Theme H – Nitrogen and amino acid metabolism Nitrogen is essential
The study of inorganic nitrogen metabolism, the assimilation of • Amino acids & proteins
ammonia in organic nitrogen compounds, and nitrogen • Nitrogen bases & nucleic acids
economy. A brief overview of amino acid anabolism and • Various coenzymes
catabolism is included. • No “reserves”
Lecture H1 – N2 cycle, N2 reduction and fixation, NH4+ • Vertebrates – nitrogen only from diet
assimilation (chapter 18)
Nitrogen metabolism • Plants – uptake of inorganic ions:
- NO3– / NO2– / NH4+
• N2 gas fixation – only some bacteria by means of
nitrogenase enzymes
• Without nitrogen – no growth
Sources of nitrogen
• Nitrogen-fixing organisms
- N2 gas in the atmosphere (80%)
- N2-fixing microorganisms
- N2 and NO3– first reduced to ammonia
• Animals
- organo-nitrogen compounds in diet
- uptake from the digestive tract
- degraded proteins and nucleic acids
- re-use/recycling of nitrogen
Essential amino acids
• Certain amino acids are essential - must eat a balanced
diet
• Animal proteins – most balanced source
, • Plant proteins – shortages and inaccessibility of certain Nitrate and nitrate reductase
amino acids (e.g. must boil soya beans for longer periods to
be digestible)
Nitrogen cycle
Tut 5
Nitrate and nitrate reductase
, Biological nitrogen fixation from N2 Nitrogenase enzyme complex
• Reduction process • Can be separated into two subunits (components)
• Ferredoxin plays an important role • Electron transport
• Series of electron transfer reactions - Fe protein – containing a [4Fe-4S] cluster that
• Very high ATP “cost” participates in the redox reactions involved in the
• Iron (Fe) and molybdenum (Mo) required conversion of N2
• Nitrogenase enzymes of microorganisms catalyze these - Fe-Mo protein – has four subunits each with two 2 redox
conversions sites ([8Fe-7S] cluster and [Fe-Mo] cluster
• N2 fixation takes place anaerobically – O2 competes (Fe-
protein is irreversibly inactivated by O2. The MoFe protein is
also inactivated by O2)
Nitrogenase • legumes: leghemoglobin binds O2
• cyanobacteria: heterocysts exclude O2
• converts one mole N2 to two moles NH3
• Ferredoxin or flavodoxin is e– donor
• Rhizobium & Bradyrhizobium in symbiosis
with legumes in root nodules (e.g. soya beans, peas, alfalfa
(South Africa, lucerne))
• also in soil bacteria (Agrobacterium, Azotobacter) Nitrogenase complex
• cyanobacteria in the ocean
The study of inorganic nitrogen metabolism, the assimilation of • Amino acids & proteins
ammonia in organic nitrogen compounds, and nitrogen • Nitrogen bases & nucleic acids
economy. A brief overview of amino acid anabolism and • Various coenzymes
catabolism is included. • No “reserves”
Lecture H1 – N2 cycle, N2 reduction and fixation, NH4+ • Vertebrates – nitrogen only from diet
assimilation (chapter 18)
Nitrogen metabolism • Plants – uptake of inorganic ions:
- NO3– / NO2– / NH4+
• N2 gas fixation – only some bacteria by means of
nitrogenase enzymes
• Without nitrogen – no growth
Sources of nitrogen
• Nitrogen-fixing organisms
- N2 gas in the atmosphere (80%)
- N2-fixing microorganisms
- N2 and NO3– first reduced to ammonia
• Animals
- organo-nitrogen compounds in diet
- uptake from the digestive tract
- degraded proteins and nucleic acids
- re-use/recycling of nitrogen
Essential amino acids
• Certain amino acids are essential - must eat a balanced
diet
• Animal proteins – most balanced source
, • Plant proteins – shortages and inaccessibility of certain Nitrate and nitrate reductase
amino acids (e.g. must boil soya beans for longer periods to
be digestible)
Nitrogen cycle
Tut 5
Nitrate and nitrate reductase
, Biological nitrogen fixation from N2 Nitrogenase enzyme complex
• Reduction process • Can be separated into two subunits (components)
• Ferredoxin plays an important role • Electron transport
• Series of electron transfer reactions - Fe protein – containing a [4Fe-4S] cluster that
• Very high ATP “cost” participates in the redox reactions involved in the
• Iron (Fe) and molybdenum (Mo) required conversion of N2
• Nitrogenase enzymes of microorganisms catalyze these - Fe-Mo protein – has four subunits each with two 2 redox
conversions sites ([8Fe-7S] cluster and [Fe-Mo] cluster
• N2 fixation takes place anaerobically – O2 competes (Fe-
protein is irreversibly inactivated by O2. The MoFe protein is
also inactivated by O2)
Nitrogenase • legumes: leghemoglobin binds O2
• cyanobacteria: heterocysts exclude O2
• converts one mole N2 to two moles NH3
• Ferredoxin or flavodoxin is e– donor
• Rhizobium & Bradyrhizobium in symbiosis
with legumes in root nodules (e.g. soya beans, peas, alfalfa
(South Africa, lucerne))
• also in soil bacteria (Agrobacterium, Azotobacter) Nitrogenase complex
• cyanobacteria in the ocean
