Mioglobina - [email protected] 17.8 kDa ca. 6% Fe tot Emoglobina tetramero a 2b 2 141 e 146...
Transcript of Mioglobina - [email protected] 17.8 kDa ca. 6% Fe tot Emoglobina tetramero a 2b 2 141 e 146...
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Mioglobina
17.8 kDa
ca. 6% Fetot
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Emoglobina
tetramero a2b2 141 e 146 a.a. 64.5 kDa
ca. 65% Fetot
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tetramero a2b2
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Istidina
prossimale
Istidina
distale
Coordinazione O2 end-on
Angolo O−O−Fe ~ 120°
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Effetto cooperativo
ed effetto Bohr
(effetto allosterico)
Curva di saturazione
sigmoidale, pH
dipendente
Curva di saturazione
iperbolica
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eg
t2g
Fe(II), d6 paramagnetico
desossi:
S = 2
paramagnetica
ossi:
S = 0
diamagnetica
Pauling Weiss
Fe(II) d6 basso spin
+ 1O2 legato
Fe(III) d5 basso spin
+ 2O2
.- legato
Fe O
O
Fe O
O
. +
-.
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Modello di Pauling: Fe(II) l.s. + 1O2
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Fe(II) h.s.: 0.92 Å
Fe(II) l.s.: 0.75 Å
Fe(III) l.s.: 0.55 Å
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Allosterismo Omotropico
Allosterismo Eterotropico
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Catena della respirazione cellulare
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Citocromo c da cuore di cavallo
EFe(III)/Fe(II) = +260 mV
12 kDa, ca. 100 a.a.
mappa del potenziale elettrostatico
potenziale positivo
= elettron-povere
potenziale negativo
= elettron-ricche
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Gruppo eme nel citocromo c
EFe(III)/Fe(II) = +260 mV
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Gruppo eme nel citocromo b
EFe(III)/Fe(II) = +20 mV
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Citocromo c554 (nitrificazione catalizzata da batteri)
Distanze Fe∙∙∙Fe ≈ 950, 1220, 920 pm
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Ossidazioni catalizzate da eme-proteine
ossigenasi
perossidasi
ossidare non sempre implica anche ossigenare i substrati
H2O2 = intermedio indesiderato
nell’ossidazione fotosintetica di H2O o per
riduzione incompleta di O2 durante la
respirazione
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Perossidasi e Catalasi
H2O2(aq) + AH2 → 2 H2O(l) + A
H2O2(aq) + H2O2(aq) → 2 H2O(l) + O2
e.g. lignina perossidasi, horse-radish perossidasi,
Citocromo-c perossidasi
Substrati: acidi grassi, ammine, fenoli, tossine xenobiotiche,….
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Sito attivo della
Citocromo c Perossidasi
gruppo guanidinio
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Fe(III) h.s.
Ciclo catalitico di
una perossidasi
E° > 1 V
ossoferrile
H2O2(aq) + AH2 →
2H2O(l) + A
da AH2 o citocromo c
Rottura
eterolitica
legame O-O
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Formazione dell’osso-ferrile
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citocromo c perossidasi
e citocromo c
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Citocromo P-450 (da Pseudomonas putida)
Monoossigenasi
50 kDa
individuati più di
18.000 P450, di
cui 57 nell’uomo
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Attivazione (involontaria) di
substrati da parte di P450
Ossigenazione di substrati
specifici da parte di P450
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osso-ferrile
Fe(IV)=O
Citocromo P-450
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h.s.
l.s.
h.s.
osso-ferrile
Fe(IV)=O
Citocromo P-450
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O2 + e-
Rottura eterolitica del legame O–O e
protonazione dell’ossigeno distale
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FeIV
O.
C
H
R H
R'
FeIV
O
H
R
R'
H
.
FeIII
C
OH
R H
R'
estrazione
di H
Oxygen Rebound Mechanism
Meccanismo di ossidazione del substrato
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P450
perossidasi
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H-bond donors = increase E°
H-bond acceptors = decrease E°
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FeII
Y
X
FeIII
Y
X
O2
O
X
FeIV
HRP: X = His
HRP I
FeIII
O
X
-O
P-450
FeIV
O
X
e-
citocromi b, cX, Y = His, Met
Mb, Hb: X = His
P-450: X = Cys-
e-
solo P-450
X = Cys-
+e-, +2H
+
-H2O
+ H2O2
- H2O
+