CHMI 2227E Biochemistry I - cellbiochem.ca

CHMI 2227E Biochemistry I - cellbiochem.ca

CHMI 2227F Biochimie I Acides amins: -Structure -Proprits chimiques gnrales CHMI 2227 - E.R. Gauthier, Ph.D. 1 Protines: Blocs de base des tre vivants! Les protines sont des molcules trs complexes; Les protines sont impliques dans de nombreux phnomnes retrouvs chez les tre vivants:

Soie Toile daraigne Cornes/ongles Antibiotiques Toxines Enzymes Anticorps Hormones Transporteurs membranaires Hmoglobine

Les blocs de base servant fabriquer les protines sont les acides amins. CHMI 2227 - E.R. Gauthier, Ph.D. 2 Acides amins C COOH Les acides amins sont des molcules au sein desquelles un atome de carbone (C) est li simultanment un group dacide carboxylique (COOH) et un groupe amine (NH2);

H C NH2 R Des centaines dacides amins existent dans la nature; Cependant, seulement 20 acides amins sont impliqus dans la synthse des protines. Chaque acide amin diffre des autres par une chane latrale R. Ces chanes latrales diffrent les unes des autres de diffrentes faon: Taille Charge Solubilit dans leau

CHMI 2227 - E.R. Gauthier, Ph.D. 3 Proprits gnrales des acides amins. Notez que le C est li 4 groupes diffrents: il est donc chiral; Deux stroisomres sont distingus: L-acides amins D-acides amins

Seuls les acides amins de la srie L servent faire les protines. L = le groupe amino est gauche dans la reprsentation de Fisher. L et D: convention. Na rien voir avec la direction de rotation de lumire CHMI 2227 - E.R. Gauthier, Ph.D. polarise. 4 Proprits gnrales des acides amins. Chaque acide amin possde au moins deux groupes ionisables: Acide

carboxylique (COOH) Amine (NH2) Charge nette: +1 pH acide pKa 1 (pH 2-2.5) Charge nette: 0 Zwitterion CHMI 2227 - E.R. Gauthier, Ph.D. pKa 2 (pH 9-10.5) Charge nette: -1 pH alcalin 5 Titrage des acides amins 1. scnario le plus simple:

pI = point isolectrique= pH o 100% zwitterion pI = pKa1 + pKa2 2 La fraction de groupe COOH ou NH2 qui est charg un pH donn peut tre trouv par lquation dHendersonHasselbach: Tampon! Tampon! pH = pKa + log AHA

CHMI 2227 - E.R. Gauthier, Ph.D. 6 Les 20 acides amins CHMI 2227 - E.R. Gauthier, Ph.D. 7 Structure des 20 acides amins 1. acides amins hydrophobes Tous ces acides amins sont insoluble dans leau. Note: la glycine nest pas optiquement active. Pourquoi? CHMI 2227 - E.R. Gauthier, Ph.D. 8 Structure des 20 acides amins

1. acides amins hydrophobes Proline est le seul acide amin avoir une chane latrale cyclique; CHMI 2227 - E.R. Gauthier, Ph.D. 9 Structure des 20 acides amins 2. Acides amins aromatiques CHMI 2227 - E.R. Gauthier, Ph.D. 10 Structure des 20 acides amins 2. Acides amins aromatiques Tyr, Trp et Phe sont trs utiles:

Leur noyau aromatique absorbe lUV 260-280 nm. Constitue la base de la dtection des protines 280 nm. Absorbance max (nm) Molar Absorbance (M-1 cm-1) Phenylalanine 257.4 197

Tyrosine 274.6 1420 Tryptophan 279.8 5690 CHMI 2227 - E.R. Gauthier, Ph.D. 11 Structure des 20 acides amins 3. acides amins polaires, non-chargs CHMI 2227 - E.R. Gauthier, Ph.D. 12 Structure des 20 acides amins

Cystine et cystine En biochimie: -oxidation: perd H -Rduction: gagne H Pont disulfure CHMI 2227 - E.R. Gauthier, Ph.D. 13 Structure des 20 acides amins Tyr, Ser, Thr Ser PO4-3 ATP Protine kinase Protine phosphatase

ADP H2O O- P=O O Phospho-Ser CHMI 2227 - E.R. Gauthier, Ph.D. 14 Structure des 20 acides amins 4. Acides amins chargs ngativement CHMI 2227 - E.R. Gauthier, Ph.D. 15 Structure des 20 acides amins 4. Acides amins chargs ngativement 100% D

pH 9.5 pKa2 100% C pKa1 A pKaR B 4.1 pKa2 C D 2.1 Quel est le pI du glutamate?

pKaR pKa1 100% B 100% A NaOH CHMI 2227 - E.R. Gauthier, Ph.D. 16 Structure des 20 acides amins 5. Acides amins chargs positivement CHMI 2227 - E.R. Gauthier, Ph.D. 17 Structure des 20 acides amins 5. Acides amins chargs positivement:His pKa2 pKaR

CHMI 2227 - E.R. Gauthier, Ph.D. 18 Structure des 20 acides amins Valeurs des pKa des acides amins CHMI 2227 - E.R. Gauthier, Ph.D. 19 Analyse des acides amins 1. Chromatographie change dions pH >> pI pH << pI + + lution

Il existe diffrents types de rsines changeuses dions: change de cations: charge ngativement/sparation de cations change danions: charge positivement/sparation danions. videmment, le type de rsine utiliser dpendra de la charge de CHMI 2227 - E.R. Gauthier, Ph.D.pH de la solution. lacide amin, ce qui dpend son tour du 20 Analyse des acides amins 2. Dtection des acides amins

O COOH OH OH C NH3+ R-HC=O CO2 O O R N O Ninhydrine

2 Acide amin O O Pourpre!! Alors quon peut dtecter Trp, Phe et Tyr grce leur A260-280nm, cest impossible pour les autres acides amins; La ninhydrine ragit avec le groupe amine des acides amins, gnrant un produite de couleur pourpre (jaune dans le cas de Pro). La raction la ninhydrine permet alors de dtecter et quantifier (A570nm) les acides amins luant des fractions lors

de 2227 la chromatographie CHMI - E.R. Gauthier, Ph.D. par change dions. 21

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