Tertiary structure, stress-70 proteins

The interactions, such as hydrogen bonding, that dictate the tertiary structure of proteins are not as strong as covalent chemical bonds. Because these interactions are rather weak, they can be disrupted with relatively modest stresses. 

Another superfamily is formed by bacterial di-heme CCP (with over 110 entries in PeroxiBase) that are periplasmic enzymes providing protection from oxidative stress. These homodimeric enzymes have a conserved tertiary structure containing two type-c hemes covalently attached to two predominantly a-helical domains via a characteristic binding motif. One heme acts as a low redox-potential center where H2O2 is reduced, and the other as a high redox-potential center that feeds electrons to the peroxidatic site from soluble electron-shuttle proteins such as cytochrome c [24]. In the crystal structure of the Geobacter sulfurreducens enzyme shown in Fig. 3.1g, the first heme appears as a bis-histidinyl-coordinated form (and... 

The stress-70 proteins interact with a broad spectrum of polypeptide substrates, but they have some degree of specificity in their interactions. In several instances, it has been shown that a stress-70 protein can bind to proteins [e.g., bovine pancreatic trypsin inhibitor (BPTI), a-lactalbumin] that have been stabilized in a nonnative, or denatured, form by reduction and carboxymethylation of the cysteines that would normally form disulfides at the same time, they will not bind to the native forms of the same proteins (Liberek et al., 1991b Palleros et ai, 1991, 1992). This suggests that the peptide-binding activity of the stress-70 proteins discriminates in favor of polypeptides in a denatured, and possibly extended, conformation over those in a compact secondary and tertiary structure. NMR experiments demonstrating that the E. coli dnaK... 

Two intriguing activities of PrP emerged from studies with transgenic mice expressing mutants lacking the internal hydrophobic domain (HD). First, deleting residues 105-125 from the HD was sufficient to convert PrP from a stress-protective into a neurotoxic protein [52]. Second, co-expression of wild-type PrPc completely blocked the toxic activities of PrPAHD mutants [48, 51, 52] (Fig. 1). The toxic activity of PrPAHD mutants could be related to that of Doppel, a neurotoxic protein with a tertiary structure similar to that of the C-terminal domain of PrPc [140]. Notably, Doppel-induced neurodegeneration is also rescued by the co-expression of PrPc [141-144]. A comprehensive review of Doppel is provided by David West-away in this book. 

Aggregation. The existence of 1 (folding intermediate) states in several proteins leads to the inactivation of the protein by aggregation. Moderate amounts of stress can generate 1 states, which retains secondary structure but tertiary structural... 

The denaturation of protein involves loss of their tertiary and secondary structures. This typically occurs by application of some external stresses, out of which thermal, interfacial and dehydration-related stresses are the most important stresses causing denaturation of proteins in drying processes. These stresses disrupt the tertiary structure, and subsequently the a-helix and j3-sheets of native proteins are turned into unfolded random shapes. When a protein is unfolded, the hydrophobically buried sites are exposed to the solvent and subsequently interact with interfaces and other unfolded polypeptides. The unfolded protein allows subsequent cross-linking interactions such as protein-protein hydrophobic, electrostatic, and disulfide-sulfhydryl interactions. These interactions result in aggregation, coagulation, and, finally, precipitation (Pelegrine and Gasparetto, 2005 Anandharamakrishnan et al, 2007). 

A more scientific definition is given by Veis [423] The gelatins are a class of proteinaceous substance that have no existence in nature, but are derived from the parent protein collagen, by any one of a number of procedures involving the destruction of the secondary structure of the collagen and, in most cases, some aspects of the primary and tertiary structures. Collagen is the principle proteinaceous component of the white fibrous connective tissues, which serve as the chief, tensile stress-bearing elements for all vertebrates, whereas related protons are found in any of the lower phyla . 

The denaturation of proteins is a conformational transformation of the macromolecule that induces a loss of its specific properties. It can result from either a rise in temperature, a change in pH of the medium, a mechanical stress, or a chemical action. The denaturation is primarily the result of a transformation from an a-helix to a -structure which instantaneously modifies the tertiary structure. 

All these studies seem to indicate that the cooperativity of the folding process is due to the specific pattern of tertiary interactions and/or the specific interplay between short- and long-range interactions. This may appear to be a trivial statement, but detailed analysis of the results from the simple, exact model, protein-like models and reduced models of real proteins show several specific requirements for the protein folding cooperativity. It is very encouraging that over the entire spectrum of theoretical model studies of the protein folding process, these requirements essentially overlap [35-37,51,83,92,95]. Naturally, various models place different stress on the specific interactions that may control protein folding and structural uniqueness. 

Protein products are especially prone to changes of the tertiary and quaternary structure of the molecules and thereby loss of activity. Degradation is induced by heat and shear stress. Rigorous shaking and administration by peristaltic pumps should be avoided. 

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