75px, Lipopolysaccharide. The core includes both the inner and outer core. Core oligosaccharide (or Core-OS) is a short chain of sugar residues within Gram-negative lipopolysaccharide (LPS). Core-OS are highly diverse among bacterial species and even within strains of species

Structure

600px, Core-OS Structure and Metabolic Pathways: oligosaccharide from '' Escherichia coli'' R1. Inner core is represented in green and outer core is represented in blue. The core domain always contains an oligosaccharide component which attaches directly to lipid A and commonly contains sugars such as heptose and 3-deoxy-D-mannooctulosonic acid (also known as KDO or keto-deoxyoctulosonate). The LPS Cores of many bacteria also contain non-carbohydrate components, such as phosphate,

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...

s, and ethanolamine substituents. Many core structures have been described in the literature, this description is based on the ''traditional'' general structure (as found in

enteric bacteria Gut microbiota, gut microbiome, or gut flora, are the microorganisms, including bacteria, archaea, fungi, and viruses that live in the digestive tracts of animals. The gastrointestinal metagenome is the aggregate of all the genomes of the gut m ...

and ''

Pseudomonas ''Pseudomonas'' is a genus of Gram-negative, Gammaproteobacteria, belonging to the family Pseudomonadaceae and containing 191 described species. The members of the genus demonstrate a great deal of metabolic diversity and consequently are able to ...

''). See the figure above for an overview of the structure found in '' E. coli'' R1.

Inner core

The "base" of the inner core is 1–3 KDO residues. The last KDO is often modified with a phosphate or ethanolamine group. From the KDOs, there are attached 2–3 heptoses (i.e. L-glycero-D-''manno''heptulose) that are usually phosphorylated. These KDO and heptoses comprise the "inner core". The ketosidic bond between KDO and lipid A (α2→6) is especially susceptible to acid cleavage. LPS researchers use a weak acid treatment to separate the lipid and polysaccharide portions of LPS. An LPS molecule that includes only a lipid A and an inner core (or less. See example) is referred to as "deep-rough LPS".

Outer core

The outer core is made of hexose residues that are attached to the last heptose residue in the inner core. Hexoses often found in the outer core include: D- glucose, D-

mannose Mannose is a sugar monomer of the aldohexose series of carbohydrates. It is a C-2 epimer of glucose. Mannose is important in human metabolism, especially in the glycosylation of certain proteins. Several congenital disorders of glycosylat ...

, D-

galactose Galactose (, '' galacto-'' + ''-ose'', "milk sugar"), sometimes abbreviated Gal, is a monosaccharide sugar that is about as sweet as glucose, and about 65% as sweet as sucrose. It is an aldohexose and a C-4 epimer of glucose. A galactose molec ...

, ''etc.''. There are usually at least three hexoses bound β1→3, with the O antigen being ligated to the third hexose. Other hexose are often found attached to the outer core, branching from the main oligomer. LPS that include lipid A and a complete core oligosaccharide (inner and outer) is referred to as "rough LPS."

Biosynthesis

The enzymes involved in core oligosaccharide synthesis are conserved among ''

Escherichia coli ''Escherichia coli'' (),Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. also known as ''E. coli'' (), is a Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus '' Esc ...

'' and '' Salmonella''. '' Pseudomonas aeruginosa'' has some unique enzymes.

Function

The mechanism whereby the core oligosaccharide of lipopolysaccharide affect the membrane behavior is not well understood.

References