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| − | [[Image:Fried egg, sunny side up.jpg|thumb|250px|The protein in this [[Egg (food)|egg]] has undergone denaturation and loss of solubility, caused by the high temperature of the [[cooking]] process.]]
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| − | If proteins in a living cell are denatured, this results in disruption of cell activity and possibly cell death. Denatured proteins can exhibit a wide range of characteristics, from loss of solubility to communal aggregation. [[Denatured alcohol]] is an exception to this definition, as the term refers not to any alteration of the substance's structure but to the addition of toxins and other things to render it undrinkable.
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| − | == Common examples ==
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| − | When food is cooked, some of its proteins become denatured. This is why boiled eggs become hard and cooked meat becomes firm.
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| − | A classic example of denaturing in proteins comes from [[egg white]]s, which are largely egg [[ovalbumin|albumin]]s in water. Fresh from the eggs, egg whites are transparent and [[liquid]]. Cooking the [[Thermostability|thermally unstable]] whites turns them opaque, forming an interconnected [[solid]] mass. The same transformation can be effected with a denaturing chemical. Pouring egg whites into a beaker of [[acetone]] will also turn egg whites opaque and solid. The skin which forms on [[curdled]] milk is another common example of denatured protein. And the traditional Peruvian cold appetizer known as [[ceviche]] is prepared by chemically "cooking" raw fish and shellfish in an acidic citrus marinade, without heat.
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| − | Although denaturating egg whites is irreversible, in many other cases, denaturing is reversible.
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| − | == Protein denaturation ==
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| − | Denatured proteins can exhibit a wide range of characteristics, from loss of [[solubility]] to [[communal aggregation]]. Communal aggregation is the phenomenon of aggregation of the hydrophobic proteins to come closer and form the bonding between them, so as to reduce the total area exposed to water. It is a very common problem with the hydrophobic protein to make aggregates. Such aggregates hamper the filtration process by cake formation.
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| − | === Background ===
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| − | [[Protein]]s are very long strands of [[amino acid]]s linked together in specific sequences. A protein is created by [[ribosome]]s that "read" mRNA that is encoded by [[codon]]s in the gene and assemble the requisite amino acid combination from the [[DNA|genetic]] instruction, in a process known as [[Translation (genetics)|translation]]. The newly created protein strand then undergoes [[posttranslational modification]], in which additional [[atom]]s or [[molecule]]s are added, for example [[copper]], [[zinc]] or [[iron]]. Once this post-translational modification process has been completed, the protein begins to fold (spontaneously, and sometimes with [[enzymatic]] assistance), curling up on itself so that [[hydrophobic]] elements of the protein are buried deep inside the structure and [[hydrophilic]] elements end up on the outside. The final shape of a protein determines how it interacts with its environment.
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| − | When a protein is denatured, the [[Secondary, tertiary and quaternary structure|secondary]] and [[tertiary structure]]s are altered but the [[peptide bond]]s between the amino acids are left intact. Since the structure of the protein determines its function, the protein can no longer perform its function once it has been denatured. This is in contrast to [[intrinsically unstructured proteins]], which are unfolded in their [[native state]], but still functionally active.
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| − | === How denaturation occurs at levels of protein structure ===
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| − | {{seealso|Protein structure}}
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| − | * In '''[[quaternary structure]]''' denaturation, protein sub-units are dissociated and/or the spatial arrangement of protein subunits is disrupted.
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| − | * '''[[Tertiary structure]]''' denaturation involves the disruption of:
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| − | :* [[Covalent]] interactions between amino acid [[side chain]]s (such as [[disulfide bridge]]s between [[cysteine]] groups)
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| − | :* Noncovalent [[dipole]]-dipole interactions between polar amino acid side chains (and the surrounding [[solvent]])
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| − | :* [[Van der Waals force|Van der Waals (induced dipole) interactions]] between nonpolar amino acid side chains.
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| − | * In '''[[secondary structure]]''' denaturation, proteins lose all regular repeating patterns such as [[alpha helix|alpha-helices]] and [[beta sheet|beta-pleated sheets]], and adopt a [[random coil]] configuration.
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| − | * '''[[Primary structure]]''', such as the sequence of amino acids held together by covalent [[peptide bond]]s, is not disrupted by denaturation.
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| − | ==== Loss of function ====
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| − | Most biological proteins lose their biological function when denatured. For example, [[enzyme]]s lose their [[catalysis|activity]], because the substrates can no longer bind to the [[active site]], and because amino acid residues involved in stabilizing substrates' [[transition state]]s are no longer positioned to be able to do so.
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| − | ==== Reversibility and irreversibility ====
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| − | In many proteins (unlike egg whites), denaturation is reversible (the proteins can regain their native state when the denaturing influence is removed). This was important historically, as it led to the notion that all the information needed for proteins to assume their native state was encoded in the primary structure of the protein, and hence in the [[DNA]] that codes for the protein.
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| − | == Nucleic acid denaturation ==
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| − | The denaturation of [[nucleic acid]]s such as [[DNA]] due to high temperatures, is the separation of a double strand into two single strands, which occurs when the [[hydrogen bond]]s between the strands are broken. This may occur during [[polymerase chain reaction]]. Nucleic acid strands realign when "normal" conditions are restored during [[annealing (biology)|annealing]]. If the conditions are restored too quickly, the nucleic acid strands may realign imperfectly.
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| − | == Denaturants ==
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| − | === Acids ===
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| − | [[Acid]]ic protein denaturants include:
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| − | * [[Acetic acid]]
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| − | * [[Trichloroacetic acid]] 12% in water
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| − | * [[Sulfosalicylic acid]]
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| − | === Solvents ===
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| − | Most organic [[solvent]]s are denaturing, including:
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| − | * [[Ethanol]]
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| − | * [[Methanol]]
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| − | * [[Acetone]]
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| − | === Cross linking reagents ===
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| − | [[Cross link]]ing agents for proteins include:
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| − | * [[Formaldehyde]]
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| − | * [[Glutaraldehyde]]
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| − | === Chaotropic agents ===
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| − | [[Chaotropic agent]]s include:
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| − | * [[Urea]] 6 - 8 [[molarity|mol/l]]
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| − | * [[Guanidinium chloride]] 6 mol/l
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| − | * [[Lithium perchlorate]] 4.5 mol/l
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| − | === Disulfide bond reducers ===
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| − | Agents that break [[disulfide bond]]s by reduction include:
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| − | * [[2-Mercaptoethanol]]
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| − | * [[Dithiothreitol]]
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| − | * [[TCEP]] (tris(2-carboxyethyl)phosphine)
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| − | === Other ===
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| − | * [[Picric acid]]
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| − | == See also ==
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| − | * [[Amino acid]]
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| − | * [[Peptide]]
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| − | * [[Protein]]
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| − | * [[Protein folding]]
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| − | * [[Random coil]]
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| − | * [[Fixation (histology)]]
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| − | == References ==
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| − | == External links ==
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| − | [[Category:Protein structure]]
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| − | [[Category:Nucleic acids]]
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| − | {{credit|247514644}}
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