Describe how microbes build large biomolecules from either (1) the products of catabolism (i.e. in heterotrophy) or (2) the fixation of CO2 (i.e. in autotrophy). What is the role of ATP and reducing equivalents like NADH/NADPH?

Question

What will be an ideal response?

Answer 1

Heterotrophy breaks down organic molecules. Pathways of breakdown include hydrolytic and oxidative reactions that produce small monomers that can enter the glycolysis pathway, then the TCA cycle or fermentation pathways. Many of the intermediates from glucose catabolism and the tricarboxylic acid (TCA) cycle can be used as substrates for biosynthesis reactions in the cell. In addition, microbes reverse these pathways to build larger molecules (carbon skeletons) from the smaller molecular products of catabolism (sugars, amino acids). For example, products of heterotrophy can enter reverse glycolysis (gluconeogenesis).

Autotrophy fixes CO2 into sugar. This happens through cycles like the Calvin cycle and reverse TCA cycles. In the Calvin cycle, there are three main phases: carboxylation of ribulose 1,5-bisphosphate to a 6-C molecule and splitting into 3-phosphoglycerate (PGA): 6C ? 2[3C]; reduction of PGA to glyceraldehyde 3-phosphate (G3P); and regeneration of ribulose 1,5-bisphosphate through the consumption of additional ATP. One of every six G3P is converted to glucose. The other five molecules enter a series of reactions that regenerate three molecules of ribulose 1,5-bisphosphate. Reverse TCA, on the other hand, involves fixation of CO2 by several intermediates, including succinyl-CoA, 2-oxoglutarate, and acetyl-CoA. Reduction (addition of 2H+ + 2e-) is performed by NADPH or NADH and by reduced ferredoxin (FDH2). In addition, ATP is consumed when citrate is converted to acetyl-CoA, and the acetyl group is then used to create pyruvate. Pyruvate is then used to re-generate oxaloacetate, consuming a 3rd ATP molecule.

Biosynthesis usually reduces the substrate by hydrogenation and by removing oxygen. Cell components such as lipids and amino acids are more reduced than substrates such as CO2 and acetate, so their biosynthesis requires a reducing agent such as NADPH. All biosynthesis requires energy from ATP and electrons from reducing cofactors such as NADPH.