CHAPTER 27 AMINO ACIDS, PEPTIDES AND PROTEINS NUCLEIC ACIDS he relationship between structure and function reaches its ultimate expression in istry of amino acids, peptides, and pre Amino acids are carboxylic acids that contain an amine function Under cer- tain conditions the amine group of one molecule and the carboxyl group of a second can react, uniting the two amino acids by an amide bond HaNCHCO hINCHCO NHCHCO + H Two a-amino acids Amide linkages between amino acids are known as peptide bonds, and the product of peptide bond formation between two amino acids is called a dipeptide. The peptide chain may be extended to incorporate three amino acids in a tripeptide, four in a tetrapep- tide, and so on. Polypeptides contain many amino acid units. Proteins are naturally occurring polypeptides that contain more than 50 amino acid units--most proteins are polymers of 100-300 amino acids. The most striking thing about proteins is the diversity of their roles in living sys- tems: silk, hair, skin, muscle, and connective tissue are proteins, and almost all enzymes are proteins. As in most aspects of chemistry and biochemistry, structure is the key to function. We'll explore the structure of proteins by first concentrating on their funda- mental building block units, the a-amino acids. Then, after developing the principles of peptide structure, we'll see how the insights gained from these smaller molecules aid our of protein 1051 Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
1051 CHAPTER 27 AMINO ACIDS, PEPTIDES, AND PROTEINS. NUCLEIC ACIDS The relationship between structure and function reaches its ultimate expression in the chemistry of amino acids, peptides, and proteins. Amino acids are carboxylic acids that contain an amine function. Under certain conditions the amine group of one molecule and the carboxyl group of a second can react, uniting the two amino acids by an amide bond. Amide linkages between amino acids are known as peptide bonds, and the product of peptide bond formation between two amino acids is called a dipeptide. The peptide chain may be extended to incorporate three amino acids in a tripeptide, four in a tetrapeptide, and so on. Polypeptides contain many amino acid units. Proteins are naturally occurring polypeptides that contain more than 50 amino acid units—most proteins are polymers of 100–300 amino acids. The most striking thing about proteins is the diversity of their roles in living systems: silk, hair, skin, muscle, and connective tissue are proteins, and almost all enzymes are proteins. As in most aspects of chemistry and biochemistry, structure is the key to function. We’ll explore the structure of proteins by first concentrating on their fundamental building block units, the -amino acids. Then, after developing the principles of peptide structure, we’ll see how the insights gained from these smaller molecules aid our understanding of proteins. Amide (peptide) bond H3NCHCO R O H3NCHCO R O Two -amino acids NHCHCO H3NCHC R R O O Dipeptide Water H2O Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CHAPTER TWENTY-SEVEN Amino Acids, Peptides, and Proteins. Nucleic Acids The chapter concludes with a discussion of the nucleic acids, which are the genetic material of living systems and which direct the biosynthesis of proteins. These two types of biopolymers, nucleic acids and proteins, are the organic chemicals of life 27.1 CLASSIFICATION OF AMINO ACIDS Amino acids are classified as a, B, y, and so on, according to the location of the amine group on the carbon chain that contains the carboxylic acid function 1-Aminocyclopropanecarboxylic acid an a-amino acid that is the biological precursor to ethylene in plants 3-Aminopropanoic acid: known as B-alanine, H3NCH? CH2CO It is a p one the structural units of coenzyme A 4-Aminobutanoic acid: known as HNCHCHCHCO Y-aminobutyric acid (GABA), acid and is involved in the transmission of Although more than 700 different amino acids are known to occur naturally, group of 20 of them commands special attention. These 20 are the amino acids that are normally present in proteins and are shown in Figure 27. 1 and in Table 27.1. All the amino acids from which proteins are derived are a-amino acids, and all but one of these contain a primary amino function and conform to the general structure HCO The one exception is proline, a secondary amine in which the amino nitrogen is incor- porated into a five-membered ring HH Table 27.1 includes three-letter and one-letter abbreviations for the amino acids. Both enjoy wide use. Our bodies can make some of the amino acids shown in the table. The others which are called essential amino acids, we have to get from what we eat. 27.2 STEREOCHEMISTRY OF AMINO ACIDS Glycine is the simplest amino acid and the only one in Table 27. 1 that is achiral. The The graphic that opened a-carbon atom is a stereogenic center in all the others. Configurations in amino acids this chapter is an electrostatic potential map of glycine. are normally specified by the D, L notational system. All the chiral amino acids obtained from proteins have the L configuration at their a-carbon atom. Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
1052 CHAPTER TWENTY-SEVEN Amino Acids, Peptides, and Proteins. Nucleic Acids The chapter concludes with a discussion of the nucleic acids, which are the genetic material of living systems and which direct the biosynthesis of proteins. These two types of biopolymers, nucleic acids and proteins, are the organic chemicals of life. 27.1 CLASSIFICATION OF AMINO ACIDS Amino acids are classified as , , , and so on, according to the location of the amine group on the carbon chain that contains the carboxylic acid function. Although more than 700 different amino acids are known to occur naturally, a group of 20 of them commands special attention. These 20 are the amino acids that are normally present in proteins and are shown in Figure 27.1 and in Table 27.1. All the amino acids from which proteins are derived are -amino acids, and all but one of these contain a primary amino function and conform to the general structure The one exception is proline, a secondary amine in which the amino nitrogen is incorporated into a five-membered ring. Table 27.1 includes three-letter and one-letter abbreviations for the amino acids. Both enjoy wide use. Our bodies can make some of the amino acids shown in the table. The others, which are called essential amino acids, we have to get from what we eat. 27.2 STEREOCHEMISTRY OF AMINO ACIDS Glycine is the simplest amino acid and the only one in Table 27.1 that is achiral. The -carbon atom is a stereogenic center in all the others. Configurations in amino acids are normally specified by the D, L notational system. All the chiral amino acids obtained from proteins have the L configuration at their -carbon atom. N CO2 H H Proline RCHCO2 NH3 1-Aminocyclopropanecarboxylic acid: an -amino acid that is the biological precursor to ethylene in plants CO2 NH3 H3NCH2CH2CO2 3-Aminopropanoic acid: known as -alanine, it is a -amino acid that makes up one of the structural units of coenzyme A H3NCH2CH2CH2CO2 4-Aminobutanoic acid: known as -aminobutyric acid (GABA), it is a -amino acid and is involved in the transmission of nerve impulses The graphic that opened this chapter is an electrostatic potential map of glycine. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
27.2 Stereochemistry of Amino Acids Amino acids with nonpolar side chains Leucine Isoleucine Phenylalanine Tryptophan Amino acids with polar but nonionized side chains Asparagine Glutamine Serine Threonine e chains Aspartic acid Glutamic acid Tyrosine Amino acids with basic side chains FIGURE 27.1 Electro 20 common amino acids listed in Table 27.1. each acid is oriented so that its side chain is in the hains affect the sha properties of the Lysine Histidine Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
Amino acids with nonpolar side chains Amino acids with polar but nonionized side chains Amino acids with acidic side chains Amino acids with basic side chains Glycine Alanine Valine Isoleucine Leucine Methionine Proline Phenylalanine Tryptophan Asparagine Serine Threonine Glutamine Aspartic acid Tyrosine Cysteine Glutamic acid Lysine Arginine Histidine FIGURE 27.1 Electrostatic potential maps of the 20 common amino acids listed in Table 27.1. Each amino acid is oriented so that its side chain is in the upper left corner. The side chains affect the shape and properties of the amino acids. 27.2 Stereochemistry of Amino Acids 1053 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CHAPTER TWENTY-SEVEN Amino Acids, Peptides, and Proteins. Nucleic Acids TABLE 27.1 a-Amino Acids Found in Proteins contains electrostatic potential maps of all the amino acids in Name Abbreviation Structural formula* Amino acids with nonpolar side chains NH3 Gly(G) H CHCO Alanine Ala(a) CH2-CHCO Val (V (CH3)2CH—CHCO Leucine Leu(l) (CH3)2 CHCH2 -CHCO soleucinet le(①) NH thionine Met(M) CH3SCH, CH 2-CHCO2 Proline Pro(P) H2O NH3 Phen Phe(F) CH2--CHcO2 Tryptophan Trp (w) CH-CHCO Amino acids with polar but nonionized side chains Asparagine Asn(N CH2--CHCOz "All amino acids are shown in the form present in greatest concentration at pH7. amino acid, which must be present in the diet of animals to ensure normal Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
1054 CHAPTER TWENTY-SEVEN Amino Acids, Peptides, and Proteins. Nucleic Acids TABLE 27.1 -Amino Acids Found in Proteins Name Glycine Alanine Valine† Leucine† Isoleucine† Methionine† Proline Phenylalanine† Tryptophan† (Continued) Amino acids with nonpolar side chains Asparagine Amino acids with polar but nonionized side chains Gly (G) Ala (A) Val (V) Leu (L) Ile (I) Met (M) Pro (P) Phe (F) Trp (W) Asn (N) Abbreviation Structural formula* H NH3 CHCO2 CH3 NH3 CHCO2 (CH3)2CH NH3 CHCO2 CH3CH2CH NH3 CHCO2 CH3 (CH3)2CHCH2 NH3 CHCO2 H2C H2C H2C NH2 CHCO2 CH3SCH2CH2 NH3 CHCO2 CH2 NH3 CHCO2 N H CH2 NH3 CHCO2 O H2NCCH2 NH3 CHCO2 *All amino acids are shown in the form present in greatest concentration at pH 7. † An essential amino acid, which must be present in the diet of animals to ensure normal growth. Learning By Modeling contains electrostatic potential maps of all the amino acids in this table. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
27.2 Stereochemistry of Amino Acids TABLE 27.1 a-Amino Acids Found in Proteins(Continued) Name Abbreviation Structural formula Amino acids with polar but nonionized side chains NH Glutamine H2NCCH2 CH2--CHCO NH3 Ser(S) HOCH-CHCO Threonine CHaCH-CHCO2 Amino acids with acidic side chains NH Aspartic acid OCCH一CHCo H Glutamic acid Glu(e OCCH, CH2--CHCO H3 Tyrosine Tyr Y) HO CH2-CHCO? HSCH2--CHCO Amino acids with basic side chains Arg(r) H,, CH2 CH2-CHCO2 Histidine His(H) CHCO Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
27.2 Stereochemistry of Amino Acids 1055 TABLE 27.1 -Amino Acids Found in Proteins (Continued) Name Serine Threonine† Aspartic acid Glutamic acid Tyrosine Cysteine Amino acids with acidic side chains Amino acids with polar but nonionized side chains Lysine† Arginine† Histidine† Amino acids with basic side chains Ser (S) Thr (T) Asp (D) Glu (E) Tyr (Y) Cys (C) Lys (K) Arg (R) His (H) Abbreviation Structural formula* CH3CH NH3 CHCO2 OH HSCH2 NH3 CHCO2 H3NCH2CH2CH2CH2 NH3 CHCO2 O OCCH2 NH3 CHCO2 H2NCNHCH2CH2CH2 NH3 NH2 CHCO2 O OCCH2CH2 NH3 CHCO2 HOCH2 NH3 CHCO2 CH2 NH3 CHCO2 HO CH2 NH3 CHCO2 N N H Glutamine Gln (Q) O H2NCCH2CH2 NH3 CHCO2 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website