生物化学bi课件4protein function

1、liguofu4 Protein Function4.1 Hemoglobin4.2 Muscle contraction 4.3 ConclusionWhat we really want to know is how proteins work. Let us learn from Hemoglobin and Muscle contraction. liguofu4.1 Hemoglobin1. Structure2. Binding with O23. Factors effect on the binding with O24. Mutation & disease ligu

2、ofu1. Structure146 residues141 residuesMr = 64,500liguofu30 residues19 residues19 residuesHydrophobic interactionsH bondsSalt bondsInteractions in the four subunitsliguofuThis is called Tense state (deoxyHb)Some ion pairs are not shown hereIon pairs at the 1/ 2 and 2/ 1 interface (1)liguofuIon pairs

3、 at the 1/ 2 and 2/ 1 interface (2)Ion pairs stabilize the T state of deoxyHbliguofuconserved in all known globinsconserved27 positions are identical, 18% 82143liguofuHb subunits are structurally similar to myoglobin subunit lacks the short D helixD helixliguofuIron-porphyrin in hemoglobin; Copper-p

4、orphyrin in hemocyanin; Magnesium-porphyrin in chlorophyllliguofuPyrrole ringMethene bridgeliguofuHeme = Protoporphyrin IX + Fe2+ 没有任何氨基酸没有任何氨基酸残基是可以和残基是可以和 O2 可逆地结合可逆地结合prosthetic group过渡金属离子如过渡金属离子如 Fe2+、 Cu2+具有具有比较强的比较强的 O2结合能结合能力力needneedliguofuHeme = Protoporphyrin IX + Fe2+ l 自由的自由的Fe2+易于与易于与O

5、2反反应形成应形成O自自由基由基l 与与 N 的配位的配位可阻止可阻止 Fe2+ 氧化为氧化为Fe3+liguofu4.1 Hemoglobin1. Structure2. Binding with O23. Factors effect on the binding with O24. Mutation & disease liguofu“Breathing” : Molecular motionsThe binding of O2 to heme in myoglobin depends on its “breathing” In myoglobin 2. Binding with

6、 O2liguofuNOCOAlso22000OaCOakkCOdOdkk20002In free hemeor2200OaCOakkCOdOdkk2002In myoglobin hemeorliguofuQuantitative description of Hb binding O2liguofuQuantitative description of Hb binding O2(Hill equation)(Hill plot)liguofudkOO225 . 02Okd50225 . 0222ppOpOOOOQuantitative description of Mb binding

7、O2Why?liguofuQuantitative description of interaction (1)Protein + nLigand PLnnntiondisassociaPLLPknnntotalsitesbindingoccupiedsites bindingnnPPLPL nnnassociatioLPPLkdnnkLLliguofuQuantitative description of interaction (2)dnnkLLIf kd is constantliguofuQuantitative description of interaction (3)dnnkLL

8、If kd is not constantKdliguofuHb undergoes a structural change on binding O2 (1)Tense state is more stable and thus the predominant conformation of deoxyHbRelaxed state is the predominant conformation of higher affinity to O2Binding O2Val E11liguofuIn the T state, the porphyrin is slightly puckered,

9、 causing the heme iron to protrude somewhat on the proximal His (His F8) side. The binding of O2 causes the heme to assume a more planar conformation, shifting the position of His F8 and F helix.Hb undergoes a structural change on binding O2 (2)liguoful Allosteric protein is one in which the binding

10、 of a ligand to one site affects the binding properties of another site on the same protein. The term “allosteric” derives from the Greek allos, “other,” and stereos, “solid” or “shape.” Allosteric proteins are those having “other shapes,” or conformations, induced by the binding of ligands referred

11、 to as modulators.l Modulator is a molecule that binds to an allosteric protein and affects its binding properties.l Homotropic: the normal ligand and modulator are identical.l Heterotropic: the normal ligand and modulator are not identical.Hemoglobin binds oxygen cooperativelyliguofuTwo model for c

12、ooperative bindingConcerted ModelMWC ModelSequential Modelliguofu4.1 Hemoglobin1. Structure2. Binding with O23. Factors effect on the binding with O24. Mutation & disease liguofuHemoglobin also transports H+ Bohr effectH+ binds to:lNH of His146 in l4 N-terminal NH2lOthers: Arg, Lys liguofuO2 bin

13、ding to Hb is regulated by BPG (1)liguofu O2 binding to Hb is regulated by BPG (2)liguofuBinding of BPG to deoxy-Hb stabilizes the T-state, thus lowers the O2 affinity of Hb. Only one BPG binds to each deoxy-Hb tetramerThe BPG binding pocket exists in the T-state, but disappears in the R-state of Hb

14、.T-state (deoxy-Hb)R-state (oxy-Hb)Positively charged groups that BPG interacts withIn fetus hemoglobin (a2g g2), the conversion of His143 to g g ser143 results in the decrease of the affinity of BPGliguofu4.1 Hemoglobin1. Structure2. Binding with O23. Factors effect on the binding with O24. Mutatio

15、n & disease liguofuDistribution of mutations in human hemoglobin GluValSickle cell anemialiguofuliguofuSickle cell anemia is caused by a single amino acid replacement on the subunit: Glu6ValliguofuGluValliguofuliguofuElectron micrographof deoxy-Hb S fibersspilling out of a ruptured erythrocyte.T

16、he sickled cells are fragile. Their breakdown leads to an anemia that leaves the victim susceptible to infections and diseases. liguofuThe elongated cells tend to block capillaries, causing inflammation and considerable pain liguofuBy By Linus PaulingLinus Pauling in 1949 in 1949. 19031987NormalTrai

17、tPatient(Normal)(defective)liguofuThe HbS allele confers a small but significant resistance to malaria in heterozygous individualsThe HbS allele (i.e., sickle cell trait) was found common in certain parts of Africa.Plasmodium (a protozoan causing malaria) increases the acidity of erythrocyte, favori

18、ng the formation of deoxy-HbS (the Bohr effect), thus probably allowing the spleen to preferentially remove the infected red cells.liguofu4 Protein Function4.1 Hemoglobin4.2 Muscle contraction 4.3 ConclusionliguofuMuscle structureliguofuMuscle structureliguofuMuscle structureliguofuMuscle contractio

19、nH bandliguofuMuscle contractionliguofu2.Actin molecules3.Tropomyosin molecules4.Troponin moleculesCross bridges (head groups of myosin molecules)Actin Tropomyosin Troponin1. Myosin moleculehead groups Split by papainThin filament Thin filament Thick filament liguofu1.Myosin (1)(Two heavy chain)(fou

20、r)Mr = 540, 000MrH = 220, 000MrL = 20, 000325 nmliguofu1.Myosin (2)liguofu1.Myosin (3)：S1 structureliguofu1.Myosin (3)：S1 structureliguofu2.G actin F actin (1)liguofu2.G actin F actin (2)= Ca2+ or Mg2+liguofu2.G actin F actin (3)liguofuInteraction between myosin and F-actin liguofuStep 1Step 2The “p

21、ower stroke” model of muscle contraction(ATP-consumingMotor)Two conformations of the cross-bridge were detected in insect muscle (1965)liguofuStep 3Step 4Step 1liguofuTropomyosin: lying along the groove in the F-actin helix.Troponin: TnC, TnI, TnTTnC: the Ca2+binding subunitTnI: the inhibitory subun

22、itTnT: the Tm-binding subunitTm & Tn inhibit the binding of myosin heads to actin unless Ca2+ is about 10-5M. In resting muscle, Ca2+ is about 10-7M.The interaction between actin and myosin are regulated mainly by tropomyosin and troponin.liguofu3. Tropomyosin：coiled-coilliguofu4. Troponin (1)TnC: redTnI: light blueTnC binding region: blue TnT: yellowBound Ca2+: blackRH: regulatory headIT arm: TnI-TnT armliguofuCa24. Troponin (2)C-TnI: C-terminal region of TnIC-TnT: C-terminal region of TnTTnIreg: regulatory region of TnI:Regions where the structures are not well defined liguo