Allows intracellular utilization of 4-hydroxyproline, one of the major constituents of host collagen, by converting 4-hydroxy-L-proline to 4-hydroxy-D-proline, which can be further metabolized by intracellular 4-hydroxy-D-proline oxidases. trans-4-hydroxy-L-proline = cis-4-hydroxy-D-proline Homodimer. Belongs to the proline racemase family.