Multimer tetramer

The minimal functional unit (quite well conserved among all rubiscos) is a homodimer in which the active sites are located at the subunit interface. Residues from both subunits contribute to each active site, which is illustrated in Color Plate 8. All known forms (at present, four different types) consist of these basic dimeric units which are arranged into various larger multimer arrays—dimers, tetramers and even pentamers. The different forms of rubisco all have a common evolutionary origin and existing solid-state structures of the active sites are nearly superimposable. ... 

Several lines of evidence demonstrate that the active unit of integrase is a multimer. It is clear, as an isolated protein in solution, that integrase forms dimers [6,10-12], and it has been shown by sedimentation equilibrium studies that Rous sarcoma virus (RS V) integrase exists in reversible equilibrium between monomeric, dimeric, and tetrameric forms [13]. Protein-protein cross-linking studies of HIV-1 [14] and RSV [15] integrases confirm the existence of protein dimers and tetramers in solution, and in vivo, the yeast GAL4 two-hybrid system has demonstrated that HIV-1 integrase can interact with itself [16]. 

In addition to the multiplicity of receptor sites for glutamate, the NMDA receptors bear their own complexity as they are constructed as multimers from three distinguishable subunit classes (i.e. NR1, NR2 and NR3 subunit class). With regard to the stoichiometry of the NMDA receptor there is still some debate as to whether a native NMDA-gated ion channel within the cell membrane consists of either a tetramer or pentamer. More recently, it has been suggested that the tetramic stoichiometry is more probable (Laube et al., 1998 Hollmann, 1999). 

Generally, specific proteins can bind to each other In the body to form dimers (duplex structures), trimers, tetramers, or even larger multiples. These subunit proteins may be of identical or different structure. The different proteins in these mullimeric structures arc bound to each other by hydrogen bonds and other weak interactions. Ihese multimers often perform physiological functions that cannot be carried out by the individual separated proteins. 

Let us consider a system of N molecules with one proton donor and one acceptor site (-0-H groups), which self-associate forming bonds. However, the first dimer bond, now, is weaker than any subsequent bond in a multimer (trimer, tetramer, etc.) complex. In fact, it is not important for the enumeration of bonds whether the dimer bond is weaker or stronger than the others it suffices for it to be distinguishable. 

Ibbitson and Moore (13) conclude that the maximum in the curve of polarization vs. concentration for ethanol in carbon tetrachloride is caused by linear multimers, and the subsequent fall in polarization is caused by an increasing amount of cyclic multimer (Figure 1). The concentration at which the maximum occurs coincides with that at which the 3350-cm.-1 band first appears in the infrared spectrum, so they have suggested that this band arises from cyclic multimers. They have fitted their data to a system containing linear dimer and trimer and cyclic tetramer only and have evaluated association constants for these species. 

The large positive saturation in dilute solutions is caused by the high concentration of dimers, in which the dielectric saturation is shown by Piekara to lead to the largest increase in dipole moment. From curves which show the variation in mole fraction of each multimer with concentration it appears that in 1-hexanol there are 65 mole % pentamers, 25 mole % tetramers, and 10 mole % trimers approximately in the pure liquid. Malecki has also calculated that 60% of dimers and 37% of trimers are cyclic, the tetramers and pentamers revealing no cyclic structures. The forms of the curves showing variations in concentration of monomer and the various multimers with total concentration of hexanol are similar to... 

The properties of the crystalline enzyme from calf liver have been studied by Levine et al. (1969). Despite several recrystallizations the enzyme preparations showed up to three minor components on analytical ultracentrifugation, sucrose density gradient centrifugation, or polyacrylamide gel electrophoresis. This was not due to the presence of impurities, but rather to the occurence of multimers of the enzyme, i.e., monomer, dimer, trimer, and tetramer. Evidence to this effect was obtained from the close correspondence between protein content and enzyme activity in the fractions isolated by density gradient centrifugation and gel electrophoresis. Furthermore, the separated fractions slowly redistributed to yield analytical patterns similar to that of the native enzyme, indicating that interconversion between the various molecular species was taking place. 

Enoate reductase. Enoate reductase isolated from Clostridium tyrobutyricum catalyses the NADH or methylviologen-dependent reduction of the a,p carbon-carbon double bond of non-activated 2-enoates and in a reversible way that of 2-enals. The enzyme appears to be a multimer of identical subunits. The total molecular mass is 940 kDa ( 73 kDa per subunit). Sedimentation equilibrium experiments, molecular mass data, and electron microscopy indicate that the native enzyme is composed of a tetramer of trimers. Each enzyme subunit contains one FAD, 0.6 FMN and one [4Fe-4S] cluster. 

Figure 7 shows IR spectra of methanol in various phases including an argon matrix. The bands observed are all from O-H stretching and demonstrate the advantage of the matrix technique. The vapour-phase spectra are dominated by rotational side bands only H-bonded multimer can be seen in pure liquid and solid, though solution shows dimer as well. Spectra from the matrix at two different dilutions can however distinguish dimer, trimer, tetramer and multimer. 

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