Core MCAT biochemistry: enzyme kinetics, protein structure levels, and central metabolism concepts.
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- What is the Michaelis constant (Km) in enzyme kinetics?
- The substrate concentration at which the enzyme reaction rate is half of Vmax; approximates the dissociation constant of the enzyme-substrate complex.
- Define Vmax in enzyme kinetics.
- The maximum velocity of an enzyme-catalyzed reaction, reached when all enzyme active sites are saturated with substrate.
- The Michaelis-Menten equation is ____
- v = (Vmax * [S]) / (Km + [S])
- A low Km value indicates what about an enzyme?
- High affinity for its substrate; the enzyme reaches half-Vmax at low substrate concentration.
- How does increasing substrate concentration affect reaction rate after the enzyme is saturated?
- No change; the rate remains at Vmax because all active sites are occupied.
- What is kcat (turnover number)?
- The number of substrate molecules converted to product per enzyme molecule per unit time at saturating substrate concentration.
- Define catalytic efficiency.
- The ratio kcat / Km; measures how well an enzyme discriminates between substrate and other molecules.
- How does competitive inhibition affect Km and Vmax?
- Increases apparent Km (requires more substrate to reach half-Vmax); Vmax is unchanged.
- How does non-competitive inhibition affect Km and Vmax?
- Decreases Vmax; Km remains unchanged.
- What is a Lineweaver-Burk plot used for?
- A double reciprocal plot (1/v vs 1/[S]) used to determine Km and Vmax, and to distinguish types of enzyme inhibition.
- On a Lineweaver-Burk plot, how does competitive inhibition appear?
- Increased x-intercept (less negative, i.e., closer to zero, because apparent Km is higher so 1/Km is lower) and same y-intercept (same 1/Vmax); lines intersect on the y-axis.
- What conditions must be met for Michaelis-Menten kinetics to apply?
- Enzyme concentration is much lower than substrate concentration, steady-state assumption holds, and product concentration is negligible.
- When does an enzyme-catalyzed reaction exhibit zero-order kinetics?
- When substrate concentration is very high and the enzyme is saturated; reaction rate is independent of substrate concentration.
- When does an enzyme-catalyzed reaction exhibit first-order kinetics?
- When substrate concentration is very low (much less than Km); reaction rate depends directly on substrate concentration.
- Why do allosteric enzymes exhibit sigmoidal (S-shaped) kinetic curves rather than hyperbolic curves?
- Allosteric binding of effector molecules to sites other than the active site causes cooperative binding of substrate; this increases the Hill coefficient above 1.