Protein | Definition, Structure, & Classification | Britannica (2024)

biochemistry

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Also known as: macromolecular peptide

Written by

Felix Haurowitz Distinguished Professor of Chemistry, Indiana University, Bloomington. Author of Chemistry and Function of Proteins.

Felix Haurowitz,

Daniel E. Koshland Emeritus Professor of Biochemistry, University of California, Berkeley. Editor, Science magazine, 1985–95.

Daniel E. KoshlandAll

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Top Questions

What is a protein?

A protein is a naturally occurring, extremely complex substance that consists of amino acid residues joined by peptide bonds. Proteins are present in all living organisms and include many essential biological compounds such as enzymes, hormones, and antibodies.

Where does protein synthesis take place?

Protein synthesis occurs in the ribosomes of cells. In eukaryotic cells, ribosomes are found as free-floating particles within cells and are also embedded in the rough endoplasmic reticulum, a cell organelle.

Where is protein stored?

Proteins are not stored for later use in animals. When an animal consumes excess proteins, they are converted into fats (glucose or triglycerides) and used to supply energy or build energy reserves.If an animal is not consuming sufficient protein, the body begins to break down protein-rich tissues, such as muscles, leading to muscle wasting and eventually death if the deficiency is severe.

What do proteins do?

Proteins are essential for life and are essential for a wide range of cellular activities. Protein enzymes catalyze the vast majority ofchemical reactionsthat occur in thecell. Proteins provide many of the structural elements of a cell, and they help to bind cells together into tissues. Proteins, in the form ofantibodies, protect animals from disease, and manyhormonesare proteins. Proteins control the activity ofgenes and regulate gene expression.

protein, highly complex substance that is present in all living organisms. Proteins are of great nutritional value and are directly involved in the chemical processes essential for life. The importance of proteins was recognized by chemists in the early 19th century, including Swedish chemist Jöns Jacob Berzelius, who in 1838 coined the term protein, a word derived from the Greek prōteios, meaning “holding first place.” Proteins are species-specific; that is, the proteins of one species differ from those of another species. They are also organ-specific; for instance, within a single organism, muscle proteins differ from those of the brain and liver.

A protein molecule is very large compared with molecules of sugar or salt and consists of many amino acids joined together to form long chains, much as beads are arranged on a string. There are about 20 different amino acids that occur naturally in proteins. Proteins of similar function have similar amino acid composition and sequence. Although it is not yet possible to explain all of the functions of a protein from its amino acid sequence, established correlations between structure and function can be attributed to the properties of the amino acids that compose proteins.

Plants can synthesize all of the amino acids; animals cannot, even though all of them are essential for life. Plants can grow in a medium containing inorganic nutrients that provide nitrogen, potassium, and other substances essential for growth. They utilize the carbon dioxide in the air during the process of photosynthesis to form organic compounds such as carbohydrates. Animals, however, must obtain organic nutrients from outside sources. Because the protein content of most plants is low, very large amounts of plant material are required by animals, such as ruminants (e.g., cows), that eat only plant material to meet their amino acid requirements. Nonruminant animals, including humans, obtain proteins principally from animals and their products—e.g., meat, milk, and eggs. The seeds of legumes are increasingly being used to prepare inexpensive protein-rich food (see human nutrition).

The protein content of animal organs is usually much higher than that of the blood plasma. Muscles, for example, contain about 30 percent protein, the liver 20 to 30 percent, and red blood cells 30 percent. Higher percentages of protein are found in hair, bones, and other organs and tissues with a low water content. The quantity of free amino acids and peptides in animals is much smaller than the amount of protein; protein molecules are produced in cells by the stepwise alignment of amino acids and are released into the body fluids only after synthesis is complete.

The high protein content of some organs does not mean that the importance of proteins is related to their amount in an organism or tissue; on the contrary, some of the most important proteins, such as enzymes and hormones, occur in extremely small amounts. The importance of proteins is related principally to their function. All enzymes identified thus far are proteins. Enzymes, which are the catalysts of all metabolic reactions, enable an organism to build up the chemical substances necessary for life—proteins, nucleic acids, carbohydrates, and lipids—to convert them into other substances, and to degrade them. Life without enzymes is not possible. There are several protein hormones with important regulatory functions. In all vertebrates, the respiratory protein hemoglobin acts as oxygen carrier in the blood, transporting oxygen from the lung to body organs and tissues. A large group of structural proteins maintains and protects the structure of the animal body.

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Protein | Definition, Structure, & Classification | Britannica (2024)
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