Amino Acid Synthesis

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  • 31.1 The Nitrogenase Complex Fixes Nitrogen

  • 31.2 Amino Acids Are Made from Intermediates of Major Pathways

  • 31.3 Feedback Inhibition Regulates Amino Acid Biosynthesis

Nitrogen is a key component of amino acids. The atmosphere is rich in nitrogen gas (N2), a very unreactive molecule. Certain organisms, such as bacteria that live in the root nodules of yellow clover, can convert nitrogen gas into ammonia, which can be used to synthesize, first, glutamate and, then, the other amino acids.

The major source of nitrogen for the biosphere is the gaseous form of nitrogen, N2, which makes up about 80% of Earth’s atmosphere. However, this form of nitrogen is virtually unusable by most forms of life. Only a few prokaryotes, most notably the nitrogen-fixing bacteria, are able to convert N2 gas into NH3 (ammonia), a form of nitrogen that the rest of the biosphere can use. This process, called nitrogen fixation, is one of the most remarkable reactions in biochemistry.

DID YOU KNOW?

A verse from The Rime of the Ancient Mariner by Samuel Taylor Coleridge:

Water, water, every where, And all the boards did shrink; Water, water, every where, Nor any drop to drink.

Just as the Ancient Mariner was surrounded by water, none of it drinkable, so we are surrounded by nitrogen, but none is accessible to us.

What is so difficult about fixing nitrogen? The extremely strong N ≡ N bond, which has a bond energy of 940 kJ mol−1 (225 kcal mol−1), is highly resistant to chemical attack. Indeed, the eighteenth-century French chemist Antoine Lavoisier named nitrogen gas “azote,” meaning “without life,” because it is so unreactive. Nitrogen can, however, be reduced industrially. Industrial processing is in fact the source of the majority of nitrogen used for fertilizer. The industrial process for nitrogen fixation devised by Fritz Haber in 1910 is still being used today.

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Figure 31.1: Lightning nitrogen fixation.

The fixation of N2 is typically carried out by mixing it with H2 gas over an iron catalyst at about 500°C and a pressure of 300 atm, conditions with which even our hardiest microoganismic colleagues cannot cope. In fact, 25% of Earth’s yearly fixed nitrogen is obtained by industrial processes. Lightning and ultraviolet radiation fix another 15% in the form of nitrogen oxides (Figure 31.1).

Interestingly, despite the severe conditions of the Haber process and of lightning strikes, the conversion of nitrogen and hydrogen to ammonia is actually thermodynamically favorable; the reaction is difficult kinetically because intermediates along the reaction pathway are unstable and rapidly decay to biochemically useless molecules. Some bacteria and archaea, diazotrophic (nitrogen-fixing) microorganisms, have ways of reducing the inert molecule N ≡ N (nitrogen gas) to two molecules of ammonia under conditions compatible with life. An important diazotrophic microorganism is the symbiotic Rhizobium bacterium, which invades the roots of leguminous plants and forms root nodules in which they fix nitrogen, supplying both the bacteria and the plants. The amount of N2 fixed by nitrogen-fixing microorganisms has been estimated to be 1011 kg per year, about 60% of Earth’s newly fixed nitrogen.

In this chapter, we will first examine the process of nitrogen fixation and how the nitrogen is incorporated into glutamate. We will then examine how glutamate becomes a nitrogen source for the other amino acids as well as how the carbon skeletons for amino acid synthesis are derived.