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?
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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.
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The maximum velocity of an enzyme-catalyzed reaction, reached when all enzyme active sites are saturated with substrate.- The Michaelis-Menten equation is ____
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v = (Vmax * [S]) / (Km + [S])- A low Km value indicates what about an enzyme?
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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?
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No change; the rate remains at Vmax because all active sites are occupied.- What is kcat (turnover number)?
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The number of substrate molecules converted to product per enzyme molecule per unit time at saturating substrate concentration.- Define catalytic efficiency.
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The ratio kcat / Km; measures how well an enzyme discriminates between substrate and other molecules.- How does competitive inhibition affect Km and Vmax?
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Increases apparent Km (requires more substrate to reach half-Vmax); Vmax is unchanged.- How does non-competitive inhibition affect Km and Vmax?
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Decreases Vmax; Km remains unchanged.- What is a Lineweaver-Burk plot used for?
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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?
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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?
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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?
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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?
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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?
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Allosteric binding of effector molecules to sites other than the active site causes cooperative binding of substrate; this increases the Hill coefficient above 1.
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