Structure of Proteins I.OVERVIEW CHa The 20 amino acids commonly found in proteins are joined together by ree di analyzed by considering the molecule in terms of four organizational levels,namely,primary.secondary,tertiary,and quaternary(Figure 2.1). se hierarchies of opnoiens.aug9estnghatherearegenraleseegarainghneaway in which proteins achieve their native,functional form.These repeated structural IIL.PRIMARY STRUCTURE OF PROTEINS ry structure tant because many genetic diseases result in prote amino acid sequen s which nghabnol d loss o and the mutated proteins are known.this information may be used to diagnose or study the disease. A.Peptide bond and the o-amino group of another.For example,valine and alanine can form the dipep ond(Figure 2.2) Yegal3nmethroughtheormationofapepitde (see p.20).Prolonged exposure tostron o acid or base at elevated Figure 2.1 temperatures is required to hydrolyze these bonds nonenzymically. Four hierarchies of protein structure.Structure of Proteins 2 I. OVERVIEW The 20 amino acids commonly found in proteins are joined together by peptide bonds. The linear sequence of the linked amino acids contains the information necessary to generate a protein molecule with a unique three-dimensional shape. The complexity of protein structure is best analyzed by considering the molecule in terms of four organizational levels, namely, primary, secondary, tertiary, and quaternary (Figure 2.1). An examination of these hierarchies of increasing complexity has revealed that certain structural elements are repeated in a wide variety of proteins, suggesting that there are general “rules” regarding the ways in which proteins achieve their native, functional form. These repeated structural elements range from simple combinations of α-helices and β–sheets forming small motifs, to the complex folding of polypeptide domains of multifunctional proteins (see p. 18). II. PRIMARY STRUCTURE OF PROTEINS The sequence of amino acids in a protein is called the primary structure of the protein. Understanding the primary structure of proteins is impor￾tant because many genetic diseases result in proteins with abnormal amino acid sequences, which cause improper folding and loss or impairment of normal function. If the primary structures of the normal and the mutated proteins are known, this information may be used to diagnose or study the disease. A. Peptide bond In proteins, amino acids are joined covalently by peptide bonds, which are amide linkages between the α-carboxyl group of one amino acid and the α-amino group of another. For example, valine and alanine can form the dipeptide valylalanine through the formation of a peptide bond (Figure 2.2). Peptide bonds are not broken by conditions that denature proteins, such as heating or high concentrations of urea (see p. 20). Prolonged exposure to a strong acid or base at elevated temperatures is required to hydrolyze these bonds non enzymically. Figure 2.1 Four hierarchies of protein structure. N C C H H N C C H O CH3 H N H C O C O N C C N H H O C C C N H O C C O O H N C C N H N H R R C C R C R Quaternary 4 structure Tertiary 3 structure 2 Secondary structure Primary 1 structure H 13 168397_P013-024.qxd7.0:02 Protein structure 5-20-04 2010.4.4 11:31 AM Page 13