Catalyzes the two-step oxidative conversion of choline to glycine-betaine with betaine aldehyde as an intermediate. Glycine-betaine accumulates to high levels in the cytoplasm of cells to prevent dehydration and plasmolysis in adverse hyperosmotic environments. Accepts either choline or the reaction intermediate betaine-aldehyde as substrate. choline + H2O + 2 O2 = glycine betaine + H(+) + 2 H2O2 kcat is 13.4 sec(-1) with choline as substrate and 11.6 sec(-1) with betaine aldehyde as substrate. Amine and polyamine biosynthesis; betaine biosynthesis via choline pathway; betaine from choline: step 1/1. Homodimer. Mass of the protein with one linked FAD moiety. Belongs to the GMC oxidoreductase family.