AMINO ACID

Nguy n Ph c Long

Thu c tnh ha h c t nhin c a amino acid

T t c c t o nn t qu trnh trng hp -amino acid. Trong

-amino acid tham gia t ng h p nn cc protein c i. M t vi amino acid

n t d ng t do hay h p ch o thnh peptide hay

protein. Cc amino acid khng to protein ny c nhng ch c bi

cc amino acid thi t y , tyrosine c trong thnh phn hormone

tuy n gip, glutamate l mt ch t d n truy n th

Hnh 6.1: Cng th c phn t c a Alanine, Glutamine, Phenylalanine

Cc -amino acid trong peptide hay protein (ngoi tr proline) c mt nhm COOH v mt

nhm NH2, hai nhm ch t nguyn t carbon b i x ng c cu

trc t di n, hay cn gi l C . Ngoi ra, m u c m t g c R ring bit, chng

(ngo i l i v i glycine, g c R c a n l mt nguyn t Hydro).

Hnh 6.2: C u trc hai hnh th t n t i c a glycine tr a

nguyn t nguyn t carbone.

Nh ng nghi p c a mnh i h

ra m t d n xu t c a lysine l pyrrolysine trong mt s loi sinh v t c , (archaeal species)

Methanosarcina barkeri ch ng h n c tm th y trong

c u trc t nhin c a protein nh vo ho ng c a cc phn t c bi t.

Chnh s hi n di n 2 amino acid ny trong mt s protein khi n cho cc nh khoa hc th c

m c, li u h c thuy t v protein c i amino

acid khc na c th hi n di n tr c?

Phn lo i cc amino acid

i ta d a vo g phn lo i cc amino acid. Nu phn chia da vo tnh cht c a

g c v nhm k c.

Nhm amino acid k c khng ho c, do v y ph n l n cc amino acid

ny s n m ph n n i (interior) c

hnh thnh lin k c c lin k t ch t ch v ng

ng t o lin k t hydro v ng v gi a cc amino acid vi nhau. Do tnh

ch c bi c t n t i m m t ngo i c a amino acid hay trong

trung tm phn ng c a cc enzyme.

Hnh 6.3: ci Hy L i v i cc amino acid.

Lin k t peptide

Lin k c hnh thnh t ph n t o thnh peptide

v protein c dng R-CO-NH- t s hormone hay neurotransmitter, khng sinh v tc

nhn chng u (antitumor agents) c bn ch t l peptide m t chu i ng n g m m t s amino

acid.

Hnh 6.4: M t s hnh thnh lin k t peptide( trn), m t tetrapeptide (hnh gi a) v d ng

c ng c a n i.

Nhm ch c -COOH v -NH2 c a amino acid c kh c,

hai nhm ny tn t i d c bi u di v d

ng h p glycine trn, c Ka l 1,6 x 10-10

v do v c Kb c a n khi bi t

Ka x Kb = 10-14

trong dung d ng s acid Ka c a

ion NH3+ v Kb l h ng s base ca ion COO

-.

Hnh 6.5: ng cong chu c a enzyme ribonuclease A ti 25oC. N c a KCl l

ng mu ng mu xanh l cy).

T u ki , pH = 7,39, nhm carboxyl s t n t i d ng anion cn nhm

amine s c proton ha tr thnh dng cation. Ngoi ra, amino acid c gc R khng phn

c n t i d ng trung ha pH ny. Do v y, amino acid gi l ch ng c c

(zwitterion):

- C nhi nng chy cao v b phn h y.

-

M i amino acid s c m t h ng s acid ring c c vi i d ng Ka hay

pKa. T i s n tch c a m i amino acid, peptide hay protein s ph thu c vo pH ca

ng lo nh

chng b n di. m pH m t i s n tch c a amino acid bng 0

i ta g n (isoelectric poitn, pl)

Hnh 6.6: Minh h ng h p c a hi ng t o l p tr ng thi ion ca cc ch ng c

anode l c c m.

- N u dung dch ki m m nh ta s c n II > III, amino acid s d ch chuyn v pha

anod.

- N u dung dch l acid mnh th n III > II, amino acid dch chuyn v catod.

- N u n II v III b ng nhau th khng c s d ch chuyn no v hai c c c n

ha tan nh nh ng hi u l t n tch b ng 0)

n c n c th tnh theo cng thc sau:

Hnh 6.7: M t ng cong chu c a glycine

ng h p ph c t p, v d ng h p c a acid aspartic, ta phi vi t cc qu

trnh cn bng c n c a n l cn

b ng no r i m i p d ng cng thc trn:

n c a n nm gi a cn bng (1) v (2), do v c tnh

Hnh 6.8: n c a m t s amino acid.

Hnh 6.9: Minh h a s n tch theo pH ca Glutamate v Lysine theo chi

d n c a n [OH-].

a g c R

Hnh 6.10: Kh o lin k t disulfide c a cysteine t o thnh cystine.

G c R quy nh tnh cht v vai tr ca b t k amino acid no. Cc amino acid khng phn

c c s c u thnh nn li c c b o v kh c

l i, cc amino acid phn cc s t o nn phn v v trung tm phn ng c a cc phn t

protein c hot tnh xc tc. Vng imidazole ca histamine l mt v d . N c th

tr l ch t cho hay nhn proton ng xu t hi n t i vng

ho ng c a cc enzyme. Mt trong nh ng kh c bi t khc n a c a histamine l tr

thnh ch m pH trong hemoglobin (H+ n t s phn ly c a carbonic acid), nh v y m

hemoglobin c th m b c ch i kh c a mnh. Hay nhm OH c a

SH c a cysteine khin cho cc amino acid ny c th

t i nhn (nucleophiles) trong qu trnh xc tc. Nhm SH c a cysteine

cn c kh o c u n i -S-S- v i phn t cysteine khc, to nn vng lin kt ph c vai

tr quan trng trong vi nh hnh cu tr u trc vng ho ng c a

th c a insulin chng h n.

Hnh 6.11: M t s chu i peptide quan tr .

Thu c tnh quang h c c a amino acid

Hnh 6.12: Thi t b v nguyn l quay cc c i quang (chiral) ni chung.

Nguyn t carbon c 4 nhm lin kt khc nhau to thnh hnh t di c g i l m ng

phnchiral. Trong cc amino acid thit y u, ch c glycine khng phi l m ng phn

chiral v n c 2 nguyn t hydro trong phn t (ph u ki ng

phn chiral c kh c bi t l xoay mt ph ng nh sng lch sang phi (dextrorotatory)

ho c sang tri (levorotatory). Tt c cc amino acid trong protein c h u c c u hnh

thu c h L- c h y chng l cc L-amino acid.

Cc D-amino acid tn t i trong t u trc c a protein, c khi

c tm th y trong cc khng nhm khng sinh polypeptide. i

d ng danh php R&S (rectus & sinister), tuy t g c s d ng

trong phn t c nhi u nguyn t i quang. i t t c ng

c l i, khng phi t t c ng phn R. Hai

cch nh ng phn ny khng thc s V d , L-threonine c th g i l

(2S, 3R)-threonine; L-isoleucine l (2S,3S)-isoleucine.

Hnh 6.13: (trn) Minh h aL amino acid v D amino acid. i)L-amino acid (bn tri)

v L-glyceraldehyde (bn phi). Hnh bi u di ng phn D-L l d ng Fisher

p th tia c c tm

v h p thu t t nh t c sng 280nm.

Hnh 6.14: p c a cc gc v nhm ha hc i quang S &

R. Hnh trnl y H lm chun v p theo th t t NH2 n CO2- v cu i cng l