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The aldehyde tag is a short peptide tag which can be introduced into fusion proteins and by subsequent treatment with the formylglycine-generating enzyme (FGE) a reactive aldehyde group is generated for further coupling. Since there already is an array of aldehyde-specific reagents commercially available (such as aminooxy or hydrazide reagents), possible applications are diverse and include the conjugation of fluorophores, glycans, PEG (polyethylene glycol) chains or reactive groups for further synthesis (see applications). == Development == The aldehyde tag is an artificial peptide tag recognized by the formylglycine-generating enzyme (FGE). Formylglycine is a glycine with a formyl group (-CHO, an aldehyde) at the α-carbon. The sulfatase motif is the basis for the sequence of the peptide which results in the site-specific conversion of a cysteine to a formylglycine residue. The peptide tag was engineered after studies on FGE recognizable sequences in sulfatases from different organisms. Carrico et al. discovered a high homology in the sulfatase motif in bacteria, archaea as well as eukaryotes.〔Carrico, I. S., Carlson, B. L. and Bertozzi, C. R. (2007) Introducing genetically encoded aldehydes into proteins. Nat Chem Biol 3, 321–322.〕 Aldehydes and ketones find use as chemical reporters due to their strong electrophilic properties. This enables a reaction under mild conditions when using a strong nucleophilic coupling partner. Typically, hydrazides and aminooxy probes are used in bioconjugation. They form stabilized addition products with carbonyl groups that are favoured under the physiological reaction conditions. At neutral pH, the equilibrium of Schiff base formation, is lying far to the reactant’s side. To make more product named compounds are used to form stable hydrazones and oximes. Since the pH-optimum of 4 to 6 cannot be achieved by adding a catalyst due to associated toxicity, the reaction is slow in live cells. A typical reaction constant is 10−4 to 10−3 M−1 s−1.〔Jencks, W. P. (1959) Studies on the Mechanism of Oxime and Semicarbazone Formation1. J. Am. Chem. Soc. 81, 475–481.〕 A carbonyl group is introduced into proteins as a chemical reporter using different techniques, including modern methods like stop codon suppression and the herein discussed aldehyde tag.〔Carrico, I. S., Carlson, B. L. and Bertozzi, C. R. (2007) Introducing genetically encoded aldehydes into proteins. Nat Chem Biol 3, 321–322.〕〔Zhang, Z., Smith, B. A. C., Wang, L., Brock, A., Cho, C. and Schultz, P. G. (2003) A New Strategy for the Site-Specific Modification of Proteins in Vivo†. Biochemistry 42, 6735–6746.〕 Limiting the use of aldehydes and ketones is their restricted bioorthogonality in certain cellular environments. Limitations of aldehydes and ketones as chemical reporters are due to * competition with endogenous aldehydes or ketones in metabolites and cofactors. Lower yields and impaired specificity can occur. * side reactions like oxidation or unwanted addition of endogenous nucleophiles. * restrained set of probes that form sufficiently stable products.〔Lim, R. K. V. and Lin, Q. (2010) Bioorthogonal Chemistry: Recent Progress and Future Directions. Chem Commun (Camb) 46, 1589–1600.〕〔Prescher, J. A. and Bertozzi, C. R. (2005) Chemistry in living systems. Nat Chem Biol 1, 13–21.〕 Aldehydes and ketones are therefore best used in compartments where such unwanted side reactions are decreased. For experiments with live cells, cell surfaces and extracellular space are typical fielding areas. Nevertheless, a feature of carbonyl groups is the vast number of organic reactions that involve them as electrophiles. Some of these reactions are readily convertible to ligations for probing aldehydes. A rather exotic reaction recently employed for bioconjugation by Agarwal et al. is the adaptation of the Pictet-Spengler-reaction as a ligation. The reaction is known from natural product biosynthetic pathways 〔Stöckigt, J., Antonchick, A. P., Wu, F. and Waldmann, H. (2011) Die Pictet-Spengler-Reaktion in der Natur und der organischen Chemie. Angew. Chem. 123, 8692–8719.〕 and has the major advantage that a new carbon-carbon bond is formed. This guarantees long-term stability compared to carbon-heteroatom bonds at same reaction kinetics.〔Agarwal, P., van der Weijden, J., Sletten, E. M., Rabuka, D. and Bertozzi, C. R. (2013) A Pictet-Spengler ligation for protein chemical modification. Proc Natl Acad Sci U S A 110, 46–51.〕 The modification of cysteine or, more rarely, serine 〔Miech, C., Dierks, T., Selmer, T., Figura, K. von and Schmidt, B. (1998) Arylsulfatase from Klebsiella pneumoniae Carries a Formylglycine Generated from a Serine. J. Biol. Chem. 273, 4835–4837.〕 by FGE is a rather unusual posttranslational modification and was discovered already in the late 1990s.〔Dierks, T., Lecca, M. R., Schmidt, B. and von Figura, K. (1998) Conversion of cysteine to formylglycine in eukaryotic sulfatases occurs by a common mechanism in the endoplasmic reticulum. FEBS Letters 423, 61–65.〕 Interestingly, the deficiency of FGE leads to an overall deficiency of functional sulfatases due to a lack of α-formylglycine formation vital for the sulfatases to perform their function. FGE is essential for protein modification and need of high specificity and conversion rate is given in the native setting, which makes this reaction interesting for chemical and synthetic biology.〔Dierks, T., Dickmanns, A., Preusser-Kunze, A., Schmidt, B., Mariappan, M., von Figura, K., Ficner, R. and Rudolph, M. G. (2005) Molecular Basis for Multiple Sulfatase Deficiency and Mechanism for Formylglycine Generation of the Human Formylglycine-Generating Enzyme. Cell 121, 541–552.〕 Carrico et al. pioneered the insertion of the modified sulfatase motif peptide into proteins of interest in 2007.〔Carrico, I. S., Carlson, B. L. and Bertozzi, C. R. (2007) Introducing genetically encoded aldehydes into proteins. Nat Chem Biol 3, 321–322. Jencks, W. P. (1959) Studies on the Mechanism of Oxime and Semicarbazone Formation1. J. Am. Chem. Soc. 81, 475–481.〕 Such use of aldehydes and ketones as a chemical reporter in bioorthogonal applications has been applied in self-assembly of cell-lysing drugs,〔Rideout, D. (1994) Self-assembling drugs: a new approach to biochemical modulation in cancer chemotherapy. Cancer Invest. 12, 189–202; discussion 268–269.〕 the targeting of proteins,〔Chen, I., Howarth, M., Lin, W. and Ting, A. Y. (2005) Site-specific labeling of cell surface proteins with biophysical probes using biotin ligase. Nat Meth 2, 99–104.〕〔Zhang, Z., Smith, B. A. C., Wang, L., Brock, A., Cho, C. and Schultz, P. G. (2003) A New Strategy for the Site-Specific Modification of Proteins in Vivo†. Biochemistry 42, 6735–6746.〕 as well as glycans 〔Mahal, L. K., Yarema, K. J. and Bertozzi, C. R. (1997) Engineering Chemical Reactivity on Cell Surfaces Through Oligosaccharide Biosynthesis. Science 276, 1125–1128.〕 and the preparation of heterobifunctional fusion proteins 〔Hudak, J. E., Barfield, R. M., de Hart, G. W., Grob, P., Nogales, E., Bertozzi, C. R. and Rabuka, D. (2012) Synthesis of Heterobifunctional Protein Fusions Using Copper-Free Click Chemistry and the Aldehyde Tag. Angew. Chem. Int. Ed. 51, 4161–4165.〕 since then. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Aldehyde tag」の詳細全文を読む スポンサード リンク
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