Oligomerized protein molecule

The heat inactivation of many enzymes, and in particular of those which are composed of subunits, is not a simple process, and the In at versus t plots are more complex. Usually the oligomeric forms of enzymes are slower inactivating than their monomeric forms (Szajani, Ivoiy and Boross, 1980). Similarly, specific interactions between different protein molecules could result in higher heat resistance. 

A more useful structural classification takes into consideration shape and oligomeric structure (Table B4.1.1). In part, structure reflects biological location and origin. Simple, fairly rigid protein molecules occur in the extracellular environment, more complex and readily deactivated molecules are found intracellularly, and hydrophobic proteins are associated with membranes. 

Figure 3. Model for centrin-G-protein complex assembly in die connecting cilium of photoreceptor cell. Schematic representations of a part of the inner lumen of the photoreceptor connecting cilium. Centrins ( centrin isoforms 1 and 2) (Cen) are physically linked to the inner surface of the microtubule of the connecting cilium(CC). (A) Scenario at high free Ca " concentrations in CC Cen are specifically dephosphorylated by protein phosphatase PP2Cf. Ca "-binding to Cen induces Cen oligomerization and increases affinity of the Gtf -subunit of the visual heterotrimeric G-protein transducin (Gta-GtPy). This may result in trapping G-protein molecules in the connecting cilium and G-protein diffiision is inhibited (Barrier hypothesis, Wolfrum et al. 2002). (B) Scenario at low free Ca " concentrations in CC Cen are specifically phosphorylated by protein kinase CK2. Cen-P decreases affinity of G-protein to Cen. Arrow indicates that free difihision of G-protein is possible, (for references, please see text)...

The structure of the interdomain interface in bovine yB- and PB2-crystallins is very similar in both proteins (1-2). In yB-crystallin the inter ce is formed by domains from the same molecule, while in the PB2-crystallin dimer the interface consist of residues from swapped amino- and caiboxy-terminal domains of two different molecules which associate to form a dimer. Domain swapping can be considered as a common step for the oligomerization of proteins and may play an important role in the evolution of oligomeric proteins (3-4). The importance of surface interactions in formation of intermolecular contacts in protein crystals is shown by comparative studies of various y-crystallins (5-6). These suggest that differences in surface interactions between the domains of P- and y-crystallin may be responsible for dimer formation by 3-crystallins. 

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