Potassium is a macroelement essential to many aspects of plant life, such as photosynthesis, phloem transport or cellular electrochemistry. Many enzymes in animals or microbes are known to be stimulated or activated by potassium (K+ ions). Several plant enzymes are also strictly K+-dependent, and this can be critical when plants are under K deficiency and thus intracellular K+ concentration is low. Although metabolic effects of low K conditions have been documented, there is presently no review focusing on roles of K+ for enzyme catalysis or activation in plants. In this mini-review, we compile the current knowledge on K+-requirement of plant enzymes and take advantage of structural data to present biochemical roles of K+. This information is instrumental to explain direct effects of low K+ content on metabolism and this is illustrated with recent metabolomics data.