On a Lineweaver-Burk plot of 1/v versus 1/[S], what is the y-intercept?

• A. 1/Vmax
• B. Km
• C. Vmax
• D. 1/[S]

Answer: (A)

In a Lineweaver-Burk plot, you plot 1/v versus 1/[S], which linearizes the Michaelis–Menten equation to a line with slope Km/Vmax and y-intercept 1/Vmax. This comes from starting with v = Vmax[S]/(Km + [S]), then taking reciprocals: 1/v = (Km/Vmax)(1/[S]) + 1/Vmax. So when 1/[S] is zero (that is, at infinite substrate), 1/v equals 1/Vmax. Therefore the y-intercept is 1/Vmax. This value reflects the reciprocal of the maximum velocity, and changes in Vmax shift the intercept accordingly. It’s not Km (which affects the slope) and it’s not Vmax itself (the intercept is the reciprocal of Vmax, not Vmax).