Glycerol-3-phosphate is an important intermediate both in glycolysis and in phospholipid biosynthesis.
Glycerol uptake in bacteria is mediated by the glycerol diffusion facilitator, an integral membrane protein catalyzing the rapid equilibration of concentration gradients of glycerol across the cytoplasmic membrane. Intracellular glycerol is converted to Glycerol-3-Phosphate (G3P) by the enzyme glycerol kinase that uses ATP as phosphoryl donor. Glycerol-3-P is not a substrate of the glycerol diffusion facilitator and thus remains in the cell, where it is further metabolized. As a result, the driving force for the uptake of glycerol is generated by the phosphorylation of glycerol by glycerol kinase.
In addition, many organisms possess two systems the salvage of glycerophosphoryl diesters, the Glp system and the Ugp system.
The Ugp and the GlpT transport systems, both of which can transport G3P against the concentration gradient, are well adapted to the two different physiological situations:
a) phosphate starvation;
b) the availability of glycerol and glycerol-containing compounds as a carbon source.

======== VARIANT CODES: =================

Vaiants 1, 2 or 3 - digit before dot shows how many Glycerol uptake systems the organism possesses one (1), two (2) or three(3).

Digits after dot show what kind of transport is available:
1 for GlpF - Glycerol uptake facilitator protein,
2 for GlpT transport system,
3 for Ugp transport system.

Variant 7 - no uptake systems known, but just utilization of intracellular glycerol and glycerol-3-P(G3P) to glyceron phosphate (GAP) -like in all Shewanellas (except Shewanella putrefaciens CN-32)

======Examples: =============

Code 3.123 for Escherichia coli K12 - means that all three transport system are available in this organism.

Code 2.12 for Bacillus subtilis - shows that there are two glycerol transport systems: Glycerol uptake facilitator protein and GlpT transport system.

====== Glp system:=====

When used as a carbon source, G3P is taken up exclusively by the GlpT transport system. This system is part of the glp regulon, a number of proteins whose genes are transcribed in response to the presence of glycerol and G3P in the medium and which are geared for the uptake and the metabolism of glycerol, G3P, and glycerol phosphoryl diesters. GlpT and all of the other glp-encoded proteins are under the control of GlpR, the repressor of the system, with G3P as the effective inducer.
The GlpT transport system can function in two modes:
In the exchange mode, G3P is taken up in exchange with internal Pi. Uptake of G3P without Pi exchange is essentially by proton symport. Under these conditions, GlpT-mediated uptake of G3P can serve as the sole source of Pi. The GlpT permease is a tightly membrane-bound oligomeric complex of identical polypeptide subunits.
The glpT gene is repressed by the GlpR repressor encoded by the glpEGR operon. By binding to the operators near the glpT promotor, GlpR blocks its expression.
In the Glp system, the glpQ gene encodes a periplasmic glycerophosphoryl diester phosphodiesterase (periplasmic GDP) which hydrolyzes deacylated phospholipids to an alcohol plus Glycerol-3-P. The Glycerol-3-P is then transported into the cell by the GlpT transporter. Periplasmic GDP is specific for the glycerophospho- moiety of the substrate, while the alcohol can be any one of several alcohols. This provides the cell with the capability of channeling a wide variety of glycerophosphodiesters into the glpQT-encoded dissimilatory system.

==== Ugp (uptake of glycerol phosphate) system:==========

In the Ugp system the diesters are hydrolyzed during transport at the cytoplasmic side of the inner membrane to Glycerol-3-P plus alcohol by a cytoplasmic GDP, an enzyme encoded by the ugpQ gene. The Ugp system is induced when the cells are starved for inorganic phospate, which is generates phosphate by the system.
Internal glycerophosphodiesters are not substrates of the cytoplasmic GDP.

In E. coli glycerol-3-P can be further metabolized to dihydroxyacetone phosphate (DHAP) by either of two membrane-bound enzymes, depending on the growth conditions. The presumed role of this process is the salvage of glycerol and glycerol phosphates generated by the breakdown of phospholipids and triacylglycerol.


====Aerobic and anaerobic glycerol-3-P dehydrogenases: =============

Under aerobic conditions, a homodimeric aerobic glycerol-3-P dehydrogenase (encoded by the glpD gene) is produced, which can accept either oxygen or nitrate as the electron acceptor.
Under anaerobic conditions, a different glycerol-3-P dehydrogenase is preferentially expressed. This tri-heteromeric protein complex, which is encoded by the glpACB opron, channels the electrons from glycerol-3-P to either fumarate or nitrate.

==== Divergence of glpD and glpO genes( roles GlpD and GlpO): ==============================

There are actually two enzymes very close structurally (and functionally) - they are the products of glpD and glpO genes:" the GlpO sequence is 30-43% identical to those of the alpha-glycerophosphate dehydrogenases (GlpDs) from mitochondrial and bacterial sources"(see Ref.4), but they produse different reactions:

Aerobic glycerol-3-phosphate dehydrogenase (EC 1.1.99.5)- glpD gene product – reaction R00848: sn-Glycerol 3-phosphate + FAD <=> Glycerone phosphate + FADH2
and
Alpha-glycerophosphate oxidase (EC 1.1.3.21) – glpO gene product
with a reaction R00846:
sn-Glycerol 3-phosphate + Oxygen <=> Glycerone phosphate + H2O2.
From Ref.4:
“The FAD-dependent alpha-glycerophosphate oxidase (GlpO) from Enterococcus casseliflavus and Streptococcus sp. was originally studied as a soluble flavoprotein oxidase; surprisingly, the GlpO sequence is 30-43% identical to those of the alpha-glycerophosphate dehydrogenases (GlpDs) from mitochondrial and bacterial sources."
=================================================

There are 2 isozymes of glycerophosphoryl diester phosphodiesterase in E.coli: a periplasmic isozyme (glpQ) and a cytosolic isozyme (ugpQ).

------=============OBSERVATIONS=======================

I included in this SS a Conservative protein of unknown function(DUF482)- COGs COG3146 - because it's in very strong cluster with major enzyme of this pathway UdpQ - Glycerophosphoryl diester phosphodiesterase (EC 3.1.4.46). For af example see the genomes :
Bru.mel.,Bru.abo.bi, Bru.abo., Bru.sui.13, Mes.lot.MA, Mes.sp.BNC, Agr.tum.st, Agr.tum.st, Rhi.leg.bv, Sin.mel., Rho.sph..
Same story with YjgF -Putative translation initiation inhibitor, yjgF family

**************REFERENSES:*******************

1. Lemieux MJ, Huang Y, Wang DN. Glycerol-3-phosphate transporter of Escherichia coli: structure, function and regulation. Res Microbiol. 2004 Oct;155(8):623-9. Review.
PMID: 15380549

2. Danilova LV, Gel'fand MS, Liubetskii VA, Laikova ON. Computer analysis of regulating metabolism of glycerol-3-phosphate in proteobacteria genome. Mol Biol (Mosk).
2003 Sep-Oct; 37(5):843-9. PMID: 14593921

3. Brzoska P, Rimmele M, Brzostek K, Boos W. The pho regulon-dependent Ugp uptake system for glycerol-3-phosphate in Escherichia coli is trans inhibited by Pi. J Bacteriol. 1994 Jan;176(1):15-20. PMID: 8282692;

4. Colussi T, Parsonage D, Boles W, Matsuoka T, Mallett TC, Karplus PA, Claiborne A. Structure of alpha-glycerophosphate oxidase from Streptococcus sp.: a template for the mitochondrial alpha-glycerophosphate dehydrogenase. Biochemistry. 2008 Jan 22;47(3):965-77. PMID: 18154320.

5. Joseph B, Mertins S, Stoll R, Schär J, Umesha KR, Luo Q, Müller-Altrock S, Goebel W. Glycerol metabolism and PrfA activity in Listeria monocytogenes. J Bacteriol. 2008 Aug;190(15):5412-30.PMID: 18502850