Enzyme velocity and reaction order

To measure the rate of reactions involving enzymes, we often utilize a graph representing concentration changes over time for reactants and products. For a simple reaction: A + B → P (product) with time on the x-axis and concentration on the y-axis, we track the decrease in concentrations of reactants A and B and the increase in product P. Typically, we would measure these changes using a method like spectrophotometry, which allows us to observe changes in absorbance corresponding to concentration changes of either reactant or product.

The rate of reaction can be expressed as:

$ext{Rate} ext{ } \beta ext{ } [A]^f [B]^g$

Where:

• [A] and [B] are the concentrations of the reactants,

• f and g denote the reaction orders for A and B respectively, which must be determined experimentally.

Understanding Reaction Order:

• The concept of reaction order assesses how the rate of reaction changes with varying concentrations of reactants.

• When experimentally testing, if the concentration of A is doubled and the rate also doubles, we determine that A is a first order reactant (f=1).

• If doubling the concentration of another reactant results in quadrupling the rate, it confirms that this reactant is of second order (g=2).

• Conversely, if the rate remains unchanged as we alter reactant concentration, it indicates zero order for that reactant (0).

Implications in Industrial Applications:

• Knowing reaction orders helps optimize conditions for reactions, thereby enhancing efficiency. For instance, if A is second order and B is zero order, increasing B’s concentration won’t affect the reaction rate, but increasing A’s will.

Initial Velocity Measurement:

• The initial velocity (V₀) of an enzyme reaction is typically measured within the first 60 seconds to avoid complications from reverse reactions, ensuring clarity in rate measurements.

• As substrate concentration increases, reaction velocity also increases until it reaches a plateau, which indicates enzyme saturation — at this point, the reaction operates at zero order kinetics where additional substrate doesn't affect the rate.

This framework will guide our exploration of enzyme kinetics throughout the topic.