## PROTEINS

Proteins are large, complex molecules that play many critical roles in the body. - They do most of the work in cells and are required for the structure, function, and regulation of the body’s tissues and organs. - Proteins are made up of hundreds or thousands of smaller units called amino acids, which are attached to one another in long chains. - There are 20 different types of amino acids that can be combined to make a protein. - The sequence of amino acids determines each protein’s unique 3-dimensional structure and its specific function.

## Types of Proteins

A protein molecule that consists of but a single polypeptide chain is said to be monomeric; proteins made up of more than one polypeptide chain, as many of the large ones are, are called oligomeric. - Based upon chemical composition, proteins are divided into two major classes: • simple proteins, which are composed of only amino acids • conjugated proteins, which are composed of amino acids and additional organic and inorganic groupings, certain of which are called prosthetic groups. - Conjugated proteins include glycoproteins, which contain carbohydrates. lipoproteins, which contain lipids; and nucleoproteins, which contain nucleic acids. - Classified by biological function: • proteins include the enzymes, which are responsible for catalyzing the thousands of chemical reactions of the living cell. • keratin, elastin, and collagen, which are important types of structural, or support, proteins • hemoglobin and other gas transport proteins; ovalbumin, casein, and other nutrient molecules • antibodies, which are molecules of the immune system (see immunity); protein hormones, which regulate metabolism and • proteins that perform mechanical work, such as actin and myosin, the contractile muscle proteins.

## Protein sources

Proteins provide the 20 amino acids which make up the cells, tissues and organs of your body. - Your body makes over half the amino acids needed, but your diet must provide the rest, the essential amino acids. Some foods are “complete” sources of protein and contain all the essential amino acids. - Animal-based foods are the main sources of complete protein and include meat, fish, poultry, eggs, milk and cheese. - Plant-based foods also contain protein, but are “incomplete” sources of protein, meaning they contain only some of the essential amino acids. - These incomplete proteins include nuts and seeds, rice, beans, corn and tofu. But eating complementary plant-based foods can provide your complete essential amino acid requirement. - For example, beans and peas complement rice or nuts and seeds, each providing the essential amino acid the other lacks. Soybeans are the only plant-based food considered a complete protein, providing all essential amino acids.