The enzyme S-ribosylhomocysteine lyase (EC 4.4.1.21) catalyzes the reaction :''S''-(5-deoxy-D-ribos-5-yl)-L-homocysteine = L-homocysteine + (4''S'')-4,5-dihydroxypentan-2,3-dione

Nomenclature

This enzyme belongs to the family of

lyase In biochemistry, a lyase is an enzyme that catalyzes the breaking (an elimination reaction) of various chemical bonds by means other than hydrolysis (a substitution reaction) and oxidation, often forming a new double bond or a new ring structure. ...

s, specifically the class of carbon-sulfur lyases. The

systematic name A systematic name is a name given in a systematic way to one unique group, organism, object or chemical substance, out of a specific population or collection. Systematic names are usually part of a nomenclature. A semisystematic name or semitrivial ...

of this enzyme class is ''S''-(5-deoxy-D-ribos-5-yl)-L-homocysteine L-homocysteine-lyase 4''S'')-4,5-dihydroxypentan-2,3-dione-forming''. Other names in common use include ''S''-ribosylhomocysteinase, and LuxS. This enzyme participates in

methionine metabolism Methionine (symbol Met or M) () is an essential amino acid in humans. As the precursor of other amino acids such as cysteine and taurine, versatile compounds such as SAM-e, and the important antioxidant glutathione, methionine plays a critical ro ...

Structure and function

LuxS is a

homodimeric In biochemistry, a protein dimer is a macromolecular complex formed by two protein monomers, or single proteins, which are usually non-covalently bound. Many macromolecules, such as proteins or nucleic acids, form dimers. The word ''dimer'' has ...

iron-dependent

metalloenzyme Metalloprotein is a generic term for a protein that contains a metal ion cofactor. A large proportion of all proteins are part of this category. For instance, at least 1000 human proteins (out of ~20,000) contain zinc-binding protein domains al ...

containing two identical

tetrahedral In geometry, a tetrahedron (plural: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular faces, six straight edges, and four vertex corners. The tetrahedron is the simplest of all the o ...

metal-binding sites similar to those found in

peptidase A protease (also called a peptidase, proteinase, or proteolytic enzyme) is an enzyme that catalyzes (increases reaction rate or "speeds up") proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the fo ...

s and

amidase In enzymology, an amidase (, ''acylamidase'', ''acylase (misleading)'', ''amidohydrolase (ambiguous)'', ''deaminase (ambiguous)'', ''fatty acylamidase'', ''N-acetylaminohydrolase (ambiguous)'') is an enzyme that catalyzes the hydrolysis of an am ...

s. Furthermore, LuxS is involved in the

synthesis Synthesis or synthesize may refer to: Science Chemistry and biochemistry *Chemical synthesis, the execution of chemical reactions to form a more complex molecule from chemical precursors ** Organic synthesis, the chemical synthesis of organ ...

autoinducer Autoinducers are signaling molecules that are produced in response to changes in cell-population density. As the density of quorum sensing bacterial cells increases so does the concentration of the autoinducer. Detection of signal molecules by ba ...

AI-2 (

autoinducer-2 Autoinducer-2 (AI-2), a furanosyl borate ester, borate diester or tetrahydroxy furan (species dependent), is a member of a family of signaling molecules used in quorum sensing. AI-2 is one of only a few known biomolecules incorporating boron. Firs ...

), which mediates

quorum sensing In biology, quorum sensing or quorum signalling (QS) is the ability to detect and respond to cell population density by gene regulation. As one example, QS enables bacteria to restrict the expression of specific genes to the high cell densities at ...

in roughly half of bacterial species. AI-2, a furanosyl borate diester, is a small

signaling In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The ''IEEE Transactions on Signal Processing'' ...

molecule generated by bacteria. LuxS converts S-ribosylhomocysteine to

homocysteine Homocysteine is a non-proteinogenic α-amino acid. It is a homologue of the amino acid cysteine, differing by an additional methylene bridge (-CH2-). It is biosynthesized from methionine by the removal of its terminal Cε methyl group. In the b ...

and 4,5-dihydroxy-2,3-pentanedione (DPD); DPD can then spontaneously cyclisize to active AI-2. AI-2 is a

signalling In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The ''IEEE Transactions on Signal Processing'' ...

molecule that is believed to act in cross-species communication by regulating niche-specific

gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...

s with diverse functions, such as toxin production, biofilm formation, sporulation, and virulence gene expression, in various

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...

, often in response to

population density Population density (in agriculture: standing stock or plant density) is a measurement of population per unit land area. It is mostly applied to humans, but sometimes to other living organisms too. It is a key geographical term.Matt RosenberPopul ...

. The AI-2 formation pathway begins with ''S''-adenosyl-L-homocysteine (AdoHcy), which is hydrolyzed to ''S''-adenosyl-L-homocysteine (SRH) and adenine by ''S''-adenosyl-L-homocysteine/5’-methylthioadenosine nucleosidase (SAHN or MTAN, EC 3.2.2.9) (8-10). LuxS cleaves S-ribosyl-homocysteine to form L-

(Hcy) and 4,5-dihydroxy-2,3-pentanedione (DPD), which can then spontaneously cyclisize to active AI-2 (11-15). An unequivocally AI-2 related behavior was found to be restricted primarily to bacteria bearing known AI-2 receptor genes. Thus, while it is certainly true that some bacteria can respond to AI-2, it is doubtful that it is always being produced for purposes of signaling.

Clinical significance

LuxS influences iron uptake in pneumococcal species, which also affects biofilm formation. LuxS mutant D39''luxS'' has reduced virulence when compared to wild type studies done on the intranasal channels of mice, and experiments have shown that this mutant also has significantly decreased biofilm formation capabilities.

Nomenclature

Structure and function

Clinical significance

References

Further reading