Significance of V_max:

• The values of V_max vary widely for different enzymes and can be used as an indicator of an enzymes catalytic efficiency. It does not find much clinical use.
• There are some enzymes that have been shown to have the following reaction sequence:

  

• In this situation, the formation of product is dependent on the breakdown of an enzyme-product complex, and is thus the rate-limiting step defined by k₃.

Definition and Significance of k_cat:

• The constant, k_cat (sec^-1), is also called the turnover number because under saturating substrate conditions, it represents the number of substrate molecules converted to product in a given unit of time on a single enzyme molecule. In practice, k_cat values (not V_max) are most often used for comparing the catalytic efficiencies of related enzyme classes or among different mutant forms of an enzyme.
• k_cat/K_m is the specificity constant - used to rank an enzyme according to how good it is with different substrates.

  

• Upper limit for k_cat/K_m is rate of diffusion (109 M^-1s^-1)
• Units on k₂ are amount product per amount of enzyme per unit time (also called the “turnover number”). Units on E₀ are amount of enzyme (moles, grams, units) per unit volume; Km has the same units as [S] (mole/liter)].

3.7.1.2. Experimental Rate Parameters of Michaelis-Menten Kinetics