9. Microbial Growth and Biosynthesis
9.1 Biosynthesis
Learning Objectives
- Describe the importance of substrates for biosynthesis
- Identify the roles of the central pathways in biosynthesis
- Describe the various ways autotrophic microorganisms fix carbon
- Describe the importance of nitrogen fixation
- Explain the importance of assimilatory reduction of sulphur
Cellular metabolism includes catabolic pathways, which extracts energy from the environment, and anabolic pathways, which use energy and carbon building blocks for cellular growth. Microbes obtain their energy through heterotrophy, lithotrophy (bacteria and archaea only) or phototrophy. Anabolic pathways are necessary for survival and cellular function, giving rise to larger, more complex molecules which store energy. They are, in contrast to microbial catabolism, relatively conserved among the microbes. So whether an organism is an autotroph (using CO2 as a carbon source) or heterotroph (using organic carbon sources), the anabolic pathways generally require the same small set of organic building blocks – the precursor metabolites. In heterotrophs, these molecules come from the reactions in their central catabolic pathways:
- Glycolytic pathway (= Embden Meyerhof Parnas pathway)
- Entner-Doudoroff pathway
- Pentose phosphate pathway
- TCA pathway (= tricarboxylic acid pathway, Kreb’s cycle or citric acid cycle)
Because many of the enzymatic reactions within these pathways are also used to create precursor molecules in autotrophs, the four pathways are called the central pathways and, in the heterotrophs, are referred to as amphibolic – they function in both the catabolic and anabolic directions.
Substrates for Biosynthesis
In heterotrophs, biosynthetic processes begin with diverting some of the intermediates in the central pathways, as well as the energy and reduced cofactors from those pathways. The balance between catabolic oxidation of organics, and the anabolic reduction of organics to build biomass, must therefore be tightly regulated.
TCA Cycle
The TCA cycle, also known as the citric acid cycle, or Krebs cycle, produces energy through the complete oxidation of acetate, derived from carbohydrates, fats, and proteins, to carbon dioxide. The cycle is one of the major metabolic processes utilized to generate energy, and it also provides precursors for a number of biosynthetic pathways. These precursors are used as substrates for the biogenesis of some of the amino acids, pyrimidines (nucleic acid bases), and reducing agents such as NADH.
Glycolytic pathway
The glycolytic pathway, also known as the Embden Meyerhof Parnas pathway) catalyzes a series of reactions that oxidize the 6 carbon sugar glucose, into the 3 carbon molecule pyruvate. Most of these reactions are reversible, and with the addition of a few enzymes that function only in the reductive direction, the pathway can also be reversed to synthesize glucose from pyruvate. This is called gluconeogenesis and is important for hete