The catabolism of trans-4-hydroxy-L-proline (HPRO) and proline (L-Pro) (from endogenous and dietary sources of collagen) makes a significant contribution to the glyoxylate pool in humans. The dehydrogenation (oxidation) of HPRO/L-Pro to L-1-pyrroline-5-carboxylate (1PYR-5COOH) coupled to the conversion of FAD to FADH2 is the first step in proline catabolism. Proline dehydrogenases (PRODHs), located at the inner mitochondrial membrane mediate this reaction. Whereas PRODH prefers L-Pro as substrate, PRODH2 has a clear preference for HPRO (Summitt et al. 2015).