When both K m and V max are altered, Is the enzyme inhibited or activated?

Enzyme activators lower K m (the Michaelis constant) and/or raise V max (the asymptotic reaction velocity at infinite substrate concentration); conversely, inhibitors raise K m and/or lower V max . But what if an effector moves both K m and V max in the same direction? Uncompetitive inhibitors, which decrease both K m and V max by the same factor, are the most common example of this. A less well‐known example occurs often when crowding agents are added to the buffer in order to mimic the environment commonly encountered in vivo . Crowding agents tested on different enzymes have been shown to have every conceivable effect on K m or V max , causing them to rise, fall, or stay the same. In this article, a mathematical analysis is presented allowing biochemists to judge whether an effector that causes K m and V max to both move in the same direction serves as an inhibitor, an activator, or most surprising, as both. © 2019 International Union of Biochemistry and Molecular Biology, 47(4):446–449, 2019.