Glycoproteins avidin

A biochemical curiosity is the presence in egg white of the glycoprotein avidin.ab Each 68-kDa subunit of this tetrameric protein binds one molecule of biotin tenaciously with Kf 1015 M 1. Nature s purpose in placing this unusual protein in egg white is uncertain. Perhaps it is a storage form of biotin, but it is more likely an antibiotic that depletes the environment of biotin. A closely similar protein streptavidin is secreted into the culture medium by Streptomyces avidinii.c Its sequence is homologous to that of avidin. It has a similar binding constant for biotin and the two proteins have similar three-dimensional structures.3/d i Biotin binds at one end of a P barrel formed from antiparallel strands and is held by multiple hydrogen bonds and a conformational alteration that allows a peptide loop to close over the bound vitamin. 

Water-soluble chelating diphosphines. This amine hydrochloride has been used to prepare water-soluble ligands for transition metals, particularly Rh(I). Thus, the complex Rh(I) -2 is a highly active catalyst for homogeneous hydrogenation, and Rh(I)-3 combines with the glycoprotein avidin to form an effective asymmetric catalyst lor hydrogenation of -acetamidoacrylic acid. 

The low-molecular-weight vitamin biotin is easily conjugated to antibodies and enzyme markers. Up to 150 biotin molecules can be attached to one antibody molecule, and the strong affinity of the biotin for the glycoprotein avidin allows its use as complex-ing secondary reagents. Biotin labeling of the primary (direct) or secondary (indirect) antibody can be used in the avidin-biotin methods. In the labeled avidin method the tracer is attached directly to the avidin molecule. In the avidin-biotin bridge method a biotinylated enzyme such as peroxidase is allowed to bind after attachment of avidin to the biotin-labeled antibody. 

Biotin was originally discovered as part of the complex called bios, which promoted the growth of yeast, and separately, as vitamin H, the protective or curative factor in egg white injury - the disease caused by diets containing large amounts of uncooked egg white. The glycoprotein avidin in egg white binds biotin with high affinity. This has been exploited to provide a variety of extremely sensitive assay systems. 

Higgins et al. synthesized a biotin-functionalized terthiophene 2.210 by reaction of biotin hydrazide and a terthiophene which was functionalized by a succinimidyl active ester (Chart 1.43) [299]. The copolymerization of 2.210 with terthiophene on a Pt electrode formed poly(terthiophene) films containing intact biotin moieties. The binding of 5 x 10 " mol of the glycoprotein avidin to the pendant biotin units of the polymer film resulted in a positive shift of the oxidation potential due to specific inferactions and blocking of the ion transport to and from the polymer by the bound protein. 

Native ovoflavoprotein (49 kDa, pf=5.1) has, as does ovomucoid, certain antinutritional effects, as it inhibits serine proteases (trypsin, chymotrypsin and also microbial proteases) and has antiviral activity. Ovomacroglobulin (ovostatin) is an inhibitor of serine, cysteine, thiol and metalloproteases and shows antimicrobial activity. Some antinutritional effects are also seen in the basic glycoprotein avidin in raw egg white (relative molecular weight of the monomer is 15.6 kDa). It contains four identical subunits (pf = 9.5), each of which binds one molecule of biotin to give an unavailable complex. However, the denatured avidin, for example in hard-boiled eggs, does not interact with biotin. The interaction of riboflavin with flavoprotein (32 kDa, pf = 4.0) has, on the contrary, a positive influence on vitamin stability. Cystatin acts as cysteine protease inhibitor, and shows antimicrobial, antitumor and immunomodulating activities. 

Paul Goulet, Nicholas Pieczonka, and Ricardo Aroca report the fabrication and characterization of layer-by-layer (LbL) films composed of the glycoprotein avidin and metallic Ag nanoparticles. These structures show strong bio-specific interactions and are employed as selective substrates for surface-enhanced Raman and resonance Raman scattering. 

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