marwa Sedira Lecturer of Medical Biochemistry & Molecular Biology 2025 ILOs: isoenzymes 1- Define enzymes, substrate, catalytic site and(isozymes) 2- Nomenclature of enzymes. 3- General properties of the enzymes. 4- Classification of Enzymes. 5- Examples of clinically important enzymes and isoenzymes 1.Which of the following clinical conditions shows high concentration of aspartate transaminase? a. Acute parotitis b. Myositis c. Myocardial infarction d. Acute pancreatitis 2. Which of the following enzymes is more specific for liver diseases? a. Alkaline Phosphatase b. Alanine Transaminase c. Aspartate Transaminase d. Creatine kinase  Definitions: Enzymes: are protein catalysts that increase the rate of reactions without being changed in the overall process. Substrate: It is the substance or substances upon which an enzyme acts e.g. lactose is the substrate for lactase enzyme. Substrate binding site or catalytic site: It is a special pocket or cleft on the surface of the enzyme that binds specifically to the Isoenzymes (Isozymes): substrate. They are different forms of the same enzyme having the same catalytic activity but differ in their chemical, immunological structure and tissue sources. Nomenclature (Terminology) of enzymes: 1-Some enzymes retain their old name which give no hint of the associated enzymic reaction, for example, trypsin and pepsin. 2-Most commonly used enzyme names have the suffix "- ase" attached to: The substrate of the reaction (e.g. glucosidase acts on glucose and urease acts on urea). To a description of the action performed (for example, lactate dehydrogenase and adenylyl cyclase). General properties of the enzymes: (1) They are protein in nature with few exceptions, so they are water soluble. (2) They are denaturated by ↑ temperature (heat labile), concentrated salt solutions, extremes of PH. General properties of the enzymes: (3) They require optimum temperature and optimum PH to act maximally. (4) They are specific in their action, act on one or two substrates with few exceptions. (5) They remain chemically unchanged during reaction. Classification of Enzymes: Enzymes are classified according to: 1-Functional Plasma Enzymes with the following characters: 1. They have a specific function in the blood. 2. They are present at any time in the plasma. 3. They are present at much higher concentration in the blood than at tissues. 4. Functional plasma enzymes are often synthesized in the liver. 5. Their substrates are present in the blood. 6. E.g., lipoprotein lipase, coagulation factors & fibrinolytic enzymes 2- Non-functional plasma enzymes with the following characters: 1. Have no specific function in blood. 2. Their substrates are absent from the blood. 3. They function mainly intracellular. 4. They are present in the blood in very small concentration in comparison to their tissue levels. 5. Their level increases in the plasma when there is a cellular damage e.g., AST, ALT, CK & ALP. Examples of clinically important enzymes and isoenzymes: 1. Serum Aspartate aminotransferase (AST or GOT) 2. Alanine transaminase (ALT or GPT) 3. Alkaline phosphatase. 4. Creatine kinase (CPK or CK) and its isoenzyme MB. 5. Lactate dehydrogenase (LDH) and its isoenzymes (LDH1 and LDH2).  1.Aspartate transaminase (AST the old name is GOT):  Present in high concentration in the cytosol and mitochondria of the cardiac and skeletal muscle cells and to a lesser extent in the liver, kidney and erythrocytes.  It has two isomers one in the cytosol and one in the mitochondria  Normal level of AST in serum is 5-35 U/L.  It catalyzes the following transamination reaction: Causes of raised AST:  Myocardial infarction (but it is nonspecific).  Acute viral or toxic hepatitis. N.B: AST is more specific for heart diseases as in myocardial infarction. 2.Alanine transaminase (ALT old name is GPT): o ALT is an intracellular enzyme present in the cytosol of the cells. o Present in high concentration in the liver cells and to a lesser extent in the skeletal muscles, kidney and heart. o Normal level is between 5- 40 IU/L It catalyzes the following transamination reaction: Causes of raised plasma ALT activities: 1. Acute viral or toxic hepatitis. 2. Cirrhosis (but may be normal). 3. Liver congestion secondary to congestive heart failure. N.B: ALT is more specific for liver diseases than AST. 3. Alkaline phosphatase enzyme:  It is hydrolase enzyme responsible for removing of phosphate from different chemical compounds.  It has optimal activity at pH 8.  It is secreted in different organs like intestine, kidney, bone, liver and placenta. Causes of raised plasma alkaline phosphatase activity are: Physiological: 1. During the last trimester of pregnancy, the plasma total ALP activity rises due to the contribution of the placental isoenzyme. 2. In preterm infants, plasma total ALP activity is up to five times. 3. In children, the total activity is about 2.5 times more, because of the increased osteoblastic activity in children. Pathological: 1. Jaundice caused by extrahepatic obstruction. 2. Intrahepatic cholestasis 3. Infective hepatitis. 4. Alcoholic hepatitis. 5. Hepatocellular carcinoma. Dramatically high levels of ALP (reach 25 times of upper limit) in: 6. Bone diseases as paget’s disease. 7. Rickets and osteoblastoma. 8. Metastatic carcinoma of bone and in hyperparathyroidism. 4. Creatine phosphokinase (CPK also called CK): Sources: heart, brain, and skeletal muscles. Function: It catalyzes the following reaction:  Isoenzymes of CPK: Creatine kinase consists of two subunits, M and B, which combine to form three isoenzymes.  CK-MM: It is the predominant isoenzyme in the skeletal and cardiac muscle and is detected in the plasma of normal subjects.  CK-MB: It is derived normally from the heart muscle, and less than 5% in the skeletal muscles. It has two isoforms CK-MB1 and CK-MB2.  CK-BB: It is present in high concentrations in the brain and in the smooth muscles of the gastrointestinal and genital tracts.  Causes of raised creatine kinase: Creatine kinase is markedly increase in nearly all patients when injury, inflammation, or necrosis of skeletal or heart muscles occur.  CK-MM: It increases in muscle dystrophy.  CK-MB: markedly increase in myocardial infarction (see biomarkers of myocardial infarction).  CK-BB: It shows marked increase in brain cancer and brain injury. 5. Lactate dehydrogenase (LDH): It catalyzes the conversion of pyruvate to lactate and vice versa in different tissues. Sources: The enzyme is widely distributed with high concentrations in the cells of the liver, heart, skeletal muscles, kidney, brain and erythrocytes. Reference range of total LDH: 100-200 U/L Compostion of lactate dehydrogenase: LDH is formed of 4 subunits i.e. it is a tetramer. The subunits are H and M so it has 5 isoenzymes which are:  LDH-1 (4H): in the heart  LDH-2 (3H1M): in the reticuloendothelial system  LDH-3 (2H2M): in the lungs  LDH-4 (1H3M): in the kidneys  LDH-5 (4M): in the liver and striated muscle Causes of raised plasma total LDH activity are: 1. Myocardial infarction. Normally, LDH2 concentration in blood is greater than LDH1.  If an LDH-1 level is higher than the LDH-2 level, myocardial infarction is suggested.  LDH level will start to increase by 6-12 hours after myocardial infarction, reaches the peak by 24-48 hours and goes back to original level by about 6-8 days. 2. Viral hepatitis. 3. Skeletal muscle diseases. 4. Elevation of LDH-5 occurs after the damage of the liver or skeletal muscle. 1.Which of the following clinical 3. Which of the following enzymes conditions shows high shows increase in bone cancer? concentration of aspartate a.Alkaline Phosphatase transaminase? b.Alanine Transaminase a. Acute parotitis c.Aspartate Transaminase b. Myositis d.Creatine kinase c. Myocardial infarction d. Acute pancreatitis 2. Which of the following enzymes is more specific for liver diseases?4. Enumerate isoenzymes of creatine kinase? a. Alkaline Phosphatase b. Alanine Transaminase c. Aspartate Transaminase d. Creatine kinase 1. C. Myocardial infarction 2.b. Alanine Transaminase 3. a. Alkaline Phosphatase 4. CK-MM CK-MB CK-BB