The Metabolism of Nitrogen

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Section M Nitrogen metabolism

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Presentation transcript:

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

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

The Path of Electrons from Ferrodoxin to N2

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

Amino Acid Biosynthesis (Cont’d)

Amino Acids and The Citric Acid Cycle

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

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

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

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 23.11 (see next slide)

Structure and Reactions of Folic Acid

Serine to Cysteine (Cont’d)

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

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

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

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 23.18 (see next slide)

The Urea Cycle (Cont’d)