Functions of Proteins and Their Importance in the Human Body

1. Dietary Protein as a Source of Energy

The calorie value for protein on the Nutrition Facts labels of foods is 4 Calories per gram, which represents “metabolizable energy,” which can be used by the human body. But if we distract the energy used to digest, absorb and metabolize protein, we get 3.2 Calories per gram of “net metabolizable energy,” which can be converted to physical energy or body weight [2,3]. Net metabolizable energies for carbohydrates (4 Cal/g) and fats (9 Cal/g) are practically the same as their metabolizable energies [2,3] .

2. Proteins in the Human Body

Dietary proteins provide building blocks–amino acids–for proteins in the human body:

Structural proteins: C ollagen gives the structure to the bones, skin and connective tissue (tendons and ligaments); elastin gives elasticity to the skin; keratin gives structures to the nails and hair; keratin is also in the upper layer of the skin; actin and myosin enable muscle contraction, and cell membrane proteins regulate influx and outflux of certain substances into and out from the cells. Transport proteins: Hemoglobin carries oxygen, and transferrin carries iron in the blood; myoglobin carries oxygen in the muscles; ferritin stores iron, and cytochromes carry electrons in the respiratory chain in the cells; albumin carries certain ions, hormones, vitamins, bilirubin, drugs and toxins via the blood. Albumin also helps to retain water in the blood. Enzymes enable synthesis or breakdown of most substances in the body. Thrombin and fibrin enable blood coagulation; fibrin also enables wound healing. The hormones insulin, oxytocin, somatotropin and thyroxine are proteins. Proteins released into the blood in inflammation: C-reactive protein, prostaglandins, prostacyclins . Antibodies (immunoglobulins IgA, IgE, IgG, IgM) enable defense against microbes (immunity).

Protein deficiency can affect all above functions.

Synthesis of proteins in the human body is regulated by DNA [129] .