Faculty of Physics - Waterloo University

Hsc70 and Reorientation of its Subdomains

von

Lorenz Steinbock

2005

This report is presenting the paper of Zhang et al published in the PNAS July 13 2004 [1]. The focus of their investigations was the heat shock cognate (Hsc70) chaperone and the reorientation of its subdomains in the nucleotide-binding domain (NBD). Chaperones are critical for the refolding of denatured proteins and are suspected to prevent diseases like Alzheimer, caused by agglomeration of unfolded proteins. The NBD of Hsc70 has two main lobes with a ATP binding site inside the cleft. Their residual dipolar coupling (RDC) measurements showed that this protein is not rigid like prior x-ray studies suggested but that it is a flexible molecule, which is able to reorient its subdomains and though initiate hydrolysis of the bound ATP or even couples this to the substrate-binding domain (SBD) responsible for the binding and release of unfolded protein.

1 Folding and Chaperones

The biological object in this study was a chaperone protein. These molecules are vital and are able to help unfolded proteins to find back to their native conformation. Missfolded protein can agglomerate to macromolecules, aided by there exposed hydrophobe residues. These agglomerations damage the biochemical pathways in a cell, resulting in the death of it.

1.1 Protein Structure and Function

The function of a protein is mainly dependent from the right conformation. As an example we can look at an antibody which has epsilon structure and is able to bind with two of its branches an antigen. Therefore the binding site at the end of the branches has to be designed specifically to match with the three dimensional structure of the antigen and to form a tight binding (see right structure in Figure 1). Another example is a transmembrane protein, which is acting as a channel for ions. Therefore the transmembrane protein has to be designed like a tunnel form, with hydrophobe residues at the outside, so it can be incorporated into the membrane (see left strucuture in Figure 1).

We can clearly see, that for the proper function of a protein, a correct folding, which will enable the protein to fulfill its function, is vital for a cell and its organism.

Figure 1: Right: transmembrane protein with cylindrical shape, left: antibody [2] [3]

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1.2 Protein Folding

Christian Anfinsen (Nobel prize 1972) showed 1961 that proteins can find their proper conformation themselves and even fold back into this shape after being unfolded. He concluded that the information for the proper folding was stored in the primary structure. But how does the amino acid chain find its proper conformation? In 1969 Cyrus Levianthal calculated the time a protein would need, if it would randomly check all possible conformations. For a protein with 100 amino acids, the protein could theoretically adopt 2100 - 1030 possible conformations; this would take longer than the age of the universe (Levinthal’s paradox). But this was not seen in reality, which showed that protein fold within seconds.

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