Whether found in humans like, animals, plants or in single-celled bacteria, proteins are made of units of 20 different amino acids. Proteins consist of macromolecules called polypeptides, made from monomers called amino acids. Proteins have hundreds, thousands, or sometimes even millions of these amino acids. These amino acids are made up of carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur. Some proteins consist of only single polypeptides. In most cases it involves two or more combined polypeptide

s, sometimes with other small organic molecules or metal ions.

There are two types of proteins, fibrous and globular. The major fibrous proteins are collagen, keratin, fibrinogen, and muscle proteins. Collagen, which makes up the bones, skin, tendons, and cartilage, is the most abundant protein found in vertebrates. Keratin, which makes up the outermost layer of skin and the hair, scales, hooves, nails, and feathers of animals, twists into a regularly repeating coil called an alpha helix. Further more, fibrinogen is the blood plasma protein responsible for blood clotting. Lastly, myosin is the protein chiefly responsible for muscle contraction, combines with actin, another muscle protein, forming actomyosin, the different filaments of which shorten, causing the contracting action. Unlike fibrous proteins, globular proteins are spherical and highly soluble. They play a dynamic role in body metabolism. Examples are albumin, globulin, casein, hemoglobin, all of the enzymes, and protein hormones. Hemoglobin is a respiratory

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