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Protein Folding and the Movements of Life
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18842 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
12 / 1991 |
2,619 Words |
| Author
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Dan W. Urry
Dan W. Urry is director of the laboratory of molecular
biophysics in the school of medicine at the University of
Alabama at Birmingham. |
The discus thrower crouches motionless, discus in hand; he bursts into a powerful circular motion of increasing rotational speed; he releases the discus, which is propelled over a long arching trajectory; and then the discus thrower becomes motionless again except for the heaving of his chest. An earlier intake of food had been broken down to produce the stored chemical energy, which is the source of movement in living organisms.
In the broad sense, movements of life would include not only the flexion of muscles with their ripples visible under the skin, but also the waving of cilia on cell surfaces, the pseudopodia, oozing movement of amoebas, and even the rearrangements of components within a cell. How do these movements occur? The working premise of most scientists today is that the root source of movement at all levels is the shape change that takes place within individual protein molecules. Then, if chemical energy is the energy source for movement in living organisms, the obvious question is "how does chemical energy direct shape changes (folding) in proteins to produce movements?"
Although there are several ways in which protein folding might produce movement, a growing body of evidence suggests that the forces responsible for the movement can be demonstrated in model proteins that behave somewhat like coil springs. As an analogy, consider a wire from one end of which a weight is suspended. If this wire is stimulated to wrap up into a helical spring it performs mechanical work by lifting the weight. The original question can now be rephrased: "What is the nature of protein such that chemical energy could cause it to fold up like a spring?"
To begin to answer this question, we must first review the basic building blocks of protein.
The building blocks of protein
The basic building blocks of protein are amino acids represented in chemical notation as H2NCHR-COOH (H=hydrogen, N=nitrogen, C=carbon, O=oxygen and R=one of 20 different chemical groups). An amino acid minus a water molecule (HOH) is called an amino acid residue. These residues occur linked together by the peptide bond (CO-NH) in chains called polypetides. Since there are 20 different R groups that may form an amino acid, there are 20 different naturally occurring amino acids that may be incorporated into a polypeptide chain. A large single polypetide chain or a combination of several chains constitutes a protein. Each amino acid residue not only has a full name,
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