Defective protein

They act in part as a quality control or editing mechanism for detecting misfolded or otherwise defective proteins... 

More than 160 different mutations have been described for the protein C gene, which has nine exons ranging in size from 53 to 587 base pairs, separated by 8 in-trons ranging in size from 92 to 2668 base pairs (89). These mutations can result in a defective or even absent protein C molecule. Mutations that lead to reduced amounts of protein C molecule without accompanying evidence for an abonormal protein C molecule in the circulation are characterized as causing type I deficiency. Type II deficiency is represented by mutations that lead to the production of more or less normal amounts of defective protein C molecule (89). 

When a disease occurs, it is often the defective proteins that are involved. Most drugs also target receptors and enzymes, which are themselves proteins. Through an understanding of the proteins and their functions, better and more specific drugs can be developed (refer to Appendix 2, Section A2.3 for more information about proteins and Exhibit 3.13 for protein extraction and studies). 

One type of the constituent metallocenters in the MoFe protein has the properties of a somewhat independent structural entity. This component, referred to as the FeMo cofactor (FeMo-co), was first identified by Shah and Brill (1977) as the stable metallocluster extracted from acid-denatured MoFe protein. The FeMo-co was able to fully activate a defective protein in the extracts of mutant strain UW45, a protein which subsequently was shown to contain the P clusters but not the EPR-active center. The isolated cofactor accounted for the total S = t system observed by EPR and Mdssbauer spectroscopies of the holo-MoFe protein (Rawlings et al., 1978). Elemental analysis indicated a composition of Mo Fee-8 Se-g for the cofactor, which, if there are two FeMo-co s per a2 2> accounts for all the molybdenum and approximately half the iron in active enzyme (Nelson etai, 1983). Although FeMo-co has been extensively studied [reviewed in Burgess (1990)] the structure remains enigmatic. To date, all attempts to crystallize the cofactor have failed. This is possibly due to the instability and resultant heterogeneity of the cofactor when removed from the protein. Also, there is a paucity of appropriate models for spectral comparison (see Coucouvanis, 1991, for a recent discussion). Final resolution of this elusive structure may require its determination as a component of the holoprotein. 

Sphingolipidosis OMIM numbers Enzyme deficiency/defective protein Source of enzyme for postnatal diagnosis Main glycosphingolipid storage products Sample for storage product analysis... 

First, as we have already noted, proteins with different functions always have different amino acid sequences. Second, thousands of human genetic diseases have been traced to the production of defective proteins. Perhaps one-third of these proteins are defective because of a single change in their amino acid sequence hence, if the primary structure is altered, the function of the protein may also be changed. Finally, on comparing functionally similar proteins from different species, we find that these proteins often have similar amino acid sequences. An extreme case is ubiquitin, a 76-residue protein involved in regulating the degradation of other proteins. The amino acid sequence of ubiquitin is identical in species as disparate as fruit flies and humans. 

Defective proteins and those with characteristically short half-lives are generally degraded in both bacterial and eukaryotic cells by selective ATP-dependent cytosolic systems. A second system in vertebrates, operating in lysosomes, recycles the amino acids of membrane proteins, extracellular proteins, and proteins with characteristically long half-lives. 

In E. coli, many proteins are degraded by an ATP-dependent protease called Lon (the name refers to the long form of proteins, observed only when this protease is absent). The protease is activated in the presence of defective proteins or those slated for rapid turnover two ATP molecules are hydrolyzed for every peptide bond cleaved. The precise role of this ATP hydrolysis is not yet clear. Once a protein has been reduced to small inactive peptides, other ATP-independent proteases complete the degradation process. 

Ubiquitin-dependent proteolysis is as important for the regulation of cellular processes as for the elimination of defective proteins. Many proteins required at only one stage of the eukaryotic cell cycle are rapidly degraded by the ubiquitin-dependent pathway after completing their function. The same pathway also processes and presents class I MHC antigens (see Fig. 5-22). Ubiquitin-dependent destruction of cyclinis critical to cell-cycle regulation (see Fig. 12-44). The E2 and E3 components of the ubiquitination cascade pathway... 

Mechanism for degrading defective proteins Proteins that are defective or destined for rapid turnover are marked for destruction by the attachment of a small, highly conserved protein called ubiquitin. Proteins marked in this way are rapidly degraded by a cellular component known as the proteasome. 

Recently a variety of modifiers of ubiquitin ligases have been discovered33 1 as have ubiquitin-like domains in other proteins. These findings elucidate the complexity of the sorting of proteins and removal of improperly folded and otherwise defective proteins from the secretory pathway and return to the proteasomes in the cytosol.dd ee They also suggest important roles for ubiquitination in a broad range of metabolic controls. 

Section C Thomas, P.J., Qu, B.H. and Pederson, P.L. (1995) Defective protein folding as a basis of human disease. Trends Biochem. Sci. 20, 456-459. Wahl, M.C. and Sundaralingam, M. (1997) C-H...O hydrogen bonding in biology. Trends Biochem. Sci. 22, 97-102. Hampton, R., Dimster-Denk, D. and Rine, J. (1996) The biology of HMG-CoA reductase the pros of contra-regulation. Trends Biochem. Sci. 21, 140-145. Kantrowitz, E.R. and Lipscomb, W.N. (1990) Escherichia coli aspartate trans-carbamoylase the molecular basis for a concerted allosteric transition. Trends Biochem. Sci. 15, 53-59. 

The consequences of errors in protein synthesis are not as serious. A single defective protein molecule will, in general, not cause deleterious effects such a protein may not function properly or may be unstable, and may represent an energy wastage to the cell however, such errors do not become perpetuated in future generations. 

In general, null alleles are associated with the classic early-onset phenotype, whereas missense mutations which lead to defective proteins that exhibit residual enzyme activity lead to attenuated phenotypes (Froissart et al., 2002). However, studies of genotype-phenotype correlation have revealed a lack of perfect concordance, which suggests other factors may be involved that influence disease outcome (Froissart et al., 2002). At present, the putative factors that modify LSD-phenotypes among patients with identical genotypes remain obscure. 

Zhon Z, Gong Q, Jannary CT. Correction of defective protein trafficking of a mntant HERG potassinm channel in human long QT syndrome. Pharmacological and temperatnre effects. J. Biol. Chem. 1999 274 31123-31126. 

Graff I, Schram-Doumont A, Szpirer C. Defective protein kinase C-mediated actions in cystic fibrosis neutrophils. Cell Signal (1991) 259-66. 

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