The pathways by which allantoin is processed and converted to ureidoglycolate, and then into metabolically useful substrates. E. coli are able to utilize allantoin as a sole nitrogen source under anaerobic conditions by converting it to ureidoglycolate; this may be further metabolized to produce glyoxylate and thence 3-phosphoglycerate, or alternatively oxidized to oxolureate, which can converted into oxamate and carbamoylphosphate. This may then be further metabolized to CO2, NH4+ and ATP.