Oriented immobilization of immunoglobulin G onto the cuvette surface of the resonant mirror biosensor through layer-by-layer assembly of multilayer films

Enzymatic labeling of proteins: techniques and approaches

Site-specific modification of proteins is a major challenge in modern chemical biology due to the large number of reactive functional groups typically present in polypeptides. Because of its importance in biology and medicine, the development of methods for site-specific modification of proteins is an area of intense research. Selective protein modification procedures have been useful for oriented protein immobilization, for studies of naturally occurring post-translational modifications, for creating antibody–drug conjugates, for the introduction of fluorophores and other small molecules on to proteins, for examining protein structure, folding, dynamics, and protein–protein interactions, and for the preparation of protein–polymer conjugates. One of the most important approaches for protein labeling is to incorporate bioorthogonal functionalities into proteins at specific sites via enzymatic reactions. The incorporated tags then enable reactions that are chemoselective, whose functional groups not only are inert in biological media, but also do not occur natively in proteins or other macromolecules. This review article summarizes the enzymatic strategies, which enable site-specific functionalization of proteins with a variety of different functional groups. The enzymes covered in this review include formylglycine generating enzyme, sialyltransferases, phosphopantetheinyltransferases, O-GlcNAc post-translational modification, sortagging, transglutaminase, farnesyltransferase, biotin ligase, lipoic acid ligase, and N-myristoyltransferase.

Chemoenzymatic reversible immobilization and labeling of proteins without prior purification

Site-specific chemical modification of proteins is important for many applications in biology and biotechnology. Recently, our laboratory and others have exploited the high specificity of the enzyme protein farnesyltransferase (PFTase) to site-specifically modify proteins through the use of alternative substrates that incorporate bioorthogonal functionality including azides and alkynes. In this study, we evaluate two aldehyde-containing molecules as substrates for PFTase and as reactants in both oxime and hydrazone formation. Using green fluorescent protein (GFP) as a model system, we demonstrate that the purified protein can be enzymatically modified with either analogue to yield aldehyde-functionalized proteins. Oxime or hydrazone formation was then employed to immobilize, fluorescently label, or PEGylate the resulting aldehyde-containing proteins. Immobilization via hydrazone formation was also shown to be reversible via transoximization with a fluorescent alkoxyamine. After characterizing this labeling strategy using pure protein, the specificity of the enzymatic process was used to selectively label GFP present in crude E. coli extract followed by capture of the aldehyde-modified protein using hydrazide-agarose. Subsequent incubation of the immobilized protein using a fluorescently labeled or PEGylated alkoxyamine resulted in the release of pure GFP containing the desired site-specific covalent modifications. This procedure was also employed to produce PEGylated glucose-dependent insulinotropic polypeptide (GIP), a protein with potential therapeutic activity for diabetes. Given the specificity of the PFTase-catalyzed reaction coupled with the ability to introduce a CAAX-box recognition sequence onto almost any protein, this method shows great potential as a general approach for selective immobilization and labeling of recombinant proteins present in crude cellular extract without prior purification. Beyond generating site-specifically modified proteins, this approach for polypeptide modification could be particularly useful for large-scale production of protein conjugates for therapeutic or industrial applications.

Grading the commercial optical biosensor literature-Class of 2008: 'The Mighty Binders'

Optical biosensor technology continues to be the method of choice for label-free, real-time interaction analysis. But when it comes to improving the quality of the biosensor literature, education should be fundamental. Of the 1413 articles published in 2008, less than 30% would pass the requirements for high-school chemistry. To teach by example, we spotlight 10 papers that illustrate how to implement the technology properly. Then we grade every paper published in 2008 on a scale from A to F and outline what features make a biosensor article fabulous, middling or abysmal. To help improve the quality of published data, we focus on a few experimental, analysis and presentation mistakes that are alarmingly common. With the literature as a guide, we want to ensure that no user is left behind.