Proteine - Biochimie
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Transcript of Proteine - Biochimie
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AMINOACIZI ŞI PROTEINE(parţile 1-2)
STRUCTURA FUNCŢIE
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Structura generala a unui aminoacid
Radicalul R este extrem de IMPORTANT!
DE CE?
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RADICALUL R
determină hidrofobicitatea-polaritatea- sarcina electrica
a aa
Structura tridimensională
a proteinelor
Ex. PROTEINE CITOPLASMATICE
R polar, încărcat electric- la suprafaţa proteinei
R non-polar, hidrofob- la interiorul proteinei
PROTEINE MEMBRANARE
-invers-
DE CE?
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Aminoacizi alifatici cu R fără grupe funcţionale
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Aminoacizi alifatici cu grupe OH
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Aminoacizi alifatici dicarboxilici şi amidele lor
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Aminoacizi bazici (alifatici Lys, Arg şi aromatici Hys)
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Aminoacizi alifatici care conţin sulf
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Iminoacizi
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Aminoacizi aromatici
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Alte clasificări ale aminoacizilor I
I. aminoacizi proteinogeni• fundamentali (codificaţi genetic; 20; configuraţia L)• nefundamentali („derivaţi”: care nu sunt codificaţi genetic): 4 HO-Pro, 5 HO-Lys, GLA (acid gama-carboxi-Glu)
II. aminoacizi neproteinogeniex. β-Ala, ornitina, citrulina, GABA, homocisteina, homoserina, ac. para-aminobenzoic
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Alte clasificări ale aminoacizilor II
• Aminoacizi esenţiali- 9 (Leu, Ile, Val, Trp, Phe, Met, Thr, Lys, Hys); aminoacizi limitanţi pt. o proteină
• Aminoacizi neesenţiali ex Glu, Ala, Asp
• Aminoacizi semiesenţiali (ex. Arg, Pro, Ser, Tyr, Cys, Gly, taurina)
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Proprietăţile aminoacizilor• Chiralitatea. Definiţia?
AMINOACIZI NATURALI
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Calculul ph isolectric pentru arginina (Arg) prin metoda 1
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Calculul pI prin metoda 2REGULI DE STABILIRE A FORMEI ACIDE/BAZICE PREDOMINANTE LA UN
ANUMIT pH
1. Dacă pH < pKa (pH mai acid) predomină forma acidă 2. Dacă pH > pKa (pH mai bazic) predomină forma bazică 3. Dacă pH = pKa concentraţia formei bazice este egală cu concentraţia formei acide
-COOH COO- + H+ forma forma acidă (0) bazică (-) -NH3
+ NH2 + H+ forma forma acidă (+) bazică (0) 0 pH < pKa pKa pH > pKa 14 COOH 0 0 0 0 0 0 0 0 0 0 0 - - - - - - - - - - - - NH3+ + + + + + + + + + + + + + 0 0 0 0 0 0 0 0 0 0 0 0 0
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Metoda 2 de calcul al pI
Calculul pI al Arg prin metoda 2. Grupa / pKa 0 2 9 12 14 α-COOH 0 - - - α- NH3+ + + 0 0 guanidiniu + + + 0 TOTAL 2+ 1+ 0 1-
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Punctul isoelectricGruparea funcţională pKa aproximativ • α-carboxil 3 • non α -carboxyl (Asp, Glu) 4• imidazol (His) 6• sulfhidril (Cys) 8• α-amino 8 • non α-amino (Lys) 10• hidroxil (Tyr) 10• α-amino (Pro) 9 • guanido (Arg) 12
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Care este punctul isolectric al Lys, Arg, Hys?
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De ce Histidina face parteatat din grupul aa polari încărcaţi electric, cat si şi din grupul aa polari neîncărcaţi electric?
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Proprietăţi chimice ale aminoacizilor
• Rr ale grupei –COOH: decarboxilare (ex. Hys histamină)
• Rr ale grupei – NH2
- condensarea cu grupe carbonil (formare de baze Schiff)- alchilare (formare betaine)- acilare- carboxilare (formare de carbaminoacizi)- oxidare (ex. transformarea Lys în alizină) - transaminare (formare de cetoacizi)
• Rr ale grupelor funcţionale din radicalul R:- oxidarea blandă a –SH din Cys (formare cistina Cys-S-S-Cys)- oxidarea energică a –SH din Cys (formare acid cisteic R-SO3H)- R-SH + AgNO3 R-SAg (mercaptide) + HNO3
- esterificarea –OH din Ser, Thr sau Tyr cu H3PO4 (formare esteri fosforici)
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Legătura peptidică –CO-NH-
Legătura peptidică este
PLANARĂ şi RIGIDĂ.
DE CE?
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Ce se poate prezice despre legătura peptidică pornind de la această structură?
CARACTER PARŢIAL DE DUBLĂ LEG.
-Trans
-Rotaţie liberă împiedicată
-Capac. Crescută de a forma leg. de H
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PEPTIDE BIOACTIVE1. Glutation (GSH): gama-Glu-Cys-Gly
Roluri:
- agent reducător cu rol antioxidant
a) reacţia GPx (glutation peroxidazei):
2 GSH + LOOH GSSG + LOH + H2O
b) reface gr SH ale proteinelor
2 GSH + prot-S-S-prot GSSG + prot-SH
- catabolismul insulinei
- catabolismul xenobioticelor: reacţia glutation-transferazei:
R-X + GSH R-S-G + HX
unde R-X este xenobiotic
- transportul aminoacizilor prin membrane
(reacţia gama-glutamil-transpeptidazei)
γ-Glu-Cys-Gly + aa γ-Glu-aa + Cys-Gly
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PEPTIDE BIOACTIVE
PEPTIDE CU ROL HORMONAL
2. Kinine (peptide cu rol vasoactiv):
Bradikinina- 9aa, hipotensiv
Kalidina – 10 aa
3. Angiotenina II- 8 aa, hipertensiv, stimul pt secretia de aldosteron
4. TSH-RH- amidă internă Glu de la cap N-terminal
5. Ocitocina, vasopresina- 9aa, 1 S-S, amidă Gly, respectiv Glu la cap C
6. Endorfine, encefaline- neuropeptide cu rol analgezic (opioide endogene)