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.

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Sleator RD (2012). "Prediction of Protein Functions". Functional Genomics. Methods in Molecular Biology. Vol.815. pp.15–24. doi: 10.1007/978-1-61779-424-7_2. ISBN 978-1-61779-423-0. PMID 22130980. a b c d e f g Murray RF, Harper HW, Granner DK, Mayes PA, Rodwell VW (2006). Harper's Illustrated Biochemistry. New York: Lange Medical Books/McGraw-Hill. ISBN 978-0-07-146197-9. Mostproteins fold into unique 3D structures. The shape into which a protein naturally folds is known as its native conformation. [27] :36 Although many proteins can fold unassisted, simply through the chemical properties of their amino acids, others require the aid of molecular chaperones to fold into their native states. [27] :37 Biochemists often refer to four distinct aspects of a protein's structure: [27] :30–34 Rüdiger H, Siebert HC, Solís D, Jiménez-Barbero J, Romero A, von der Lieth CW, Diaz-Mariño T, Gabius HJ (April 2000). "Medicinal chemistry based on the sugar code: fundamentals of lectinology and experimental strategies with lectins as targets". Current Medicinal Chemistry. 7 (4): 389–416. doi: 10.2174/0929867003375164. PMID 10702616. This article is about a class of molecules. For protein as a nutrient, see Protein (nutrient). For other uses, see Protein (disambiguation).Muñoz-Huerta, Rafael F.; Guevara-Gonzalez, Ramon G.; Contreras-Medina, Luis M.; Torres-Pacheco, Irineo; Prado-Olivarez, Juan; Ocampo-Velazquez, Rosalia V. (Aug 16, 2013). "A Review of Methods for Sensing the Nitrogen Status in Plants: Advantages, Disadvantages and Recent Advances". Sensors. 13 (8): 10823–10843. Bibcode: 2013Senso..1310823M. doi: 10.3390/s130810823. PMC 3812630. PMID 23959242. Sumner JB (1926). "The isolation and crystallization of the enzyme urease. Preliminary paper" (PDF). Journal of Biological Chemistry. 69 (2): 435–41. doi: 10.1016/S0021-9258(18)84560-4. Archived from the original on 2011-03-25 . Retrieved 2011-01-16.