1. Read the article and choose from the list A-H the best phrase to fill each of the spaces 1-8.
PROTEINS[16]
Most proteins consist of linear polymers built 1.................. All proteinogenic amino acids possess common structural features, including an ?-carbon to which an amino group, a carboxyl group, and 2.................. Only proline differs from this basic structure as it contains an unusual ring to the N-end amine group, which forces the CO-NH amide moiety into a fixed conformation. The side chains of the standard amino acids have 3.................; it is the combined effect of all of the amino acid side chains in a protein that ultimately determines 4................. and its chemical reactivity.
The amino acids in a polypeptide chain are linked 5.................. Once linked in the protein chain, an individual amino acid is called a residue, and the linked series of carbon, nitrogen, and oxygen atoms are known as 6.................. . The peptide bond has two resonance forms that contribute some double-bond character and inhibit rotation around its axis, so that the alpha carbons are roughly coplanar. The other two dihedral angles in the peptide bond determine 7................. . The end of the protein with a free carboxyl group is known as the C-terminus or carboxyl terminus, whereas the end with a free amino group is known as 8................. .
A. the main chain or protein backbone
B. a great variety of chemical structures and properties
C. the N-terminus or amino terminus
D. a variable side chain are bonded
E. by peptide bonds
F. from series of up to 20 different L-alfa-amino acids
G. the local shape assumed by the protein backbone
H. its three-dimensional structure
2. Now read the sentences and complete the spaces with appropriate words:
a. Most proteins consist of _ _ _ _ _ _ polymers built from _ _ _ _ _ _ of up to 20 different L-alfa-amino acids.
b. The amino acids in a _ _ _ _ _ _ _ _ _ _ _ chain are _ _ _ _ _ _ by peptide bonds.
c. The peptide bond has two _ _ _ _ _ _ _ _ _ forms that contribute some double-bond character and _ _ _ _ _ _ _ rotation around its _ _ _ _.
d. The end of the protein with a free carboxyl group is known as the carboxyl_ _ _ _ _ _ _ _.