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**History and Etymology**:
– Proteins recognized as distinct biological molecules in the 18th century
– Noted examples include albumin, blood serum albumin, fibrin, and wheat gluten
– Described by Gerardus Johannes Mulder and named by Jöns Jacob Berzelius in 1838
– Term ‘protein’ derived from Greek word ‘πρώτειος’
– Early nutritional scientists believed protein was crucial for body structure

**Structure and Function**:
– Proteins are large biomolecules made of amino acid chains
– Functions include catalyzing metabolic reactions, DNA replication, and cell structure
Protein folding into specific 3D structures determines activity
– Proteins can have prosthetic groups or cofactors attached
Protein complexes can form to achieve specific functions

**Metabolism and Importance**:
– Essential for virtually every cellular process
– Enzymes like actin and myosin are vital for metabolism
– Roles in cell signaling, immune responses, and cell adhesion
– Animals require proteins in their diet for essential amino acids
Digestion breaks down proteins for metabolic use

**Synthesis, Degradation, and Techniques**:
– Synthesized from gene sequences and modified post-translationally
Protein turnover process degrades and recycles proteins
– Proteins can be purified using techniques like chromatography and electrophoresis
– Methods like ultracentrifugation and chromatography used for protein purification
– Techniques like X-ray crystallography and mass spectrometry study protein structure and function

**Classification, Biochemistry, and Interactions**:
– Proteins classified by sequence, structure, and function
– Amino acids have common structural features
Protein interactions crucial for cellular processes
– Protein-protein interactions key in signal transduction pathways
– Average-sized bacteria contain about 2 million proteins per cell

Protein (Wikipedia)

Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, responding to stimuli, providing structure to cells and organisms, and transporting molecules from one location to another. Proteins differ from one another primarily in their sequence of amino acids, which is dictated by the nucleotide sequence of their genes, and which usually results in protein folding into a specific 3D structure that determines its activity.

A representation of the 3D structure of the protein myoglobin showing turquoise α-helices. This protein was the first to have its structure solved by X-ray crystallography. Toward the right-center among the coils, a prosthetic group called a heme group (shown in gray) with a bound oxygen molecule (red).

A linear chain of amino acid residues is called a polypeptide. A protein contains at least one long polypeptide. Short polypeptides, containing less than 20–30 residues, are rarely considered to be proteins and are commonly called peptides. The individual amino acid residues are bonded together by peptide bonds and adjacent amino acid residues. The sequence of amino acid residues in a protein is defined by the sequence of a gene, which is encoded in the genetic code. In general, the genetic code specifies 20 standard amino acids; but in certain organisms the genetic code can include selenocysteine and—in certain archaeapyrrolysine. Shortly after or even during synthesis, the residues in a protein are often chemically modified by post-translational modification, which alters the physical and chemical properties, folding, stability, activity, and ultimately, the function of the proteins. Some proteins have non-peptide groups attached, which can be called prosthetic groups or cofactors. Proteins can also work together to achieve a particular function, and they often associate to form stable protein complexes.

Once formed, proteins only exist for a certain period and are then degraded and recycled by the cell's machinery through the process of protein turnover. A protein's lifespan is measured in terms of its half-life and covers a wide range. They can exist for minutes or years with an average lifespan of 1–2 days in mammalian cells. Abnormal or misfolded proteins are degraded more rapidly either due to being targeted for destruction or due to being unstable.

Like other biological macromolecules such as polysaccharides and nucleic acids, proteins are essential parts of organisms and participate in virtually every process within cells. Many proteins are enzymes that catalyse biochemical reactions and are vital to metabolism. Proteins also have structural or mechanical functions, such as actin and myosin in muscle and the proteins in the cytoskeleton, which form a system of scaffolding that maintains cell shape. Other proteins are important in cell signaling, immune responses, cell adhesion, and the cell cycle. In animals, proteins are needed in the diet to provide the essential amino acids that cannot be synthesized. Digestion breaks the proteins down for metabolic use.

Proteins may be purified from other cellular components using a variety of techniques such as ultracentrifugation, precipitation, electrophoresis, and chromatography; the advent of genetic engineering has made possible a number of methods to facilitate purification. Methods commonly used to study protein structure and function include immunohistochemistry, site-directed mutagenesis, X-ray crystallography, nuclear magnetic resonance and mass spectrometry.

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