Tetramerization

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... 

Cyclooctatetraene can be obtained on an industrial scale by metal carbonyl catalyzed thermal tetramerization of acetylene. If cyclooctatetraene is UV-irradiated at low temperature in the presence of acetone, it is reversibly rearranged to form semibullvalene (H.E. Zimmerman, 1968, 1970). 

Gyclooctatetraene (GOT). Tetramerization of acetylene to cyclooctatetraene [629-20-9], CgHg, although interesting, does not seem to have been used commercially. Nickel salts serve as catalysts. Other catalysts give ben2ene. The mechanism of this cyclotetramerhation has been studied (4). 

These reactions are usehil for the preparation of homogeneous difunctional initiators from a-methylstyrene in polar solvents such as tetrahydrofuran. Because of the low ceiling temperature of a-methylstyrene (T = 61° C) (26), dimers or tetramers can be formed depending on the alkaU metal system, temperature, and concentration. Thus the reduction of a-methylstyrene by sodium potassium alloy produces the dimeric dianionic initiators in THF (27), while the reduction with sodium metal forms the tetrameric dianions as the main products (28). The stmctures of the dimer and tetramer correspond to initial tail-to-tail addition to form the most stable dianion as shown in equations 6 and 7 (28). 

The methodology for preparation of hydrocarbon-soluble, dilithium initiators is generally based on the reaction of an aromatic divinyl precursor with two moles of butyUithium. Unfortunately, because of the tendency of organ olithium chain ends in hydrocarbon solution to associate and form electron-deficient dimeric, tetrameric, or hexameric aggregates (see Table 2) (33,38,44,67), attempts to prepare dilithium initiators in hydrocarbon media have generally resulted in the formation of insoluble, three-dimensionally associated species (34,66,68—72). These precipitates are not effective initiators because of their heterogeneous initiation reactions with monomers which tend to result in broader molecular weight distributions > 1.1)... 

The cychc haUdes can be converted to discrete substitution products by reaction with amines, alcohol, or alkylating agents. For example, (NPCl ) reacts with ammonia to form (NP(NH2)2)3 [13597-92-7] withy -NaOCgH CH to form (NP(OCgH4CH2)2)3 [27122-73-2] and with CH MgCl to form (NP(CH3)2)3 [6607-30-3]. Among the cychc members, the trimeric haUdes are the most inert toward substitution and tetrameric haUdes are the most active. 

Binary Compounds. The mthenium fluorides are RuF [51621 -05-7] RuF [71500-16-8] tetrameric (RuF ) [14521 -18-7] (15), and RuF [13693-087-8]. The chlorides of mthenium are RUCI2 [13465-51-5] an insoluble RuCl [10049-08-8] which exists in an a- and p-form, mthenium trichloride ttihydrate [13815-94-6], RuCl3-3H2 0, and RuCl [13465-52-6]. Commercial RuCl3-3H2 0 has a variable composition, consisting of a mixture of chloro, 0x0, hydroxo, and often nitrosyl complexes. The overall mthenium oxidation state is closer to +4 than +3. It is a water-soluble source of mthenium, and is used widely as a starting material. Ruthenium forms bromides, RuBr2 [59201-36-4] and RuBr [14014-88-1], and an iodide, Rul [13896-65-6]. 

Binary Compounds. The fluorides of indium are IrF [23370-59-4] IrF [37501-24-9] the tetrameric pentafluoride (IiF ) [14568-19-5], and JIrFg [7789-75-7]. Chlorides of indium include IrCl, which exists in anhydrous [10025-83-9] a- and p-forms, and as a soluble hydrate [14996-61-3], and IrCl [10025-97-5], Other haUdes include IrBr [10049-24-8], which is insoluble, and the soluble tetrahydrate IrBr -4H20 IrBr [7789-64-2]-, and Irl [7790-41-2], Iridium forms indium dioxide [12030-49-8], a poorly characteri2ed sesquioxide, 11203 [1312-46-5]-, and the hydroxides, Ir(OH)3 [54968-01-3] and Ir(OH) [25141-14-4], Other binary iridium compounds include the sulfides, IrS [12136-40-2], F2S3 [12136-42-4], IrS2 [12030-51 -2], and IrS3 [12030-52-3], as well as various selenides and teUurides. 

Hemoglobin is a tetramer built up of two copies each of two different polypeptide chains, a- and (5-globin chains in normal adults. Each of the four chains has the globin fold with a heme pocket. Residue 6 in the p chain is on the surface of a helix A, and it is also on the surface of the tetrameric molecule (Figure 3.13). 

Figure S.8 Schematic view down the fourfold axis of the tetrameric molecule of neuraminidase as It appeared on the cover of Nature, May 5, 1983.

The transitions between these states in each tetrameric molecule are concerted, in other words all four subunits of each molecule are in the same state, either R or T. 

The basic kinetic properties of this allosteric enzyme are clearly explained by combining Monod s theory and these structural results. The tetrameric enzyme exists in equilibrium between a catalytically active R state and an inactive T state. There is a difference in the tertiary structure of the subunits in these two states, which is closely linked to a difference in the quaternary structure of the molecule. The substrate F6P binds preferentially to the R state, thereby shifting the equilibrium to that state. Since the mechanism is concerted, binding of one F6P to the first subunit provides an additional three subunits in the R state, hence the cooperativity of F6P binding and catalysis. ATP binds to both states, so there is no shift in the equilibrium and hence there is no cooperativity of ATP binding. The inhibitor PEP preferentially binds to the effector binding site of molecules in the T state and as a result the equilibrium is shifted to the inactive state. By contrast the activator ADP preferentially binds to the effector site of molecules in the R state and as a result shifts the equilibrium to the R state with its four available, catalytically competent, active sites per molecule. 

The lac repressor monomer, a chain of 360 amino acids, associates into a functionally active homotetramer. It is the classic member of a large family of bacterial repressors with homologous amino acid sequences. PurR, which functions as the master regulator of purine biosynthesis, is another member of this family. In contrast to the lac repressor, the functional state of PurR is a dimer. The crystal structures of these two members of the Lac I family, in their complexes with DNA fragments, are known. The structure of the tetrameric lac repressor-DNA complex was determined by the group of Mitchell Lewis, University of Pennsylvania, Philadelphia, and the dimeric PurR-DNA complex by the group of Richard Brennan, Oregon Health Sciences University, Portland. 

The polypeptide chain of the lac repressor subunit is arranged in four domains (Figure 8.21) an N-terminal DNA-hinding domain with a helix-turn-helix motif, a hinge helix which binds to the minor groove of DNA, a large core domain which binds the corepressor and has a structure very similar to the periplasmic arablnose-binding protein described in Chapter 4, and finally a C-terminal a helix which is involved in tetramerization. This a helix is absent in the PurR subunit structure otherwise their structures are very similar. 

The tetrameric structure of the lac repressor has a quite unusual V-shape (Figure 8.22). Each arm of the V-shaped molecule is a tight dimer, which is very similar in structure to the PurR dimer and which has the two N-termi-nal DNA binding domains close together at the tip of the arm. The two dimers of the lac repressor are held together at the other end by the four carboxy-terminal a helices, which form a four-helix bundle. 

Figure 9.17 Schematic diagram illustrating the tetrameric stmcture of the pS3 oligomerization domain. The four subunits have different colors. Each subunit has a simple structure comprising a p strand and an a helix joined by a one-residue turn. The tetramer is built up from a pair of dimers (yellow-blue and red-green). Within each dimer the p strands form a two-stranded antiparallel p sheet which provides most of the subunit interactions. The two dimers are held together by interactions between the four a helices, which are packed in a different way from a four-helix bundle. (Adapted from P.D. Jeffrey et al.. Science 267 1498-1502, 1995.)...

Jeffrey, P.D., Gorina, S., Pavletich, N.P. Crystal structure of the tetramerization domain of the p53 tumor suppressor at 1.7 Angstroms. Science 267 1498-1502, 1995. 

The K+ channel is a tetrameric molecule with one ion pore in the interface between the four subunits... 

Figure 12.9 Schematic diagram of the stmc-ture of a potassium channel viewed perpendicular to the plane of the membrane. The molecule is tetrameric with a hole in the middle that forms the ion pore (purple). Each subunit forms two transmembrane helices, the inner and the outer helix. The pore heJix and loop regions build up the ion pore in combination with the inner helix. (Adapted from S.A. Doyle et al., Science 280 69-77, 1998.)...

All K channels are tetrameric molecules. There are two closely related varieties of subunits for K channels, those containing two membrane-spanning helices and those containing six. However, residues that build up the ion channel. Including the pore helix and the inner helix, show a strong sequence similarity among all K+ channels. Consequently, the structural features and the mechanism for ion selectivity and conductance described for the bacterial K+ channel in all probability also apply for K+ channels in plant and animal cells. 

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. 

In the case of phenyllithium, it has been possible to demonstrate by NMR studies that the compound is tetrameric in 1 2 ether-cyclohexane but dimeric in 1 9 TMEDA-cyclohexane. X-ray crystal structure determinations have been done on both dimeric and tetrameric structures. A dimeric structure crystallizes from hexane containing TMEDA. This structure is shown in Fig. 7.1 A. A tetrameric structure incorporating four ether molecules forms from ether-hexane solution. This structure is shown in Fig. 7.IB. There is a good correspondence between the structures that crystallize and those indicated by the NMR studies. 

Tetrameric structures based on distorted cubic structures are also found for (CH3Li)4 and (C2H5Li)4. These tetrameric structures can also be represented as being based on a... 

Fig. 7.1. Crystal structures of phertyllithium (A) dimeric structure incorporating tetra-methylethylenediamine (B) tetrameric structure incorporating dietl l ether., (Reproduced ftom Refs. 28 and 29 with permission of Wiley-VCH and the American Chemical Society.)...

The THF solvate of lithium i-butylacetylide is another example of a tetrameric structure. ... 

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