The high rates of photosynthesis and the carbon-concentrating mechanism (CCM) in C-4 plants are initiated by the enzyme phosphoenolpyruvate (PEP) carboxylase (PEPC). The flow of inorganic carbon into the CCM of C-4 plants is driven by PEPC’s affinity for bicarbonate (K-HCO3), which can be rate limiting when atmospheric CO2 availability is restricted due to low stomatal conductance. We hypothesize that natural variation in K-HCO3 across C-4 plants is driven by specific amino acid substitutions to impact rates of C-4 photosynthesis under environments such as drought that restrict stomatal conductance. To test this hypothesis, we measured K-HCO3 from 20 C-4 grasses to compare kinetic properties with specific amino acid substitutions. There was nearly a twofold range in K-HCO3 across these C-4 grasses (24.3 +/- 1.5 to 46.3 +/- 2.4 mu m), which significantly impacts modeled rates of C-4 photosynthesis. Additionally, molecular engineering of a low-HCO3- affinity PEPC identified key domains that confer variation in K-HCO3. This study advances our understanding of PEPC kinetics and builds the foundation for engineering increased-HCO3- affinity and C-4 photosynthetic efficiency in important C-4 crops.