First product of photorespiration is

The first product of photorespiration is Phosphoglycolate.
Photorespiration refers to a process in plant metabolism where the enzyme RuBisCO oxygenates RuBP, wasting some of the energy produced by photosynthesis.
Addition of molecular oxygen to ribulose-1,5-bisphosphate produces 3-phosphoglycerate (PGA) and 2-phosphoglycolate (2PG, or PG). PGA is the normal product of carboxylation and productively enters the Calvin cycle.
Phosphoglycolate, however, inhibits certain enzymes involved in photosynthetic carbon fixation.