Hmp

PTS = Phosphoenolpyruvate­dependent phosphotransferase system.  TPP = thiamine pyrophosphate.

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 ones probably being to provide NADPH. (2 molecules per molecule of glucose converted to ribulose 5-phosphate) necessary for biosyntheses (e.g. of fatty acids), and to provide precursors for various biosynthetic pathways (e.g. 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 re-enter 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 equivalents from NADPH can be transferred to NAD+ (see TRANSHYDROGENASE) 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.