Protein Denaturation and Folding:- Amino Acid Sequence Determines Tertiary Structure

The tertiary structure of a globular protein is determined by its amino acid sequence. The most important proof of this came from experiments showing that denaturation of some proteins is reversible. Certain globular proteins denatured by heat, extremes of pH, or de naturing reagents will regain their native structure and their biological activity if returned to conditions in which the native conformation is stable. This process is called renaturation. A classic example is the denaturation and renaturation of ribonuclease. Purified ribonuclease can be completely denatured by exposure to a concentrated urea solution in the presence of a reducing agent. The reducing agent cleaves the four disulfide bonds to yield eight Cys residues, and the urea disrupts the stabilizing hydrophobic interactions, thus freeing the entire polypeptide from its folded conformation. Denaturation of ribonuclease is accompanied by a complete loss of catalytic activity. When the urea and the reducing agent are removed, the randomly coiled, denatured ribonuclease spontaneously refolds into its correct tertiary structure, with full restoration of its catalytic activity (Fig. 4–27). The refolding of ribonuclease is so accurate that the four intrachain disulfide bonds are re-formed in the same positions in the renatured molecule as in the native ribonuclease. As calculated mathematically, the eight Cys residues could recombine at random to form up to four disulfide bonds in 105 different ways. In fact, an essentially random distribution of disulfide bonds is obtained when the disulfides are allowed to reform in the presence of denaturant, indicating that weak bonding interactions are required for correct positioning of disulfide bonds and assumption of the native conformation. This classic experiment, carried out by Christian Anfinsen in the 1950s, provided the first evidence that the amino acid sequence of a polypeptide chain contains all the information required to fold the chain into its native, three-dimensional structure. Later, similar results were obtained using chemically synthesized, catalytically active ribonuclease. This eliminated the possibility that some minor contaminant in Anfinsen’s purified ribonuclease preparation might have contributed to the renaturation of the enzyme, thus dispelling any remaining doubt that this enzyme folds spontaneously.

FIGURE 4–27 Renaturation of unfolded, denatured ribonuclease. Urea is used to denature ribonuclease, and mercaptoethanol (HOCH₂CH₂SH) to reduce and thus cleave the disulfide bonds to yield eight Cys residues. Renaturation involves reestablishment of the correct disulfide cross-links.

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قسم الشؤون الفكرية يقيم معرضًا وثائقيًا ضمن مهرجان (الشهيد) في جامعة الكوفة

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سر غذائي بسيط لحماية القلب مع التقدم في العمر

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مواضيع ذات صلة

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Protein Denaturation and Folding:- Some Proteins Undergo Assisted Folding

Protein Denaturation and Folding:- Polypeptides Fold Rapidly by a Stepwise Process

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Protein Tertiary and Quaternary Structures: -Analysis of Many Globular Proteins Reveals Common Structural Patterns

Protein Tertiary and Quaternary Structures:- Globular Proteins Have a Variety of Tertiary Structures

Protein Tertiary and Quaternary Structures:- Myoglobin Provided Early Clues about the Complexity of Globular Protein Structure

Protein Tertiary and Quaternary Structures: -Structural Diversity Reflects Functional Diversity in Globular Proteins

Protein Tertiary and Quaternary Structures: -Fibrous Proteins Are Adapted for a Structural Function