This domain is found in Proline utilization A (PutA) proteins present in Geobacter sulfurreducens. PutA are bifunctional peripheral membrane flavoenzymes that catalyze the oxidation of l-proline to l-glutamate and couple the oxidation of imported proline imported to the reduction of membrane-associated quinones. This domain is located at the N-terminus and is referred to as the alpha domain. The hydrocarbon tail of Zwittergent 3-12 binds to an exposed hydrophobic patch of the alpha domain which contains aromatic and nonpolar residues. The domain may be involved in membrane association [1].

1: Structures of the PutA peripheral membrane flavoenzyme reveal a dynamic substrate-channeling tunnel and the quinone-binding site. Singh H, Arentson BW, Becker DF, Tanner JJ; Proc Natl Acad Sci U S A. 2014;111:3389-3394. PMID:24550478