Chapter 26: Protein Metabolism

Overview of Protein Metabolism

Protein metabolism encompasses the biochemical processes involved in the breakdown, synthesis, and transformation of proteins and amino acids in living organisms. It is a central topic in organic and biological chemistry, linking the fate of dietary proteins to cellular energy production and biosynthetic pathways.

• Proteins are broken down into amino acids, which can be used for energy, biosynthesis, or converted into other metabolites.

• The amino acid pool refers to the collection of free amino acids available for metabolic processes.

• Key processes include transamination, deamination, and the urea cycle.

Dietary Protein and Caloric Contribution

• With a normal diet, approximately 10 percent of daily calories come from protein.

• Proteins are digested in the stomach, where enzymes break them into smaller peptides and amino acids.

• Most dietary protein is absorbed as amino acids after enzymatic hydrolysis.

Amino Acid Catabolism

Amino acids undergo catabolism when they are not needed for protein synthesis. The process involves removal of the amino group and conversion of the carbon skeleton into metabolic intermediates.

• Transamination: Transfer of an amino group from one amino acid to a keto acid, forming a new amino acid and a new keto acid.

• Deamination: Removal of the amino group, often producing ammonia (NH3).

• Oxidative deamination: Typically occurs with glutamate, producing α-ketoglutarate and ammonia.

Example equation:

• Transamination:

• Oxidative deamination:

Transamination Reactions

Transamination is a key step in amino acid metabolism, allowing the transfer of amino groups and the interconversion of amino acids and keto acids.

• Pyridoxal phosphate (PLP), derived from vitamin B6, is a required coenzyme.

• Common transamination pairs: glutamate/α-ketoglutarate, aspartate/oxaloacetate.

• The net effect is the collection of amino groups onto glutamate, which can then be deaminated.

Urea Cycle

The urea cycle is the primary pathway for the disposal of excess nitrogen (as ammonia) in mammals. It converts toxic ammonia into urea, which is excreted in urine.

• Key intermediates: carbamoyl phosphate, ornithine, citrulline, argininosuccinate, arginine, urea.

• Two nitrogen atoms in urea come from ammonia and aspartate.

• Overall reaction:

Classification of Amino Acids by Catabolic Fate

Amino acids are classified based on the fate of their carbon skeletons after deamination:

• Glucogenic amino acids: Degraded to pyruvate or citric acid cycle intermediates, can be used for glucose synthesis.

• Ketogenic amino acids: Degraded to acetyl CoA or acetoacetate, can be used for ketone body synthesis.

• Some amino acids are both glucogenic and ketogenic.

Key Intermediates and Products

• Carbamoyl phosphate: An intermediate in the urea cycle, formed from ammonia and CO2.

• Glutamate: Central in amino group transfer and deamination.

• Urea: Final product of nitrogen excretion in mammals.

• Bilirubin: Pigment produced from heme degradation, responsible for the yellow color of urine.

Summary Table: Amino Acid Catabolism and Urea Cycle

Additional info:

• Protein metabolism is tightly regulated and integrated with carbohydrate and lipid metabolism.

• Defects in the urea cycle can lead to hyperammonemia and metabolic disorders.

• Bile pigments such as bilirubin are important clinical markers for liver function.