Lecture 38: Nitrogen Cycle Fixation Flashcards
What are the most abundant biological elements?
H, C, N, O, P
What are the available form in the enviroment of H, O, and P
H20
O2
Pi
What is the availabe form in the environment of C?
CO2
CO2`
unreactive (very stable)
- Primarily fixed by photosynthesis
- C-O bond energy is 351 kJ/mol
What is the available form in the environment of N
N2
N2
unreactive (very stable)
-only a few strains of bacteria can convert to a metabolically useful form
What is the body energy for a N/N triple bond?
945 kJ/mol
Nitrogen
required in proteins and nucleic acids
The formation of NH3 from N2 and 3H2
- exergonic reaction
- problem is that N2 is a very table molecule so the rate of formation of NH3 is negligible except under extreme conditions
NH3 in aqueous solution
- must be made under anaerobic conditions
- those organisms that fix N2 either grow under anaerobic conditions or burn off O2 by uncoupling ATP synthesis from oxidation of carbohydrates
NH3
- plants utilize for growth
- toxic to animals and is incorporated for dietary proteins
Anammox
- unusual organism
- reduce nitrite and nitrate to N2H4 (Hydrazine) and then oxidize hydrazine to N2
Nitrogen Cycle
NH3(NH4+) is oxidized to nitrites and nitrates by other special of bacteria and archaea
-nitrates are reduced to N2 by denitrifying organisms (most are green plants, bacteria, archaea, and fungi)
Organisms that can fix atmospheric nitrogen
1) Free living bacteria
2) Cyanobacteria
3) Symbionts
Free living bacteria
- Aerobic (Azotobacter)
- Anaerobic ( Desulfovibrio)
Cyanobacteria
Nostoc (common in lichen)
-N2 fixation occurs in specialized anaerobic cells (heterocysts)
Symbionts
live in symbiosis with certain plants
- with legumes (Rhizobium)
- with other plants (Frankia)
Problem: Nitrogenase inactivated by O2
utilize anaerobic organisms or maintain anaerobic conditions for nitrogen fixation
Symbiosis
Relationship is beneficial to both microbe and plant
1) Bacteria Fixes N2
2) Plant provides anaerobic environment by making Leghemoglobin which sequesters O2
3) Plant also supplies ATP
Symbiotic relationship: Benefits to bacteria
- requires anaerobic environments since nitrogenase is inactivated by O2
- N2 fixation requires a lot of energy-provided by the plant
- Provides organic acid precursors of amino acids
Symbiotic relationships: benefits to the plant
-provides nitrogen to plant in the form of amino acids
N2 fixation (reduction) reaction
- reaction with one N2 molecule occurs at one of the MoFe7S9N centers in dinitrogenase
- dinitrogenase has to reach a highly reduced state before the reaction can begin, which occurs from a transfer of e-s coming from the oxidation of pyruvate
- an intermediary step involves dinitrogenase reductase donating e-; this only happens if the enzyme is in a conformation induced by ATP
- there is ATP hydrolysis upon passage of e-s
What three problems does the symbiotic growth of microorganisms with plants in nodules solve?
- the plant helps supply energy needed for the N2 fixation reaction and helps provide leg hemoglobin that sequesters O2
- ensures an anaerobic enviroment
- in return NH3 is provided to the plant
NH3 assimilation for mammals: what amino acid is important and how is it accomplished?
Glutamine
- high concentrations of L-Glutamate are obtained from dietary proteins which are catabolized to the level of amino acids
- amino acids are used in amino transfer reactions that transfer the NH3 to alpha-ketogluterate to make L-glutamate
- The L-glutamine will be made from L-Glutamate by glutamine synthetase
