Anaerobic oxidation of glucose

Main stages of this process

Honey contains glucose [Wikimedia]

Glu­cose (a six-atom or­gan­ic com­pound in the class of monosac­cha­rides) can be metabol­i­cal­ly trans­formed in the body by two ways – aer­o­bic (with oxy­gen mol­e­cules) and anaer­o­bic (with­out oxy­gen).

The main goal of these pro­cess­es is the syn­the­sis of ATP (adeno­sine triphos­phate, which plays a key role in me­tab­o­lism in the body). Anaer­o­bic ox­i­da­tion of glu­cose can be pre­sent­ed as a process that is the re­verse of the pho­to­syn­the­sis process (this re­verse process is usu­al­ly called ox­i­da­tion or breath­ing). Sub­stances are formed from glu­cose and oxy­gen which are ini­tial for pho­to­syn­the­sis – wa­ter and car­bon diox­ide.

Anaer­o­bic ox­i­da­tion of glu­cose

Glucose [Wikimedia]

Anaer­o­bic gly­col­y­sis is the ox­i­da­tion process of glu­cose with­out oxy­gen, in which lac­tates are formed. In this process, be­sides lac­tic acid, 2 ATP mol­e­cules are formed (it is im­por­tant that at the prepa­ra­tion stage, 2 ATP mol­e­cules are spent on the re­ac­tion, but in the course of gly­col­y­sis 4 ATP mol­e­cules form).

In gen­er­al, both aer­o­bic and anaer­o­bic gly­col­y­sis take place al­most iden­ti­cal­ly – the only stage in which these two types of ox­i­da­tion can be dis­tin­guished is the last (it de­pends on the pres­ence or ab­sence of oxy­gen in the ox­i­da­tion process).

Steps of anaer­o­bic ox­i­da­tion of glu­cose

Anaer­o­bic gly­col­y­sis is a process which in­cludes 2 steps – ac­ti­va­tion and ox­i­da­tion.

In the first step, the glu­cose mol­e­cule splits into trios­es – to ac­ti­vate the glu­cose, 2 ATP mol­e­cules are ex­pend­ed. As a re­sult, iso­mer­ic car­bonyl com­pounds form (they have an iden­ti­cal el­e­men­tal com­po­si­tion, but a dif­fer­ent po­si­tion of atoms by struc­ture) – phos­pho-di­hy­drox­y­ace­tone and 3-phos­pho-glyc­erin alde­hyde (the ke­tone can also be iso­mer­ized into an alde­hyde, as a re­sult form­ing only 2 mol­e­cules of alde­hyde as a prod­uct of the process).

D-glucose chain 3D structure [Wikimedia]

The sec­ond stage is the stage of ox­i­da­tion of the formed mol­e­cules of 3-phos­pho-glyc­erin alde­hyde with the for­ma­tion of lac­tic acid (as 2 alde­hyde mol­e­cules en­ter into the re­ac­tion, as a re­sult 2 acid mol­e­cules form). The in­ter­me­di­ate prod­uct may be pyru­vic acid.

This is why the sum­ma­ry equa­tion of anaer­o­bic gly­col­y­sis is pre­sent­ed in this form:

C₆H₁₂O₆ + 2ADP + 2H₃PO₄ = 2ATP + 2C₃H₆O₃ + 2H₂O;

The en­er­gy bal­ance of the re­ac­tion may be writ­ten down in the fol­low­ing form: 2 ATP= -2ATP + 4ATP (2 ATP mol­e­cules are ex­pend­ed, and 2 form).

Pro­cess­es that lie at the ba­sis of gly­col­y­sis

Anaer­o­bic gly­col­y­sis is a many-stage process in­clud­ing 11 re­ac­tions. In the course of ox­i­da­tion of gly­col­y­sis, these re­ac­tions take place at the first stage:

  1. phos­pho­ry­la­tion of glu­cose in re­ac­tion with ATP with for­ma­tion of glu­cose-6-phos­phate;

  2. iso­mer­iza­tion of glu­cose-6-phos­phate to fruc­tose-6-phos­phate;

  3. re­peat­ed phos­pho­ry­la­tion of the prod­uct by ATP with for­ma­tion of fruc­to­to-1,6-dis­pho­p­shate;

ATP 3D structure [Wikimedia]
  1. sep­a­ra­tion of the prod­uct into two phos­pho­triose mol­e­cules;

  2. iso­mer­iza­tion of triosephos­phates with tran­si­tion of di­hy­drox­y­ace­tone phos­phate into glyc­er­alde­hyde-3-phos­phate.

The sec­ond stage in­cludes these pro­cess­es:

  1. de­hy­dra­tion of the ob­tained alde­hyde with the for­ma­tion of 1,3-diphos­pho­glyc­er­ate;

  2. first sub­strate phos­pho­ry­la­tion of 1,3-diphos­pho­glyc­er­ate with for­ma­tion of ATP mol­e­cule and 3-phos­pho­glyc­er­ate;

  3. mi­gra­tion of phos­phate group from the 3ʳᵈ car­bon atom to the 2ⁿᵈ in 3-phos­pho­glyc­er­ate with for­ma­tion of phos­pho­enolpyru­vate;

  4. sec­ond sub­strate phos­pho­ry­la­tion of ob­tained phos­pho­enolpyru­vate with for­ma­tion of ATP mol­e­cule;

  5. for­ma­tion of pyru­vate from the pyru­vate-ki­nase en­zyme;

  6. re­duc­tion of pyru­vate to lac­tic acid with use of NADH.

4 ATP mol­e­cules are formed, and not 2 (as we can see ac­cord­ing to the ox­i­da­tion di­a­gram), as this process does not take place for one mol­e­cule, but for two, as in the first stage 2 alde­hyde mol­e­cules were formed, each of which in sub­se­quent ox­i­da­tion gives 2 ATP mol­e­cules.

Fur­ther course of re­ac­tion

Lac­tic acid is not al­ways the con­clud­ing stage of the re­ac­tion – for ex­am­ple, if af­ter anaer­o­bic ox­i­da­tion of glu­cose, lac­tic acid en­ters aer­o­bic con­di­tions with ex­cess of oxy­gen, it may trans­form into pyru­vic acid and through the coen­zyme acetyl-CoA, which is im­por­tant for bio­chem­i­cal pro­cess­es, en­ter into the me­tab­o­lism of the or­gan­ism, and name­ly the Krebs cy­cle.

More ex­per­i­ments with sug­ars you can find here.

In some cas­es, the Cori cy­cle may also take place – if lac­tic acid formed af­ter anaer­o­bic ox­i­da­tion of glu­cose en­ters the liv­er from mus­cle tis­sues, the for­ma­tion of glu­cose will take place with its par­tic­i­pa­tion.

Beta D-glucose [Wikimedia]

Al­though anaer­o­bic ox­i­da­tion of glu­cose is a process that is rather in­ef­fi­cient for the or­gan­ism en­er­gy-wise, it is nec­es­sary, as it the only path of anaer­o­bic ox­i­da­tion of glu­cose in liv­ing or­gan­isms. Anaer­o­bic gly­col­y­sis is of­ten ac­tive in con­di­tions of a deficit of oxy­gen.