Hexose Monophosphate Pathway Pathway

PTS = Phosphoenolpyruvate­dependent phosphotransferase system.  TPP = thiamine pyrophosphate.

Dr. Shiva C. Aithal, Dept. of Microbiology, Dnyanopasak College, PARBHANI [email protected]

1 | HMP 2008

HEXOSE MONOPHOSPHATE PATHWAY (HMP pathway or HMP shunt or Oxidative pentose phosphate pathway or Pentose phosphate pathway/cycle or Phosphogluconate pathway or Warburg– Dickens pathway) A metabolic pathway present in a wide range of prokaryotic and eukaryotic microorganisms as well as in plants and animals; it involves the oxidative decarboxylation of glucose 6-phosphate, via 6-phosphogluconate, to ribulose 5-phosphate,

followed

by

a

series

of

reversible,

non-oxidative

interconversions whereby hexose and triose phosphates are formed from pentose phosphates. The generally accepted scheme for the HMP pathway is shown in figure above. The HMP pathway can serve various functions. The major one is to provide two molecules of NADPH for each molecule of glucose which is converted to ribulose 5-phosphate, which in turn is necessary for biosyntheses of fatty acids. Secondly it provides precursors for various biosynthetic pathways like •

Pentoses for histidine and nucleotide biosynthesis

•

Erythrose 4-phosphate for aromatic amino acid biosynthesis.

•

Fructose 6-phosphate may be converted to glucose 6-phosphate and reenter the pathway, or may be converted to pyruvate via high-dry objective reactions of the EMBDEN–MEYERHOF–PARNAS PATHWAY.

•

Similarly, glyceraldehyde 3-phosphate may be converted to pyruvate via the latter part of the EMP pathway. In organisms with a functional TCA CYCLE, pyruvate can be oxidized to yield energy via the TCA cycle and a respiratory chain. In organisms which lack a complete TCA cycle, pyruvate may be converted to acetyl-CoA and thence to acetic acid (as in some acetic

acid

bacteria).

Alternatively,

under

certain

conditions,

glyceraldehyde 3-phosphate can be converted to glucose 6-phosphate (by reactions of GLUCONEOGENESIS) which can then re-enter the HMP pathway; in this case, for every six molecules of glucose entering the pathway, one molecule is effectively completely oxidized. If reducing Dr. Shiva C. Aithal, Dept. of Microbiology, Dnyanopasak College, PARBHANI [email protected]

2 | HMP 2008

equivalents from NADPH can be transferred to NAD+ and thence to an electron acceptor via a respiratory chain, the pathway can be used to generate energy even in the absence of a TCA cycle. Other functions of the HMP pathway include the metabolism of those pentoses which can be converted to intermediates of the pathway.

Dr. Shiva C. Aithal, Dept. of Microbiology, Dnyanopasak College, PARBHANI [email protected]

3 | HMP 2008