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| [[File:Peptide bond.png|thumb|Peptide bond]] [[File:Glycine-condensation-2-3D-balls.png|thumb|180px|Glycine condensation]] | | Cook mini carrots, available [http://rockyou.zendesk.com/entries/52064770-How-Do-Appetite-Suppressants-Work- does malt Vinegar Go bad] in the Sandy Macey's just for $1. That's why it's critically vital that you supplement your diet using a great high-quality All-Natural Multi-Vitamin created from Whole-Food [http://Sanbird.com/xe/?document_srl=193991 sources].<br><br><br><br>[http://dore.gia.ncnu.Edu.tw/88ipart/node/2770000 ncnu.edu.tw]Focus you ab workout on standing abdominal exercises and lower abdominal training exercises. - Keep [http://www.directorymanila.net/technology/2014/08/paraben-free-natual-skin-care-be-aware-of-truth/ do tea bags ever go bad] different foods in several containers, preferably well sealed.<br><br>However, the study won't specifically state that the national reading level average is a the eighth grade level. Writing workshop Ny has been magnificent locate that may make a fruitful mind, helping them to polish their writing skill. [http://support.luxmodels.com/entries/52241430-Things-To-Eat-To-Get-A-Cut-Awesome-A-Lose-Weight storing ginger root] does watermelon go bad when left out After describing your character, how to keep celery fresh in the refrigerator you can work on softening him, giving him brief descriptions and emotions through the chapter. Many of us start to pull back from your writing, we yearn to control it, along with controlling what we've pencilsto say we stuck. |
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| A '''peptide bond''' ('''amide bond''') is a [[covalent bond|covalent]] [[chemical bond]] formed between two [[molecule]]s when the [[carboxyl group]] of one molecule reacts with the [[amino group]] of the other molecule, causing the release of a molecule of [[water]] (H<sub>2</sub>O), hence the process is a [[dehydration synthesis]] reaction
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| (also known as a [[condensation reaction]]), and usually occurs
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| between [[amino acid]]s. The resulting C(O)NH bond is called a peptide bond, and the resulting molecule is an [[amide]]. The four-atom functional group -C(=O)NH- is called a '''peptide link'''. [[Polypeptide]]s and [[protein]]s are chains of [[amino acid]]s held together by peptide bonds, as is the backbone of [[Peptide nucleic acid|PNA]].
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| A peptide bond can be broken by [[hydrolysis]] (the adding of water). In the presence of water they will break down and release 8–16 [[joule|kilojoule]]/[[mole (unit)|mol]] (2–4 [[calorie|kcal]]/[[mole (unit)|mol]]) <ref>Martin RB. (1998) "Free energies and equilibria of peptide bond hydrolysis and formation", ''Biopolymers'', '''45''', 351–353.</ref> of [[Thermodynamic free energy|free energy]]. This process is extremely slow (up to 1000 years). In living organisms, the process is facilitated by [[enzyme]]s. Living organisms also employ enzymes to form peptide bonds; this process requires free energy. The [[wavelength]] of [[absorbance]] for a peptide bond is 190–230 nm<ref>Goldfarb AR et al. (1951) "The Ultraviolet Absorption Spectra of Proteins", ''J. Biological Chem.'', '''193''', 397–404.(http://www.jbc.org/content/193/1/397.long)</ref> (which makes it particularly susceptible to [[UV]] radiation).
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| [[File:Amidbildung.svg|thumb|center|upright=2.0|alt=Figure 1: Dehydration synthesis (condensation) reaction forming an amide|Figure 1: Dehydration synthesis (condensation) reaction forming an amide]]
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| ==Cis/trans isomers of the peptide group==
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| Significant delocalisation of the [[lone pair]] of electrons on the nitrogen atom gives the group a [[Amide#Structure_and_bonding|partial double bond]] character. The partial double bond renders the amide group [[Plane (geometry)|planar]], occurring in either the [[Cis-trans isomerism|cis]] or [[trans isomer]]s. In the unfolded state of proteins, the peptide groups are free to isomerize and adopt both isomers; however, in the folded state, only a single isomer is adopted at each position (with rare exceptions). The trans form is preferred overwhelmingly in most peptide bonds (roughly 1000:1 ratio in trans:cis populations). However, X-Pro peptide groups tend to have a roughly 3:1 ratio, presumably because the symmetry between the <math>\mathrm{C^{\alpha}}</math> and <math>\mathrm{C^{\delta}}</math> atoms of [[proline]] makes the cis and trans isomers nearly equal in energy (See figure, below).
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| <br clear=all/>[[Image:Cis trans isomerization kinetics X Pro peptide bonds.png|thumb|center|upright=2.7|alt=Diagram of the isomerization of an X-Pro peptide bond. The diagram shows the cis isomer on the left, the transition states in the center, and the trans isomer on the right, with bidirectional arrows between each pair of states.|Isomerization of an X-Pro peptide bond. Cis and trans isomers are at far left and far right, respectively, separated by the transition states.]]<br clear = all/> | |
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| The [[dihedral angle]] associated with the peptide group (defined by the four atoms <math>C^{\alpha}-C^{\prime}-N-C^{\alpha}</math>) is denoted <math>\omega</math>; <math>\omega=0^{\circ}</math> for the cis isomer and <math>\omega=180^{\circ}</math> for the trans isomer. Amide groups can isomerize about the C-N bond between the cis and trans forms, albeit slowly (<math>\tau \sim</math>20 seconds at room temperature). The [[transition state]]s <math>\omega= \pm 90^{\circ}</math> requires that the partial double bond be broken, so that the activation energy is roughly 80 kilojoule/mol (20 kcal/mol) (See Figure below). However, the [[activation energy]] can be lowered (and the isomerization [[catalysis|catalyzed]]) by changes that favor the single-bonded form, such as placing the peptide group in a hydrophobic environment or donating a hydrogen bond to the nitrogen atom of an X-Pro peptide group. Both of these mechanisms for lowering the activation energy have been observed in '''peptidyl prolyl isomerases''' (PPIases), which are naturally occurring enzymes that catalyze the cis-trans isomerization of X-Pro peptide bonds.
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| Conformational [[protein folding]] is usually much faster (typically 10–100 ms) than cis-trans isomerization (10–100 s). A nonnative isomer of some peptide groups can disrupt the conformational folding significantly, either slowing it or preventing it from even occurring until the native isomer is reached. However, not all peptide groups have the same effect on folding; nonnative isomers of other peptide groups may not affect folding at all.
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| ==Chemical reactions==
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| Due to its resonance stabilization, the peptide bond is relatively unreactive under physiological conditions, even less than similar compounds such as [[ester]]s. Nevertheless, peptide bonds can undergo chemical reactions, usually through an attack of an [[electronegativity|electronegative]] atom on the [[carbonyl]] [[carbon]], breaking the carbonyl double bond and forming a tetrahedral intermediate. This is the pathway followed in [[proteolysis]] and, more generally, in N-O acyl exchange reactions such as those of [[intein]]s. When the functional group attacking the peptide bond is a [[thiol]], [[hydroxyl]] or [[amine]], the resulting molecule may be called a [[cyclol]] or, more specifically, a thiacyclol, an oxacyclol or an azacyclol, respectively.
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| ==See also==
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| * [[The Proteolysis Map]]
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| ==References==
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| <references />
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| * Pauling L. (1960) ''The Nature of the Chemical Bond'', 3rd. ed., Cornell University Press. ISBN 0-8014-0333-2
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| * Stein RL. (1993) "Mechanism of Enzymatic and Nonenzymatic Prolyl cis-trans Isomerization", ''Adv. Protein Chem.'', '''44''', 1–24.
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| * Schmid FX, Mayr LM, Mücke M and Schönbrunner ER. (1993) "Prolyl Isomerases: Role in Protein Folding", ''Adv. Protein Chem.'', '''44''', 25–66.
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| * Fischer G. (1994) "Peptidyl-Prolyl cis/trans Isomerases and Their Effectors", ''Angew. Chem. Int. Ed. Engl.'', '''33''', 1415–1436.
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| {{Chemical bonds}}
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| {{Protein primary structure}}
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| [[Category:Chemical bonding]]
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| [[Category:Proteins]]
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