Biochemistry
Enzymes
Enzymes for MBBS: Michaelis-Menten kinetics, Km and Vmax, inhibitor types, isoenzymes and serum enzyme markers, mapped to NMC codes BI2.1 to BI2.7.
MedNext Academy | 3 min read
Enzymes
Enzymes for MBBS: Michaelis-Menten kinetics, Km and Vmax, inhibitor types, isoenzymes and serum enzyme markers, mapped to NMC codes BI2.1 to BI2.7.
The enzyme chapter covers how proteins catalyse the reactions of metabolism and how their activity is measured and controlled. It runs from the Michaelis-Menten model and inhibitor kinetics to isoenzymes, allosteric regulation and the serum enzymes used as clinical markers of tissue damage.
High-yield: Enzymes
- Enzymes are biological catalysts that lower activation energy without altering the equilibrium of a reaction.
- The Michaelis constant Km is the substrate concentration at which reaction velocity is half of Vmax; a low Km means high affinity for substrate.
- The Lineweaver-Burk plot is the double-reciprocal of the Michaelis-Menten equation and gives straight lines that distinguish inhibitor types.
- Competitive inhibitors raise the apparent Km but leave Vmax unchanged, since more substrate can outcompete them.
- Non-competitive inhibitors lower Vmax and leave Km unchanged, as they bind away from the active site.
- Uncompetitive inhibitors lower both Km and Vmax and bind only to the enzyme-substrate complex.
- Isoenzymes are different molecular forms of the same enzyme, such as the five lactate dehydrogenase and the creatine kinase MB, MM and BB forms.
- Creatine kinase-MB rises in myocardial infarction; cardiac troponins are the more specific and sensitive markers now used.
- A raised alanine aminotransferase and aspartate aminotransferase point to hepatocellular injury; ALT is the more liver-specific.
- Alkaline phosphatase rises in cholestatic liver disease, bone disease such as Paget disease, and in pregnancy.
- Amylase and lipase rise in acute pancreatitis; lipase stays elevated longer and is more specific.
- Coenzymes are organic cofactors, many derived from vitamins, such as NAD from niacin and FAD from riboflavin.
- Allosteric enzymes give a sigmoid curve, not a hyperbolic one, and are regulated by effectors at sites other than the active site.
- Organophosphate poisons irreversibly inhibit acetylcholinesterase; the antidote pralidoxime reactivates the enzyme if given early.
- Enzyme activity depends on optimum pH and temperature, and denaturation above the optimum permanently abolishes catalysis.
Enzyme inhibitor patterns
- **Competitive:** Km rises, Vmax unchanged. Binds the active site; overcome by more substrate.
- **Non-competitive:** Vmax falls, Km unchanged. Binds a separate site.
- **Uncompetitive:** Both Km and Vmax fall. Binds only the enzyme-substrate complex.
- **Irreversible:** Covalent inactivation, as with organophosphates on acetylcholinesterase.
NMC competencies in this chapter
- **BI2.1:** Concepts of enzyme, isoenzyme, coenzyme and cofactor
- **BI2.3:** Basic principles of enzyme activity and kinetics
- **BI2.4:** Enzyme inhibitors as poisons, drugs and therapeutic enzymes
- **BI2.5:** Clinical utility of serum enzymes as markers of disease
- **BI2.6:** Use of enzymes in laboratory investigations
- **BI2.7:** Interpretation of enzyme activities as markers of pathological conditions
Frequently Asked Questions
What does a low Km tell you about an enzyme?
A low Km means the enzyme reaches half-maximal velocity at a low substrate concentration, which reflects a high affinity for its substrate.
How do competitive and non-competitive inhibitors differ on kinetics?
Competitive inhibitors raise the apparent Km but do not change Vmax, because extra substrate can displace them. Non-competitive inhibitors lower Vmax without changing Km, since they bind away from the active site.
Which enzymes are used to diagnose a heart attack?
Creatine kinase-MB and, more importantly, the cardiac troponins. Troponins are the most sensitive and specific markers and have largely replaced older enzyme tests.
Why are isoenzymes clinically useful?
Different tissues express different isoenzyme forms, so the pattern in serum can localise the damaged organ, as with the lactate dehydrogenase and creatine kinase isoenzymes.
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