Microbiology Daily Newsletter

February 3, 2014 - Biosynthesis

The flip side of the catabolic pathways are the anabolic pathways. For cells to grow, they must produce biomass: proteins, lipids, and in the case of bacteria, peptidoglycan. Up until now, the focus of most biology classes is on the catabolic processes, with an enphasis on energy harvesting. It is important to remember that the catabolic processes also provide carbon building blocks as well as energy in the form of reducing potential. The cell must balance a need for energy with a need for carbon building blocks. Today, we are going to look at two biosynthetic (anabolic) pathways: amino acid synthesis and Polyhydroxyalkanoates (PHA) synthsis.

α-ketogluterate family: Glutamine Biosynthesis

Glutamine is an amino acid derived from α-ketogluterate.

We begin with the citric acid cycle intermediate α-ketogluterate, then by adding an amino group, we produce the amino acid glutamate. Glutamate can then converted to glutamine by adding a second amino group. NOTE that the addition of a free amino group requires a phosphorylation, and then a substitution with the phosphate. Why do you think this is required?

Oxaloacetate/Aspartate Family of Amino Acids

The citric acid cycle intermediate oxaloacetate can be used to make a wide range of amino acids. The first reaction is a transamination in which an amino group from an existing amino acid is transferred to oxaloacetate to produce the amino acid aspartate. Aspartate, as can be seen, can be used to produce isoleucine, methionine and leucine. Notice that the production of lysine will require substrate reduction using NADPH and the addition of a pyruvate.

Question: How is NADPH different from NADP? Why does a cell have two different electron carriers based on nicotinamide adenine dinucleotide?

Polyhydroxyalkanoates (PHA) Biosynthesis

Polyhydroxyalkanoates (PHA) is a large carbon polymer produced by some bacteria as an energy storage inclusion body. To the left is
Poly-(R)-3-hydroxybutyrat, and example of the PHA group. The monomers are attached via an ester bond, making this a biologically produced polyester. It is a plastic, and a common example of a bio-plastic, such as you might find used in grocery store plastic bags.

PHA pathway based upon Verlindin RAJ, Hill DJ, Kenward MA, Williams CD, and I Radecka. 2007. Bacterial synthesis of biodegradable Polyhydroxyalkanoates. Journal of Applied Microbiology 102:1437–1449

The above pathway demonstrate the production of 3-hydroxyacyl-CoA, and the the polymerization into PHA. Remember that this is a process that allows bacteria, such as Bacillus sp. and Rhodococccus sp. to have an energy storage solution. The compound though is also effective industrially.

Daily Challenge:

Examine the balance cells must maintain between energy and building blocks by looking at cellular requirements for biosynthesis. Remember that before divisions, cells must grow in biomass. Use as your example a biosynthetic pathway shown in your book or that you find online (do not use the ones provided). You can use amino acid, lipid or nucleic acid production for example, or you can look at some of the other products. A reference you may find useful is GLAMM: Genome Linked Application for Metabolic Maps. GLAMM provides an interactive map of known metabolic pathways with genomic links. The map can be limited to various microorganisms that you specify.