The present study investigates effects of rising atmospheric CO2 concentration on protein composition of maize, wheat, and barley grain, especially on the fractions prolamins and glutelins. Cereals were grown at different atmospheric CO2 concentrations to simulate future climate conditions. Influences of two nitrogen fertilization levels were studied for wheat and barley. Enriched CO2 caused an increase of globulin and B-hordein of barley. In maize, the content of globulin, α-zein, and LMW polymers decreased, whereas total glutelin, zein, δ-zein, and HMW polymers rose. Different N supplies resulted in variations of barley subfractions and wheat globulin. Other environmental influences showed effects on the content of nearly all fractions and subfractions. Variations in starch-protein bodies caused by different CO2 treatments could be visualized by scanning electron microscopy. In conclusion, climate change would have impacts on structural composition of proteins and, consequently, on the nutritional value of cereals.