Proteins globular conformations

When the polypeptide chains of protein molecules bend and fold in order to assume a more compact three-dimensional shape, a tertiary (3°) level of structure is generated (Figure 5.9). It is by virtue of their tertiary structure that proteins adopt a globular shape. A globular conformation gives the lowest surface-to-volume ratio, minimizing interaction of the protein with the surrounding environment. 

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. 

Attempts were also made to design non-protein polymers able to collapse to a globular conformation with properties complying (at least in part) with the given requirements e.g., having an active center [87]. The review of the research performed in this direction can be found in [88-92]. 

Charged polysoaps (polymer micelles) combine within a molecule structural characteristics of the conventional micelles and polyelectrolytes, and supposedly adopt globular conformations in aqueous media with the hydrophobic region inside and charged groups outside as in water-soluble proteins. Thus,... 

Typically, proteins fold to organize a very specific globular conformation, known as the protein s native state, which is in general reasonably stable and unique. It is this well-defined three-dimensional conformation of a polypeptide chain that determines the macroscopic properties and function of a protein. The folding mechanism and biological functionality are directly related to the polypeptide sequence a completely random amino acid sequence is unlikely to form a functional structure. In this view, polypeptide sequence... 

Apparently, this is due to some memory effect the core, which existed in the parent globular conformation (Fig. lb), was simply reproduced upon refolding caused by the attraction between H units. One may say that the features of the parent conformation are inherited by the protein-like copolymer. Looking at the conformations of Fig. 3, it is natural to argue that proteinlike copolymer globules could be soluble in water and thus they are open to... 

Fig. 3 Typical snapshots of the globular conformations of a protein-like, b random, and c random-block copolymers of the same length (N = 512). Hydrophobic segments are shown in light gray and hydrophilic segments in dark gray. Adapted from [18]...

The typical protein sequence contains a large number of hydrophobic amino acids. When these amino acids reach a specific concentration the protein collapses into a globular conformation, similar to the critical micellar concentration of detergents. This state is the result of a collapse of the protein driven by the minimization of the interaction of the hydrophobic amino acids with the aqueous environment. This condensed state is fluid in that the polymer is not in one well-defined conformation but sampling many different conformations until eventually the native conformation with the lowest free energy is found. This, usually ill-defined, state is referred to as the molten globular state. The manner in which... 

For polypeptides with irregular sequences, as is the usual case for enzymes and other water-soluble globular proteins, the conformational situation ranges from totally random to a mixed situation with regular stretches intermingled with random (irregular) lengths of the backbone. 

The interactions between the amino acids and the solvent (electrostatic, hydrophilic, hydrophobic, S-S) determine the globular conformation. We can give some naive picture of the folded state in terms of a liquid-hydrocarbon model where the hydrophobic core stabilizes globular proteins. The hydrophilic (polar and charged) amino acids are exposed to the solvent and the hydrophobic (polar) amino acids are less exposed to the solvent and buried in the interior of the protein. 

Caseinate is a mixture of fairly flexible polymers. Most proteins are of globular conformation, and their surface properties are not easy to interpret. The values of t)ls are much higher and tend to increase with the age of the film. It may take a day to obtain a more or less constant value, which is typically 0.1-0.5 N s m 1. However, the surface layer is clearly viscoelastic, and the apparent viscosity obtained will strongly depend on measurement conditions, especially the shear rate. Actually, it cannot always be ruled out that the proteinaceous surface layer is subject to yielding or fracture upon large deformation this would imply that slip occurs in the rheometer, leading to a greatly underestimated viscosity. 

Albeit the whole spectrum of statements, ranging from no changes to complete unfolding, is found in the literature, that globular proteins undergo conformational changes upon adsorption is a generally... 

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