Reaction Velocity Calculators

What Is Reaction Velocity?

Reaction velocity (v) is the rate at which a substrate is converted to product by an enzyme, expressed in units of concentration per time — typically μmol/min or nmol/L/s. It is not a fixed value: it changes depending on how much substrate is present, how active the enzyme is, and the conditions of the reaction.

At low substrate concentrations, velocity increases nearly linearly as more substrate becomes available. As concentration rises and enzyme active sites become saturated, velocity plateaus and approaches a maximum — a ceiling known as Vmax.

The Michaelis-Menten Equation

The standard model for enzyme reaction velocity is the Michaelis-Menten equation:

v = (Vmax × [S]) / (Km + [S])

• v — reaction velocity at a given substrate concentration [S]
• Vmax — maximum velocity when all enzyme active sites are occupied
• Km — Michaelis constant; the substrate concentration at which v = Vmax/2
• [S] — current substrate concentration

Km reflects the enzyme's affinity for its substrate. A low Km means the enzyme reaches half-maximal speed at a low substrate concentration — strong binding. A high Km means weaker binding and requires more substrate to achieve the same effect.

Initial Velocity — Why It's Used

In practice, researchers measure initial velocity (v₀) — the reaction rate at the very start, before significant product builds up or substrate is depleted. During this early linear phase, reverse reactions and product inhibition are negligible, making kinetic analysis straightforward and accurate.

Initial velocity is typically measured spectrophotometrically by tracking absorbance change over time (e.g., NADH at 340 nm). The slope of the linear portion, converted using the Beer-Lambert law, gives v₀ directly.

What Affects Reaction Velocity?

• Substrate concentration: Increases v hyperbolically until Vmax is reached
• Enzyme concentration: More enzyme raises Vmax proportionally
• Temperature: Velocity roughly doubles per 10°C rise, until denaturation occurs
• pH: Each enzyme has an optimal pH; deviations reduce activity
• Inhibitors: Competitive inhibitors raise apparent Km; noncompetitive inhibitors reduce Vmax
• Cofactors and metal ions: Required by many enzymes for full catalytic activity

Types of Enzyme Inhibition and Their Effect on Velocity

Inhibitors reduce reaction velocity, but in different ways depending on their mechanism:

• Competitive inhibition: Inhibitor binds the active site. Apparent Km increases, Vmax unchanged. High substrate can overcome inhibition.
• Noncompetitive inhibition: Inhibitor binds elsewhere on the enzyme. Vmax decreases, Km unchanged. Substrate cannot overcome this.
• Uncompetitive inhibition: Inhibitor binds only to the enzyme-substrate complex. Both Km and Vmax decrease proportionally.

Lineweaver-Burk Plot

Before nonlinear regression software was standard, the Lineweaver-Burk (double-reciprocal) plot was used to linearize Michaelis-Menten data. Plotting 1/v vs 1/[S] gives a straight line where the slope = Km/Vmax and the y-intercept = 1/Vmax. While it's still useful for visualizing inhibition type, nonlinear fitting is now the preferred method for accurate parameter estimation.