Globular proteins subunits

Consider a situation with globular protein subunits that are soluble in solution at a sufficiently low temperature and that have simple complementary hydrophobic surfaces covered with much hydrophobic hydration, as in Figure 8.2A. Raising the temperature from below to... 

The subunits of an oligomeric protein typically fold into apparently independent globular conformations and then interact with other subunits. The particular surfaces at which protein subunits interact are similar in nature to the interiors of the individual subunits. These interfaces are closely packed and involve both polar and hydrophobic interactions. Interacting surfaces must therefore possess complementary arrangements of polar and hydrophobic groups. 

Ferritin is a globular protein complex consisting of 24 protein subunits and is the main intracellular iron storage protein in both prokaryotes and eukaryotes. Ferritin is used for immunolabeling at the electron microscope level because of its distinctive shape of the crystals and their electron density. 

The coiled-coil motif is an ideal model system for the following reasons there is only one type of secondary structure present (the a-helix) the a-helical structure can be easily monitored by circular dichroism spectroscopy the two-stranded coiled coil contains two subunits stabilized by both intrachain and interchain interactions and, lastly, its small size reduces the potential complexity in the analysis and interpretation of results encountered in the analysis of globular proteins, which have multiple elements of secondary structure (a-helix, (3-sheet, (3-turns, loops, and regions of undefined structure). 

The populations of other intermediate states are very small and can be neglected. For larger more complex proteins made up of multiple subunits, and in many fibrous proteins, this conclusion cannot be supported. Complex globular proteins appear to melt cooperatively in domains in which the smaller units melt independently, and the melting in fibrous proteins is even more complex. While the molar quantities for the heat capacity are dependent upon the size of the protein, the partial specific heat capacities of many proteins are very nearly the same. 

Proteins may be fibrous or globular. The structure and polarity of the particular amino acid R groups and their sequence affect the solubility properties and tertiary structure of proteins. Quaternary structure refers to the aggregation of similar protein subunits. 

The overall structure of the a and [3 tubulin subunits is very similar and consists of a core of P-sheet surrounded by helices, forming a compact globular protein composed of three sequential domains (Fig. 2) [11]. The IV-terminal, nucleotide-binding... 

Hydrophobic and osmophobic effects are important not only in the folding of individual polypeptide chains into compact globular proteins, but also in the assembly of multiprotein complexes. Osmophobic effects are noted, for instance, in the self-assembly of subunits of the glycolytic enzyme phosphofructokinase (PFK). Self-assembly is enhanced by the presence of stabilizing organic cosolvents such as trimethylamine-A-oxide (TMAO) (Hand and Somero, 1982). As discussed later, self-assembly driven by osmophobic effects results from the thermodynamic favorability of minimizing the surface area on the proteins that is in contact with the cosolvent. 

Unfolding of globular proteins and subunits. Data on frozen storage of HMM, actin and sarcoplasmic enzymes have led us to propose that denaturation involves unfolding of the protein chain based on a decrease in enzymatic activity (myosin, HMM, and sarcoplasmic enzymes), polymerizing ability (actin) and filament forming properties (myosin) (82,99,113-116,122). 

Denaturation of myosin and actomyosin has so far been ascribed to intermolecular aggregation, but recent investigations have shown that intramolecular transconformation, the unfolding of the polypeptide chains, occurs in globular proteins and in subunits with globular structures. 

From the mere fact that CF, can be released from the membrane by EDTA treatment and the enzyme stays in solution without detergents, it is apparent that the catalytic sector has minimal, if any, direct interaction with the lipids of the chloroplast membrane. It is a globular protein that is held to the surface of the membrane via interaction with the membrane sector. Recently it was shown that the y subunit is in immediate contact with the membrane sector and the 8 and e subunits may induce proper binding for catalysis [17,18], The enzyme contains a few well-defined sites that were used for localization experiments by the method of fluorescent energy transfer [19,56-61], These studies revealed the position of those sites and helped to localize the various subunits of CF, in space relative to the chloroplast membranes (for a model of CF, see Refs. 61 and 62). These experiments are awaiting analysis of the amino acid sequence of the y subunit that is now under investigation in Herrmann s laboratory [148], Definite structural analysis could be obtained only after good crystals of the enzyme become available. 

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