CHAPER 24 AMIINO ACIDS AND PROTEINS 24.1 INTRODUCTION Of the three groups of biopolymers, protein have the most diverse function. Most of its molecular weights are much larger. Their shaps cover a range from the globular protein to the helical coils of a a-keratin. But all proteins have common features Proteins are polyamides and their monomeric units are about 20 different a-amino acids
CHAPER 24 AMINO ACIDS AND PROTEINS 24.1 INTRODUCTION Of the three groups of biopolymers, protein have the most diverse function. Most of its molecular weights are much larger. Their shaps cover a range from the globular protein to the helical coils of a α–keratin. But all proteins have common features. Proteins are polyamides and their monomeric units are about 20 different α-amino acids
Amide links R R R R R R CH CH CH H-N′C-OH CH CH H 0H OH 0 H O An a-amino acid a portion of a protien molecular Primary structure: the exact sequence of the different a-amino acids along the protein chain Second and tertiary structure: the folding of the polyamide chain which give rise to higher levels of complexity Although hydrolysis of natural occurring proteins may yield as many as 22 different amino acids, the amino acids have an important structural feature in common
N H H CH C O OH R N H CH C O R N H CH C O R' N H CH C O R'' N H CH C O R'''' N H CH C O R''' Amide links An -amino acid A portion of a protien molecular Primary structure: the exact sequence of the different α-amino acids along the protein chain. Second and tertiary structure: the folding of the polyamide chain which give rise to higher levels of complexity. Although hydrolysis of natural occurring proteins may yield as many as 22 different amino acids, the amino acids have an important structural feature in common
24.2 AMINO ACIDS 24.2A STRUCTURES AND NAMES CO,H H2N H R AbbrE- NAME VIATION Glycine Gly 6.0 CH3 Alanine Ala 6.0 -CH(CH3)2 Vali Val 6.0 CH2CH(CH3)2 Leucine eu 6.0 CH(CH3 )CH2CH3 norleucine
24.2 AMINO ACIDS 24.2A STRUCTURES AND NAMES H2 N H CO2 H R R NAME ABBREVIATION pI -H -CH3 -CH(CH3 ) 2 -CH2CH(CH3 ) 2 -CH(CH3 )CH2CH3 Glycine Alanine Valine e Leucine e Isoleucine e Gly Ala Val Leu Ile 6.0 6.0 6.0 6.0 6.1
ABBRE R NAMEⅥ Ation pI -HC Pheny lalanine Phe 5.5 CH2 CONH2 Asparagine Asn CHCH CONH Glutamine GI -CH tryptophan° Tr rp 5.9 H HOOC-CH-CH HN、CH Proline Pro (complete structure CH,OH Serine Ser 7
R NAME ABBREVIATION pI H2C -CH2CONH2 -CH2CH2CONH2 N CH2 H Phenylalanine e Asparagine Glutamine Tryptophan e Phe Asn Gln Trp 5.5 5.4 5.7 5.9 HN C H2 CH2 HOOC CH CH2 (complete structure) -CH2OH Proline Serine Pro Ser 6.3 5.7
ABBRE R NAME Ⅵ IATION -CH(CH3 )OH Threonine 6.5 2C OH Tyrosine yr 5.7 HOOCCH-CH, HN、CH Hyroxyproline yp 6.3 (complete structure) CHOSH Cysteine Cys 50 H2 aa- S Cystine ys-cyS CHoCHSCH Methionine Met 5.8 CHCOOH Aspartic acid A 3.0
R NAME ABBREVIATION pI -CH(CH3 )OH H2 C OH HN C H2 CH HOOC CH CH2 (complete structure) OH Threonine e Tyrosine Hyroxyproline Thr Tyr Hyp 6.5 5.7 6.3 -CH2 SH H2 C S S H2 C -CH2 CH2 SCH3 -CH2 COOH Cysteine Cystine Methionine e Aspartic acid Cys Cys-Cys Met Asp 5.0 5.1 5.8 3.0
ABBRE R NAME Ⅵ IATION -CH, CH, CO,H Glutamic acid Glu -CH(CH2)3NH 2 lysine Lys 9.8 CH2(CH2)2NHCNHNH2 Arginine A rg 10.8 H Histidien His 76 H The conversion of cysteine to cystine requires addition comment it can be reversed by mild reducing agents 2HO2 CCHCHOSH[01 HO, CCHCH,S-SCH, CHCO,H NH NH NH Cysteine Cystine (半胱氨酸) (胱氨酸)
The conversion of cysteine to cystine requires addition comment. it can be reversed by mild reducing agents. R NAME ABBREVIATION pI -CH2CH2CO2H -CH2 (CH2 ) 3NH2 -CH2 (CH2 ) 2NHCNHNH2 N N H2 C H Glutamic acid Lysine e Arginine Histidien Glu Lys Arg His 3.2 9.8 10.8 7.6 2HO2 CCHCH2 SH NH2 HO2 CCHCH2 S _ SCH2 CHCO2 H NH2 NH2 [0] [H] Cysteine (半胱氨酸) Cystine (胱氨酸)
24.2B ESSENTIAL AMINO ACIDS For adult humans there are eight essential amino acids. These are designated with the superscript e in above table 24.2C AMINO ACIDS AS DIPOLAR IONS Since amino acids contain both a basic group(NH2)and an acidic group(-COOH), they are amphoteric h N CHCO H -hn+CHCo HNCHCO +h +h R R R Cationic form Dipolar ion Anionic form (阳离子形式) (两极离子) (阴离子形式)
24.2B ESSENTIAL AMINO ACIDS For adult humans there are eight essential amino acids. These are designated with the superscript e in above table. 24.2C AMINO ACIDS AS DIPOLAR IONS Since amino acids contain both a basic group (-NH2) and an acidic group (-COOH) , they are amphoteric. H3 N + CHCO2 H R H3 N + CHCO2 - R - H + + H + - H+ + H+ H2 NCHCO2 - R Cationic form (阳离子形式） Dipolar ion (两极离子） Anionic form (阴离子形式）
In strongly basic solutions all amino present as anions, in acidic solutions they are present as cations. At some intermediate Ph called isoelectric point(pl)the concentration of the dipolar ion is at its maximum and the concentrations of the anions and cations are equal H3CCHCO H --H3CCHCO, OH H3CCHCO2 H CH CH CH Cationic form Dipolar ion Anionic form (pKa1=23) (pKa2=97) As the acidity reaches ph2.3, one half of the cationic form will be converted to the dipolar ion. As the ph increase to23-9.7the predominant form will be the dipolar ion. When ph rise to 9.7, the dipolar ion will be half-converted to the anionic form. As pH approached to 14, the anionic form becomes predominant form
In strongly basic solutions all amino present as anions, in acidic solutions they are present as cations. At some intermediate Ph, called isoelectric point(pI) the concentration of the dipolar ion is at its maximum and the concentrations of the anions and cations are equal. H3 CCHCO2 H CH3 H3 CCHCO2 - CH3 H3 CCHCO2 - CH3 Cationic form (pKa1 = 2.3） Dipolar ion (pKa2 = 9.7） Anionic form OH - OHH + H+ As the acidity reaches pH 2.3, one half of the cationic form will be converted to the dipolar ion. As the pH increase to2.3- 9.7 the predominant form will be the dipolar ion. When pH rise to 9.7,the dipolar ion will be half-converted to the anionic form. As pH approached to 14,the anionic form becomes predominant form
If the side chain of an amino acid contains an extra acidic or basic group, then the equilibria are more complex The isoelectric point(pl)of an amino such as the alanine is the average of pKai and pKa 2.3+97 =6.0 24 3 LABORATORY SYNTHESIS OF O-AMINO ACIDS A variety of methods have been developed for the laboratory synthesis of a-amino acids. We shall describe here three general methods
If the side chain of an amino acid contains an extra acidic or basic group, then the equilibria are more complex. The isoelectric point (pI) of an amino such as the alanine is the average of pKa1 and pKa2. pI = 2.3 +9.7 2 = 6.0 24.3 LABORATORY SYNTHESIS OF α-AMINO ACIDS A variety of methods have been developed for the laboratory synthesis of α-amino acids. We shall describe here three general methods
243A DIRECT AMMONOLYSIS OF AN O-HALO ACID R-CH,CO,H 1X2, P4 RCHCO,H_NHi(excess), RCHCO2 (2)H2O NH+ This method is probably used least often because yields tend to be poor 24.3B FROM POTASSIUM PHTHALIMIDE This method is a modification of the gabriel synthesis of amines the yields are usually high and the products are easily purified
24.3A DIRECT AMMONOLYSIS OF AN α-HALO ACID R CH2 CO2H (1) X2, P4 (2) H2O RCHCO2H X NH3 (excess) RCHCO2 - NH3 + This method is probably used least often because yields tend to be poor. 24.3B FROM POTASSIUM PHTHALIMIDE This method is a modification of the Gabriel synthesis of amines. the yields are usually high and the products are easily purified