The document discusses several alternative pathways for the utilization and metabolism of hexose sugars besides the main Embden-Meyerhof-Parnas pathway. It describes three main alternative pathways: the fructose bisphosphate aldolase pathway, the Entner-Doudoroff pathway, and the phosphoketolase pathway. Each pathway is found in different types of bacteria and has distinguishing reactions and end products.

1. ALTERNATIVE PATHWAY
S.Ramya
I M.Sc Microbiology

2.  Fructose Bisphosphate Aldolase Pathway
 Entner-Doudoroff Pathway
 Phosphoketolase Pathway

3.  Warburg and Christian provided the first
evidence for the existence of an alternative
pathway for the utilization of hexose sugars.
 They described the oxidation of glucose-6-
phosphate to 6-phosphogluconate (6-P-G) via
G-6-P dehydrogenase.
 They also described the decarboxylation of
6-P-G to form a pentose sugar.

4.  For a long time, relatively little attention was
paid to the real significance of this alternative
pathway because Meyerhof and others strongly
asserted that the FBP aldolase (EMP) pathway
was the main route of glucose catabolism.
 Subsequently, ribulose-6-phosphate was shown to
be the first product formed and was converted
to ribose-5-phosphate via an isomerase reaction.
 This series of reactions is common to several
alternate pathways of carbohydrate metabolism

5. ALTERNATIVE PATHWAYS
Alternative pathways found in different bacteria:
 Fructose Bisphosphate Aldolase Pathway
 Found in almost all organisms
 Entner-Doudoroff pathway
 Found in Pseudomonas and related genera
 Phosphoketolase pathway
 Found in Bifidobacterium and Leuconostoc

6.  This is also called as Embden-Meyerhof-Parnas
pathway or Glycolysis.
Important reactions:
 Phosphorylation of glucose and fructose-6-
phosphate by ATP.
 Cleavage of fructose-1,6-bisphosphate to trioses
by a specific aldolase.
 Structural rearrangements.
 Oxidation-reduction and Pi assimilation

7. 1. Phosphorylase. Degradation of glycogen or
starch to G-1-P
2. Phosphoglucomutase. Isomerization of G-1-
P to G-6-P
3. Hexokinase. Phosphorylation of glucose to
G-6-P, using ATP Hexokinase also
phosphorylates fructose to F-6-P using ATP

8. 4. Phosphoglucoisomerase (pgi). Isomerization
of G-6-P to F-6-P.
5. Phosphofructokinase (pfkA).
Phosphorylation of F-6-P to FBP using ATP.
6. Fructose bisphosphate (FBP) aldolase (fbaA).
Cleaves FBP to GA-3-P and
dihydroxyacetone phosphate.
7. Triose phosphate isomerase (tpi ).
Interconverts GA-3-P and
dihydroxyacetone-P.

9. 8. Glyceraldehyde-3-phosphate
dehydrogenase (gap). Oxidizes GA-3-P to
1,3-diphosphoglycerate using
NAD+(nicotinamide adenine dinucleotide)
and inorganic phosphate (Pi) to form
NADP + H+.
9. Phosphoglycerokinase (pgk) Generates ATP
from ADP.
10. Phosphoglyceromutase (pgm). Uses 2,3-
diphosphoglycerate to convert 3-
phosphoglycerate to 2-phosphoglycerate.

10. 11. Enolase (eno). Enolization of
2-phosphoglycerate forms high-energy
phosphate bond (∼P; encircled P) in
phosphoenolpyruvate (PEP).
12. Pyruvate kinase (pykA; pykF). Generates ATP
from ADP.
13. Lactate dehydrogenase. Reduces pyruvate to
lactate using NADP + H+.

11. 14. Pyruvate carboxylase. Converts pyruvate to
oxaloacetic acid (OAA) via carbon dioxide
(CO2) fixation using ATP.
15. PEP carboxykinase. Forms
phosphoenolpyruvate from OAA using GTP.
16. Fructose-1,6-bisphosphatase. Removes Pi
from F-1,6-bisP to form F-6-P.

12. 17. Glucose-6-phosphatase. Removes Pi from
G-6-P to form glucose.
18. ATP-glucose pyrophosphorylase. Forms
ADP-glucose from G-1-P and ATP.
19. Glycogen synthase. Adds α-1,4-glycan to
ADP-glucose to form glycogen or starch

13. 20. Glycerophosphate dehydrogenase. Reduces
dihydroxyacetone-P to glycerol-3-P.
21. Phosphatase. Removes Pi from glycerol-3-P
to form glycerol.
22. Glycerol kinase. Phosphorylates glycerol
using ATP.

14. FRUCTOSE
BISPHOSPHATE
ALDOLASE
PATHWAY

15.  The Entner–Doudoroff pathway branches
from the Ketogluconate pathway of hexose
oxidation.
 It consist of two enzymes: 6-
phosphogluconate dehydratase and 2-keto-3-
deoxy-6-phosphogluconate aldolase.
 Final product is ethanol.

16.  glucose-6-phosphate dehydrogenase (zwf, for
zwischenferment) oxidizes glucose-6-phosphate to
6-phosphogluconolactone.
 The lactone is dehydrated to 6-phosphogluconate via
lactonase.
 6-phosphogluconate is dehydrated by 6-
phosphogluconate dehydratase (edd) to yield 2-keto-
3-deoxy-6-phosphogluconate (KDPG).

17.  The enzyme 2-keto-3-deoxy-6-phosphogluconate
aldolase cleaves 2-keto-3-deoxy-6-phosphogluconate
to form pyruvate and GA-3-P (glyceraldehyde-3-
phosphate).
 Pyruvate decarboxylase action yields ethanol and
carbon dioxide.
 The GA-3-P is metabolized via the triose phosphate
portion of the EMP pathway to yield ethanol and
carbon dioxide.
 The net yield in the Entner-Doudoroff pathway is 2
ethanol + 2 CO2.

18. ENTNER-DOUDOROFF PATHWAY

19.  One major fermentation pathway involves
the conversion of ribose-5-phosphate to
xylulose-5-phosphate(X-5-P).
 The X-5-P is then cleaved to form a C3
compound (glyceraldehyde-3-phosphate) and
C2 compound (acetyl phosphate) by the
action of the phosphoketolase enzyme.

20. PHOSPHOKETOLASE
PATHWAY