Tetrameric protein

While a number of proteins have been crystallized in this manner, the majority of studies have focused on a robust system comprising the tetrameric protein streptavidin and the vitamin biotin. The choice of this system is primcirily motivated by the strong bond between biotin and streptavidin (having an association equilibrium constant, Ka Tbe binding properties were recently... 

Back A hand-drawn image of the potassium channel, in the same view as on the front cover, with each subunit of the tetrameric protein shown in a different color. 

Biochemical characterization of clathrin-coated vesicles revealed that their major coat components are clathrin and various types of adaptor complexes. Clathrin assembles in triskelions that consist of three heavy chains of approximately 190 kDa and three light chains of 30 40 kDa. Four types of adaptor complexes have been identified to date, AP-1, AP-2, AP-3 and AP-4 (AP for adaptor protein). Whereas AP-1, AP-3 and AP-4 mediate sorting events at the TGN and/or endosomes, AP-2 is involved in endocytosis at the plasma membrane. Each adaptor complex is a hetero-tetrameric protein complex, and the term adaptin was extended to all subunits of these complexes. One complex is composed of two large adaptins (one each of y/a/S/s and [31-4, respectively, 90-130 kDa), one medium adaptin (pi -4, <50 kDa), and one small adaptin (ol-4, <20 kDa). In contrast to AP-1, AP-2 and AP-3, which interact directly with clathrin and are part of the clathrin-coated vesicles, AP-4 seems to be involved in budding of a certain type of non-clathrin-coated vesicles at the TGN. 

All Kir channels are tetrameric proteins (see Fig. 3) of one-pore/two-transmembrane (1P/2TM) domain subunits which equally contribute to the formation of highly selective K+ channels. Most Kir channels can be assembled in functional homotetramers while some require heteromeric assembly (see Fig. 3). For example, functional GIRK channels underlying DCAch (Acetyl-choline-activated) current in atria are heteromultimers of two members ofKir3 subfamily Kir3.1 andKir3.4. 

Although biotin-fluorophores have been described in the detection of biomolecules, namely proteins, peptides, or DNA, most of these loose part of their fluorescence when binding to the tetrameric proteins avidin and streptavidin. 

Avidin was first isolated from chicken egg white by Esmond Emerson Snell (1914-2003). This tetrameric protein contains four identical subunits (homotetramer) each of which can bind to biotin (Vitamin B7, vitamin H) with a high degree of affinity and specificity. When these two molecules are in the same solution, they will bind with such high affinity that the binding is essentially irreversible. No matter how many times you wash them, they will not let go of each other. 

A challenge posed to researchers was therefore to account for diverse physiological effects emanating from the same receptor/hormone interaction. Structurally, the insulin receptor (IR) is a tetrameric protein, composed of two smaller extracellular a units and two larger transmembrane [3 units (see Figure 4.20a). 

The nature of the copper in these proteins is not totally clear. Dooley et al (1988) reported that the Achromobacter protein may have two kinds of type I sites in a total of three copper sites per dimeric protein, while the A. faecalis protein was reported to be a tetrameric protein with both type I and type II coppers (KakutanielaZ., 1981). Interestingly, the Achromobacter protein is green. Both of these nitrite reductases accept electrons from a cupredoxin. 

The binding of biotin to the tetramere proteins avidin and strepta-vidin is very specific with an extremely low dissociation constant (Kd = 10 M). Because each of the subunits of the proteins carry a binding site, it is possible to produce larger complexes when derivatives are used bearing at least two biotin molecules. 

Fig. 2.4. The symmetry model of allostery. Shown here is the succesive binding of a hgand L to a protomer of a tetrameric protein with four ligand-binding sites according to the symmetry model. T tense form, R relaxed form.

The p53 protein is a tetrameric protein that binds to DNA elements with the consensus sequence 5 -RRRC(A/T)(T/A)GYYY-3 (R = pmine, Y = pyrimidine). The struc-... 

Until rather recently there had been little to indicate that DNA actually assumes cruciform conformations in cells. However, strong experimental evidence suggests that some cruciform structures do form naturally.380 Their formation from palindromic DNA [like the formation of Z-DNA from (G + C)-rich sequences] is a way of relieving torsional strain induced by super-coiling. Whether or not cruciform structures occur frequently within cells, there is no doubt that palindromic sequences are of great importance in the interaction of nucleic acids with symmetric dimeric and tetrameric protein molecules such as the gene repressor protein shown in Fig. 5-35.381-383... 

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. 

The dyad symmetry of the operator sequence is probably important in providing tight binding to two subunits of the symmetric tetrameric protein.11-13 It is also possible that repressor molecules move along DNA chains in a one-dimensional diffusion process, and that the symmetry of the operator site facilitates recognition by a protein moving from either direction.14 15... 

Figure 10.3 The MWC model for the binding of ligands to a tetrameric protein. S = substrate T = tense conformation and R = relaxed conformation.

Figure 10.8 Variation of the Hill constant with L for a tetrameric protein. [From M. M. Rubin and J.-P. Changeux, J. Molec. Biol. 21, 265 (1966).]...

Figure 10.5 The KNF model for the binding of ligands to a tetrameric protein.

The picture that emerges indicates that of a tetrameric protein with four binding sites for each ligand, but no evidence is yet available as to the distribution of these binding sites among the various subunits. 

The Schiff-base-forming types (class I) are known only for the two former aldolases (FruA, TagA), which are found usually in mammalian or (as an exception) in specific microbial organisms, whereas the Zn2+-dependent type (class II) comprises all four DHAP aldolases which are commonly found in bacteria [43], Typically, type I FruA enzymes are tetrameric proteins composed of subunits of 40 kDa [191,192], while the type II FruA are dimers of 39 kDa subunits [193]. RhuA and FucA enzymes are homotetrameric with a subunit molecular weight of 25 kDa and 30 kDa respectively [194,195],... 

Streptavidin is another biotin binding protein isolated from Streptomyces avidinii that can overcome some of the nonspecificities of avidin (Chaiet and Wolf, 1964). Similar to avidin, streptavidin contains four subunits, each with a single biotin binding site. After some postsecretory modifications, the intact tetrameric protein has a molecular mass of about 60,000 D, slightly less than that of avidin (Bayer et al., 1986, 1989). 

The Gd-DOTP chelate (Fig. 18) has been examined as a potential allosteric effector of hemoglobin [113]. Human hemoglobin is a tetrameric protein whose quaternary structure is affected by an allosteric effector. Binding of this low molecular weight species stabilizes the so-called T-form of hemoglobin, which is characterized by a low oxygen affinity, as opposed to the R-form, a very efficient... 

The most commonly used protein, often ultilized as a versatile supramolecular building block for the synthesis of biohybrid materials, is probably streptavidin. Streptavidin was discovered in the bacterium Streptomyces avidinii 241-242-243 and is an homo-tetrameric protein with a 2-fold symmetry, and molecular weight of 60 kDa.244,245,246... 

---