Lecture 11 Pt. 2: Biosynthesis of Amino Acids, Nucleotides, and Related Molecules

Nitrogen is found in many things including…?

• cofactors

• hormones

• neurotransmitters

• pigments

• defence chemicals

What percent of the atmosphere is nitrogen?

• 80%

Non-useful form of nitrogen is what?

• very stable

• very difficult to reduce

N₂ is chemically…?

• inert

N2 and O2 → ?

• NO via lightning

N2 and H2 → ?

• NH3 via industrial processes

• requires:

  • T>400 C and P>200 atm

Some organisms can fix N2 to useful forms, this includes?

• single-celled prokaryotes i.e., archaea

• proteobacteria with legumes → symbiosis with plants

• spirochetes with termites → symbiosis with animals

How are these organims able to transform N2 to useful forms?

• enzymes that overcome high activation energy

• binding and hydrolyzing ATP

Chemical transformations maintain what with respect to nitrogen?

• balance between N2 and biologically useful forms of nitrogen

Key steps of the nitrogen cycle

• fixation

• nitrification

• assimilation

• denitrification

Fixation

• bacteria reduce N2 to NH3/NH4+

Nitrification

• bacteria oxidize ammonia into nitrite (NO2-) and nitrate (NO3-)

Assimilation

• plants and microorganisms reduce NO2- and NO3- to NH3

• via nitrite reductases and nitrate reductases

What are the 3 key points of assimilation?

• NH3 incorporated into amino acids

• organisms die returning NH3 to soil

• nitrifying bacteria again convert NH3 to nitrite and nitrate

Denitrification

• nitrate is reduced to N2 under anaerobic conditions

In the nitrogen cycle, what is the ultimate electron acceptor?

• NO3- instead of O2

Nitrate reductase

• large soluble protein

• contains Mo cofactor

• catalyzes 2e- reduction of NO3- to NO2-

• NADH → e- donor

NO3- + 2 e- → ?

NO2-

Nitrite reductase

• converts products of nitrate reductase into NH4+

• found in chloroplasts in plants

NO2- + 6e- → ?

NH4+

Siroheme

• prosthetic group involved in electron transfer for assimilation via nitrite reductase