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PROTEIN STRUCTURE - composed of amino acids that are joined together by peptide bends in a linear sequence, and then folded into a unique 3D shape that determines function. - primary,secondary, tertiary, and quaternary PRIMARY STRUCTURE — The linear sequence of amine acids in a protein is the primary structure of the protein. - Many genetic diseases result in proteins with abnormal amine acid sequences, causing improper folding and loss or impairment of normal function. - Tf the primary structures of the normal and the mutated proteins are known, this information may be used to diagnose or study the disease. Peptide bond: ~ Ih proteins, adjacent amino acids are joined covalently by peptide bonds, which are amide linkages between the o-carbexyl group of ene amine acid and the u-arninegroup of the next amine acid. For eg, valine and alanine can form the dipeptide valylalanine through the formation of a peptide bond. - Peptide bends are resistant to conditions that denature proteins, such as heat and high cone of urea. ~ Prolonged exposure +o a strong acid or base at elevated temp is required to non-enzymatically break these bonds. - All amino acid sequences are read from the N- to the C terminal end, ~ Linkage of SO or more amine acids through peptide bends results in an unbranched chain called a polypeptide, or protein. - Each component amine acid is called a residue, because it is the portion of the amine acid remaining after the water atoms are lest during the formation of the peptide bend. Peptide bond characteristics: ~ The peptide bond has a partial double-bend character,ie, it is shorter than a single bend and is rigid and planar. This prevents free rotation around the bond between the carbonyl carbon and the nitrogen of the peptide bond. ~ The bends between the a- carbons and the a-amine or a-carboxy] groups can rotate freely. -This allows the polypeptide chain te assume a variety of possible conformations. The peptide bond is almost always in the trans configuration (instead of the cis) largely because of steric interference of 
