Overview of Protein Metabolism

Absorbed amino acids are not stored; instead, they are immediately used for ATP production through oxidation or incorporated into new proteins for growth and repair. Surplus amino acids are converted into glucose through gluconeogenesis or triglycerides rather than excreted, ensuring efficient nutrient utilization and preventing waste.

Before amino acids can produce ATP, they undergo deamination in hepatocytes, removing the amino group to form molecules like acetyl CoA that enter the Krebs cycle. This process produces ammonia, which is rapidly converted to urea and excreted through urine, preventing toxic buildup in tissues.

Essential amino acids cannot be synthesized by the body and must be obtained through diet; ten of the twenty amino acids are essential. Nonessential amino acids are synthesized through transamination, where an amino group transfers from one amino acid to pyruvic acid or Krebs cycle intermediates.

Protein synthesis occurs on ribosomes in nearly every cell, directed by DNA and RNA through the formation of peptide bonds between amino acids. Growth hormone, insulin-like growth factors, thyroid hormones, insulin, estrogen, and testosterone stimulate this anabolic process to support growth and tissue repair.

Protein catabolism occurs daily through normal physiological processes including exercise, growth cycles, aging, and cellular turnover. While cortisol promotes protein breakdown, proteins from dying cells are continuously degraded into amino acids for reuse or conversion into other metabolic substrates like glucose.

Abnormal protein catabolism and anabolism lead to growth retardation, muscle atrophy, and increased cancer risk. Disorders like phenylketonuria, maple syrup urine disease, and lysinuric protein intolerance impair amino acid metabolism, while protein-energy malnutrition causes severe muscle wasting and immune system compromise.

Amino acids are metabolized by hepatocytes into fatty acids through lipogenesis, ketone bodies through ketogenesis, or glucose through gluconeogenesis, depending on the body's energy needs. Some amino acids can also be recycled and converted into other amino acids through transamination reactions.