The PTS L-Ascorbate (L-Asc) Family
TC# 4.A.7
includes porters specific for

L-ascorbate Vitamin C (also known as ascorbic acid and ascorbate) is a water-soluble vitamin found in citrus and other fruits and vegetables, also sold as a dietary supplement and as a topical 'serum' ingredient to treat melasma (dark pigment spots) ...

, and is part of the

PTS-AG superfamily Permease of phosphotransferase system (or PTS-AG superfamily according to TCDB) is a superfamily of phosphotransferase enzymes that facilitate the transport of L-ascorbate (A) and galactitol Galactitol (dulcitol) is a sugar alcohol, the red ...

. A single PTS permease of the L-Asc family of PTS permeases has been functionally characterized. This is the SgaTBA system, renamed UlaABC (utilization of L-ascorbate) by Yew and Gerlt.

The SgaTBA System

The SgaTBA permease consists of three proteins: SgaT, SgaB, and SgaA. SgaT is a 12 TMS protein, possibly very distantly related to the MFS hexuronate permease of '' Escherichia coli''
TC# 2.A.1.14.2
, which presumably functions as a PTS IIC protein. This gene product, as well as SgaB and SgaA, are all essential for anaerobic L-ascorbate utilization, transport and

phosphorylation In chemistry, phosphorylation is the attachment of a phosphate group to a molecule or an ion. This process and its inverse, dephosphorylation, are common in biology and could be driven by natural selection. Text was copied from this source, wh ...

. This is the first documented example where the two sugar-specific energy-coupling proteins of a PTS permease are more closely related to the proteins of two different families. The ''sga'' regulation is controlled by the nearby YjfQ repressor.

Homology

Homologues of SgaT, like other PTS protein homologues, have been identified in a large number of evolutionarily divergent bacteria, but not in

archaea Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archaebac ...

eukaryote Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...

s. Bacteria that encode SgaT homologues include numerous Gram-negative

Pseudomonadota Pseudomonadota (synonym Proteobacteria) is a major phylum of Gram-negative bacteria. The renaming of phyla in 2021 remains controversial among microbiologists, many of whom continue to use the earlier names of long standing in the literature. The ...

, as well as many low and high G+C

Gram-positive bacteria In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall. Gram-positive bact ...

. Except for species of ''

Corynebacterium ''Corynebacterium'' () is a genus of Gram-positive bacteria and most are aerobe, aerobic. They are bacillus (shape), bacilli (rod-shaped), and in some phases of life they are, more specifically, club (weapon), club-shaped, which inspired the gen ...

Streptomyces ''Streptomyces'' is the largest genus of Actinomycetota and the type genus of the family Streptomycetaceae. Over 500 species of ''Streptomyces'' bacteria have been described. As with the other Actinomycetota, streptomycetes are gram-positive, ...

'', and '' Bacillus,'' almost all organisms possessing SgaTBA homologues are human/animal pathogens. Several organisms have two or more SgaT paralogues, including ''E. coli,'' which has three. In some of the homologues found in other bacteria, SgaB domains are fused C-terminal to the SgaT domains. For example, this is true of putative transporters in '' Vibrio cholerae'', '' Pasteurella multocida'' and ''Mycoplasma pulmonis''. Homologues of SgaB and SgaA, but not SgaT, are also found in transcriptional activator proteins where they function in regulation rather than sugar transport.

Transport Reaction

The group translocation reaction catalyzed by SgaTBA is: : L-ascorbate (out) → L-ascorbate-6-phosphate (in)

References

{{CCBYSASource, sourcepath=http://www.tcdb.org/search/result.php?tc=4.A.7, sourcearticle=4.A.7 The PTS L-Ascorbate (L-Asc) Family, revision=699838558 Prokaryote genes