<P> Protein folding is the physical process by which a protein chain acquires its native 3 - dimensional structure, a conformation that is usually biologically functional, in an expeditious and reproducible manner . It is the physical process by which a polypeptide folds into its characteristic and functional three - dimensional structure from random coil . Each protein exists as an unfolded polypeptide or random coil when translated from a sequence of mRNA to a linear chain of amino acids . This polypeptide lacks any stable (long - lasting) three - dimensional structure (the left hand side of the first figure). As the polypeptide chain is being synthesized by the ribosome, the linear chain begins to fold into its three dimensional structure . Folding begins to occur even during translation of the polypeptide chain . Amino acids interact with each other to produce a well - defined three - dimensional structure, the folded protein (the right hand side of the figure), known as the native state . The resulting three - dimensional structure is determined by the amino acid sequence or primary structure (Anfinsen's dogma). The energy landscape describes the folding pathways in which the unfolded protein is able to assume its native state . Experiments beginning in the 1980s indicate the codon for an amino acid can also influence protein structure . </P> <P> The correct three - dimensional structure is essential to function, although some parts of functional proteins may remain unfolded, so that protein dynamics is important . Failure to fold into native structure generally produces inactive proteins, but in some instances misfolded proteins have modified or toxic functionality . Several neurodegenerative and other diseases are believed to result from the accumulation of amyloid fibrils formed by misfolded proteins . Many allergies are caused by incorrect folding of some proteins, because the immune system does not produce antibodies for certain protein structures . </P>

What is the fold of a single polypeptide chain called
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