Chaperones and Protein Structure

The amino acid sequence of a protein (primary structure) is only the beginning; the end of translation is not the end of protein synthesis. The protein now coils or folds into its secondary and tertiary structures and, in some cases, associates with other polypeptide chains (quaternary structure) or conjugates with a nonprotein moiety, such as a vitamin or carbohydrate. It is essential that these processes not begin prematurely as the amino acid sequence is being assembled, since the correct final shape may depend on amino acids that have not been added yet. Therefore, as new proteins are assembled by ribosomes, they are sometimes picked up by older proteins called

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Physiology: The Unity of Function Companies, 2003 Form and Function, Third Edition

138 Part One Organization of the Body

Amino acid-accepting end

Amino acid-accepting end

Loop 1

Amino acid-accepting end

Amino acid-accepting end

Loop 1

Trna Attaches Mrna

Anticodon

lUUAj

Anticodon

Anticodon

lUUAj

Anticodon

Figure 4.7 Transfer RNA (tRNA). (a) tRNA has an amino acid-accepting end that binds to one specific amino acid, and an anticodon that binds to a complementary codon of mRNA. (b) The three-dimensional shape of a tRNA molecule.

Figure 4.8 Several Ribosomes Attached to a Single mRNA Molecule, Forming a Polyribosome. The fine horizontal filament is mRNA; the large granules attached to it are ribosomes; and the beadlike chains projecting from each ribosome are newly formed proteins.

chaperones. A chaperone prevents a new protein from folding prematurely and assists in its proper folding once the amino acid sequence has been completed. It may also escort a newly synthesized protein to the correct destination in a cell, such as the plasma membrane, and help to prevent improper associations between different proteins. As in the colloquial sense of the word, a chaperone is an older protein that escorts and regulates the behavior of the "youngsters." Some chaperones are also called stress proteins or heat-shock proteins because they are produced in response to heat or other stress on a cell and help damaged proteins fold back into their correct functional shapes.

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