Read Enzyme Kinetics from Barrier to Rate
Read enzyme questions by separating barrier-lowering, the initial rate and Km-dependent substrate response.
Chemistry questions first require the material, particles or reaction condition to be identified before a formula or rule is selected.
Reaction Coordinate
Enzyme-substrate complex changes barrier not equilibrium. An enzyme provides an alternative reaction pathway with a lower activation free energy. It does not supply energy to the substrate and does not change the overall free-energy difference between substrate and product. Therefore the fundamental correct statement is that it lowers activation energy.
Enzyme saturation and inhibition
A low Km relative to physiological substrate concentration means an enzyme is near saturation and its rate changes little as substrate varies. A competitive inhibitor raises apparent Km but does not lower Vmax: sufficiently high substrate can outcompete it. Read both the Km comparison and inhibitor mechanism before predicting rate.
Measure the region that answers the question
For an initial reaction rate, calculate the slope only from the early straight portion of a product-versus-time curve, before substrate depletion or product accumulation bends it. Km is the substrate concentration associated with half Vmax: a higher Km means lower apparent substrate affinity and more substrate is needed to approach Vmax. When similar-Vmax enzymes compete for a substrate, the lower-Km enzyme is more nearly saturated at the same substrate concentration and is the likely preferred pathway.
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- ✓Enzyme-substrate complex changes barrier not equilibrium
- ✓Initial rate: use the early linear part of the progress curve.
Read the conditions in the stem carefully and use the stated model, labels, or assumptions before choosing an answer.
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