Sunday, April 23, 2006

Ammonia assimilation is an important part of the biosynthesis stage of growth metabolism.

After the fueling step of growth metabolism has occurred, ammonia is incorporated into metabolites. This part of biosynthesis is known as nitrogen assimilation.

Study question:

How much of the cell is nitrogen?

Two alternative assimilation routes for nitrogen:

Although some nutritional sources of nitrogen for bacterial cells (e.g. nitrate) are in a more oxidised chemical state than ammonia, these are uniformly converted to ammonia before assimilation, and almost all cell nitrogen is at the same oxidation state as ammonia. Ammonia is fully reduced, fitting in with the concept that early life evolved in an oxygen free environment.

Escherichia coli
has two different first steps for incorporating ammonia into building blocks for polymers such as proteins and nucleic acids, each alternative relying on a different enzyme.

One of these alternative uses glutamate as an acceptor of ammonia, ATP as a co-substrate, and the enzyme glutamine synthetase to catalise the reaction:

The other alternative first step for ammonia assimilation uses the metobolite 2-oxoglutarate (also called alpha ketoglurate), to accept ammonia, NADPH and the enzyme GDH.

Study Questions:

What are the products of the two alternative first steps in ammonia assimilation?

How do the two reactions differ in terms of reaction products?

More importantly, what are the different advantages and opportunities offered by the two alternatives to the cell in its survival under varous different environmental challenges?

Reference Sources:

Chapter 7, Biosynthesis, in Microbe, Schaechter 2006

EcoCyc, Encyclopedia of Escherichia coli K-12 Genes and Metabolism

Why does Escherichia coli have two primary pathways for synthesis of glutamate?
Helling RB.
J Bacteriol. 1994 Aug;176(15):4664-8. Erratum J Bacteriol 1997 Jul;179(13):4455.


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