1.B.26 The Cyclodextrin Porin (CDP) Family
A single functionally characterized protein, the CymA cyclodextrin porin of Klebsiella oxytoca, comprises the CDP family. Cyclodextrins (CDs) are cylindrically shaped oligosaccharides made up of six (α-CD), seven (β-CD) or eight (γ-CD) glucosyl residues. The hydroxyl groups border the outside of the ring forming water soluble molecules with hydrophobic cavities. They are formed from starch by microorganisms in reactions catalyzed by cyclodextrin glucanotransferases which are secreted into the medium where they make the cyclodextrins that can be taken up and used as sources of carbon and energy. They cross the outer bacterial membrane via the CD porin, CymA of K. oxytoca. This porin can also transport linear maltodextrins (Pajatsch et al., 1999).
CymA is not recognizably homologous to other characterized porins, but it resembles them in that (1) it is rich in antiparallel β-sheets, (2) it is a component of the outer membrane, and (3) it is processed from a precursor having a signal peptide. CymA, reconstituted in lipid bilayers, conducts ions blocked by the presence of cyclodextrins (CDs) (KB = 28 μM) Cyclodextrins bind with high stability, but straight chained (linear) maltooligosaccharides also bind (Orlik et al., 2003). It forms a homotetramer with a central pore, and therefore lacks the typical trimeric structure of most porins. The CDP family is distantly related to the OmpG and OmpL porins in the OmpG family (1.B.21).
The generalized transport reaction catalyzed by CymA is:
Cyclodextrin (out) or linear maltodextrin (out) cyclodextrin (in) or linear maltodextrin (in)