What types of molecules serve as C sources for catabolism and what are their similarities/differences?

Question

What will be an ideal response?

Answer 1

Major C sources for catabolism in microbes include polysaccharides, lipids, proteins, and aromatics. Some of these substrates are used more rapidly (e.g. polysaccharides) because they require less activation energy or fewer types of enzymes to break down. In addition, different C sources have different breakdown products. Polysaccharides are hydrolyzed ultimately to products like glucose. By contrast, lipids are hydrolyzed to glycerol and fatty acids. Proteins are hydrolyzed to amino acids, which are further broken down in one of two ways: decarboxylation (removal of CO2) to produce an amine, or deamination (removal of NH3) to produce a carboxylic acid. Amine products include putrecine and cadaverine, which cause noxious odors. Fungi and soil bacteria catabolize lignin to oxidized benzene derivatives such as benzoate and vanillin. Many bacteria also metabolize benzene derivatives, and even polycyclic aromatic molecules, either partly or all the way to CO2.

For all of these molecules, some fraction of their products ultimately enters central metabolic pathways. Glucose from polysaccharides and glycerol from lipids commonly enter central catabolic pathways such as glycolysis. Other molecules, like carboxylic acids from protein, fatty acids from lipids, and benzene derivatives can enter the TCA cycle when a terminal electron acceptor is available; alternatively, they enter fermentation.