Catalyzes the oxidation of glycolate to glyoxylate, with a reduction of O2 to H2O2 (PubMed:25416784). Is a key enzyme in photorespiration in green plants (Probable). glycolate + O2 = glyoxylate + H2O2 kcat is 29.86 sec(-1) (at pH 8 and 30 degrees Celsius). Photosynthesis; photorespiration; glycine from 2-phosphoglycolate: step 2/3. Homotetramer. A number of isoforms are produced. According to EST sequences. Belongs to the FMN-dependent alpha-hydroxy acid dehydrogenase family. Truncated N-terminus.