Metabolic Pathways in Microorganisms
Formation of vegetative cells in micro-organisms takes place only when there is continuous supply of energy. The cell components are synthesized by metabolism, which is "the ordered transformation of substances in the cell by a series of successive enzyme reactions through specific metabolic pathways". The metabolic pathways play dual role : (i)
it provides precursors for the cell components and (ii)
energy for energy requiring processes (Schlegel, 1986).
Fig. 14.3. EMP pathway of glucose catabolism.
Fig. 14.4. The Entner-Doudoroff pathway for oxidative catabolism of glucose.
Fig. 14.5. The pentose phosphate pathway for the oxidative catabolism of glucose.
There is a variety of micro-organism which utilize organic compounds either in simple form (C1
) or complex form (C6
) or very complex form via a number of metabolic pathways. It is a unique feature of most of heterotrophic microorganisms that they secrete extracellular enzymes which act as chemical weapon for breaking down the substrates from complex forms to simple ones. In most of the microorganisms, the initial pathways, for example, glucose oxidation are common which are oxidized through glycolysis and Kreb's cycle. However, in some microorganisms the metabolic pathways for utilization of substrates, for example, glucose differs. The metabolic pathways for the break down of hexoses are discussed in brief
During photosynthesis, carbohydrates are synthesized in an adequate amount which serve a major portion of plant body and in turn, a source of nutrients for most of the microorganisms. The macromolecules are rendered from complex form to mono or dimeric forms by extracellular enzymes secreted by microorganisms. Once glucose enters into microbial cells, it is broken down into three carbon compound (C3
) through several routes such as glycolysis also known as fructose-1:6-bisphosphate (FBP) pathway and Embden-Meyerhof-Parnas (EMP) pathway, pentose phosphate pathway or the Warburg-Dickens-Horecker pathway and 2-keto-3-deoxy-6-phospho-gluconate (KDPG) pathway or the Entner-Doudoroff pathway.
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
. 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.