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Globular proteins, also known as spheroproteins, are proteins formed by compacted amino acid chains, which are folded into intricate shapes that often roughly resemble spheres. This type of protein represents one of the three major protein groups. The two other groups are fibrous proteins, which are primarily structural proteins, and membrane proteins, which are usually found attached to the membranes of cells and their organelles. A key difference between globular and fibrous proteins is that the former type is usually soluble in water, and therefore in blood plasma, while the latter type is not. Globular protein molecules can play many roles: they can be enzymes, biological messengers and transport mechanisms, and they may also be found acting as structural proteins within animal cells.
Almost all enzymes are globular proteins that vary in size, shape, and complexity. The role of enzymes in biochemistry is to act as catalysts, substances that speed or slow the rate of chemical reactions. Some common examples are amylase and lactase. Amylase is found in the saliva of humans, where it aids the digestive process by breaking starches down into simpler sugars. Lactase is the enzyme that allows humans to digest lactose; individuals with insufficient levels of this enzyme are often lactose intolerant.
This type of protein acts as biological messengers in the form of peptide hormones, which play an important role in metabolism, growth and development. Some peptide hormones of note are prolactin and insulin. Prolactin is secreted by the pituitary gland and triggers the production of breast milk in a nursing mother. Insulin is produced by the pancreas and aids in the body's ability to regulate energy and metabolize sugars.
Another important function of globular proteins is to act as biological transport mechanisms, usually in the forms of globulin and serum albumin. These two molecules are serum proteins, also called blood proteins, found within the blood plasma of living animals. A familiar form of globulin is hemoglobin, which is synthesized within red blood cells and transports oxygen throughout the body. Serum albumin, or simply albumin, is produced by the liver and transports fatty acids and hemin, an iron-rich organic compound that gives red blood cells their characteristic color.
Although these proteins are most often messenger or transporter proteins, they can also play a structural role within animal cells. One known as tubulin is responsible for the production of microtubules in cells. Microtubules are vital components of the cytoskeleton, a sort of scaffold within the cytoplasm that allows the cell to maintain its shape and structure. Globular proteins called structure motifs are also found within the cell membrane, where they again contribute to the cell's structure.