1. The Metabolism of Nitrogen
Chapter Twenty-Three
The Metabolism of Nitrogen

2. Nitrogen Fixation Nitrogen fixation is the reduction of N2 to NH3:
Bacteria are responsible for the reduction and typically form symbiotic relationships that result in nodules on the roots of leguminous plants
Reduction is catalyzed by the nitrogenase enzyme complex
N2 to ammonium ion is a six-electron reduction

3. The Path of Electrons from Ferrodoxin to N2

4. Amino Acid Biosynthesis
Common features of amino acid biosynthesis include: transamination and one-carbon transfers
Glutamate is formed by reductive amination of -ketoglutarate and NH4+
Amidation of glutamate gives glutamine
• All amino acids are grouped into families based on their biosynthetic pathways

5. Amino Acid Biosynthesis (Cont’d)

6. Amino Acids and The Citric Acid Cycle

7. Role of Pyridoxal Phosphate in Amino Acid Reactions (Cont’d)

8. Role of Pyridoxal Phosphate in Amino Acid Reactions (Cont’d)
Transamination reactions switch amino groups form one amino acid to an -keto acid

9. Serine to Glycine Involves the Transfer of a One-Carbon Unit
Serine to glycine is an example of a one-carbon transfer
The one-carbon acceptor is tetrahydrofolate, which is derived from folic acid

10. Serine to Glycine Involves the Transfer of a One-Carbon Unit (Cont’d)
Reduction of folic acid gives tetrahydrofolic acid (THF), the reactive form of the coenzyme
Tetrahydrofolate is a carrier of the one-carbon groups shown in Figure (see next slide)

11. Structure and Reactions of Folic Acid

12. Serine to Cysteine (Cont’d)

13. Methionine
Methionine cannot be produced in animals, making it an essential amino acid
Methionine reacts with ATP to form S-adenosylmethionine (SAM)

14. Cysteine in Animals
SAM is a methyl group carrier and this methyl group can be transferred to a number of acceptors producing S-adenosylhomocysteine

15. Amino Acid Catabolism (Cont’d)
The -amino group which has been transferred to -ketoglutarate has one of two fates:
It may be used for biosynthesis
It may be excreted as a part of a nitrogen-containing product

16. The Urea Cycle
The urea cycle is the central pathway in nitrogen metabolism
The nitrogens come from several sources
Steps of the cycle are outlined in Figure (see next slide)

17. The Urea Cycle (Cont’d)