Enzymes and Regulation of Protein Function Quiz

See Graphs of enzyme kinetics. Option C is correct. If you plot the velocity (rate) of an enzyme-catalysed reaction against the substrate concentration, the curve rises and then levels off at a ceiling called the maximal velocity, or Vmax, reached when every active site is occupied (saturated). Km is the substrate concentration that gives half of Vmax. A low Km therefore means that only a small amount of substrate is needed to get the enzyme working at half its top speed, which is usually read as a sign of high affinity, meaning the enzyme grips its substrate tightly. A high Km means the reverse: the enzyme needs plenty of substrate before it reaches half its maximum, so its affinity is lower. Km is only an indicator of affinity, not a direct measure of it; it also reflects other aspects of the reaction kinetics, meaning the speeds of the individual steps. The characteristic curve for enzymes that obey the Michaelis-Menten model is a rectangular hyperbola, a curve that climbs steeply and then flattens.

• Meaning of Km: Km is the substrate concentration that gives half of Vmax.
• Low Km: only a little substrate is needed to reach half of top speed, which is usually read as high affinity.
• High Km: more substrate is needed, so the affinity is lower.
• Limit: Km is an indicator of affinity, not a direct measure of it.