Those macronutrients that are long chains (polymers) of amino acids (joined with peptide bonds).
They play several essential roles:
- structural (e.g. collagen), participating in the renewal of muscular tissue, bone matrix, skin, phanera (hair, nails), etc.
- functional, catalyzing reactions in the form of enzymes, ensuring cell signaling in the form of hormones (e.g. insulin) or neurotransmitters, transporting molecules like oxygen (e.g. hemoglobin) or nutrients, defending against pathogens in the form of antibodies (e.g. immunoglobulins), enabling muscle contraction (e.g. myosin), etc.
- energetic, as the third (and lesser) energy-yielding nutrient
Some foods high in proteins: meat, fish & shellfish, eggs, dairy, nuts & seeds, or legumes.
A distinction can be made between animal proteins and plant proteins.