Catalyzes the synthesis of GDP-perosamine from GDP-4-keto-6-deoxy-D-mannose and L-glutamate. Can use only L-glutamate as amino donor. In vitro, can also use GDP-4-keto-3,6-dideoxymannose to produce GDP-3-deoxyperosamine. Involved in the formation of S-LPS, which is required for attachment of the protein S-layer to the outer membrane surface. 2-oxoglutarate + GDP-alpha-D-perosamine = GDP-4-dehydro-alpha-D-rhamnose + L-glutamate kcat is 2.7 sec(-1) for GDP-4-keto-6-deoxy-D-mannose. kcat is 0.015 sec(-1) for GDP-4-keto-3,6-dideoxymannose. Bacterial outer membrane biogenesis; LPS O-antigen biosynthesis. Homodimer. Mutant produces altered S-LPS. Belongs to the DegT/DnrJ/EryC1 family. Truncated N-terminus.