Biochemistry and Clinical Pathology
Biochemistry and clinical pathology test biomolecules, enzyme kinetics, metabolism and laboratory diagnosis. The exam expects interpretation of common biochemical reports.
Core rules
- Enzymes lower activation energy. Km is the substrate concentration at half-maximal velocity; a lower Km means higher affinity. Competitive inhibitors increase Km but leave Vmax unchanged; noncompetitive inhibitors reduce Vmax.
- Glycolysis yields net 2 ATP per glucose; aerobic oxidation yields far more ATP through the Krebs cycle and oxidative phosphorylation. Insulin lowers blood glucose; glucagon raises it.
- Clinical pathology uses HbA1c, lipid profile, liver enzymes, bilirubin, urea and creatinine. HbA1c reflects average glycaemia over about 2 to 3 months.
Worked example
A fasting lipid profile gives total cholesterol 200 mg/dL, HDL 50 mg/dL and triglycerides 150 mg/dL. With Friedewald, LDL = total cholesterol − HDL − triglycerides/5 = 200 − 50 − 30 = 120 mg/dL. The formula is unreliable when triglycerides exceed 400 mg/dL.
Traps
Do not confuse Km with Vmax. Enzyme units and reference ranges vary by laboratory. A normal random glucose does not exclude diabetes. Bilirubin and liver enzymes indicate different aspects of hepatobiliary disease.