Glycolysis or EMP Pathway
It is most widely distributed catabolic pathway which proceeds through fructose-1 : 6-bisphosphate (FBP) hence, also known as FBP pathway (Fig. 14.3). Glucose comes in metabolically active form when is phosphorylated on carbon 6 by hexokinase and converted into glucose-6-phosphate. Thus, glucose-6-phosphatc is the starting point of all three lytic mechanisms (Schlegel, 1986). Glucose-6-phosphate is converted into fructose-1:6-bisphosphate which then is cleaved into triose phosphates. All the triose phosphates are converted into two molecules of pyruvic acid (pyruvate), and ATP and NADH2.
The Entner-Doudoroff Pathway
Glucose is converted in its active form as glucose-6-phosphate. It is dehydrogenated to 6-phosphogluconate which removes water and yields 2-Keto-3-deoxy-6-phosphogluconate (KDPG) (Fig. 14.4). Due to formation of the intermediate product, the KDPG, this pathway is also known as KDPG pathway. The KDPG is then cleaved into pyruvic acid and glyceraldehyde-3-phosphate which is finally oxidized into pyruvic acid. In overall reaction one molecule of glucose yields two molecules of pyruvic acid and one mol of ATP, NAD(P) H2 and NADH2. This pathway is widely distributed in many bacteria of the genus Pseudomonas.
The Pentose Phosphate Pathway
This pathway forms a loop into the EMP pathway, for example in heterofermenter lactobacilli. The bacteria do not synthesize aldolase which is needed to convert fructose biphosphate into two molecules of triose phosphate. Therefore, breakdown of glucose progresses through pentose phosphate pathway. Glucose-6-phosphate is converted to 6-phosj$hogluconate via dehydrogenation and hydrolysis. The 6-phosphogluconate yields ribulose 5-phosphate as the final oxidation product. Further conversion products are shown in Fig. 14.5.