Summary
Highlights
Proteins are vital for almost all bodily and cellular functions, including structural support, muscle contraction, transport, and catalysis via enzymes. They are polymers of amino acids linked by peptide bonds, forming unique three-dimensional structures crucial for their function.
Amino acids consist of an amino group, a carboxyl group, and a unique side chain. Of the 20 amino acids, nearly half are essential and must be obtained from the diet. Animal proteins are considered high-quality due to their similar amino acid composition to human proteins, though plant food combinations can also provide all necessary amino acids.
Dietary proteins are broken down into individual amino acids in the stomach and small intestine by acid and enzymes. These amino acids are absorbed into the bloodstream and transported to the liver, where they are used to synthesize new proteins, including plasma proteins, and distributed to other tissues for tissue-specific protein synthesis. Protein synthesis is based on the cell's genetic information.
Body proteins are continually renewed, with older proteins broken down and recycled into new ones. Unlike carbohydrates and lipids, proteins cannot be stored. Excess amino acids are degraded for energy or converted into glucose or fatty acids. This also occurs during energy shortages, such as prolonged exercise or fasting.
Amino acid degradation begins with the removal of the amino group via deamination or transamination, producing keto-acids. These keto-acids enter metabolic cycles for energy production. Deamination produces ammonia, which is toxic and converted to urea by the liver for excretion in urine. Extremely high protein diets can overwhelm the kidneys with nitrogenous waste, potentially causing damage.