生物化學教學課件：4 Protein function

1、liguofu,4 Protein Function,4.1 Hemoglobin 4.2 Muscle contraction 4.3 Conclusion,What we really want to know is how proteins work. Let us learn from Hemoglobin and Muscle contraction.,liguofu,4.1 Hemoglobin,Structure Binding with O2 Factors effect on the binding with O2 Mutation Copper-porphyrin in h

2、emocyanin; Magnesium-porphyrin in chlorophyll,liguofu,Pyrrole ring,Methene bridge,liguofu,Heme = Protoporphyrin IX + Fe2+,沒有任何氨基酸殘基是可以和 O2 可逆地結合,prosthetic group,過渡金屬離子如 Fe2+、 Cu2+具有比較強的 O2結合能力,need,need,liguofu,Heme = Protoporphyrin IX + Fe2+,自由的Fe2+易于與O2反應形成O自由基 與 N 的配位可阻止 Fe2+ 氧化為Fe3+,liguofu,4.1

3、 Hemoglobin,Structure Binding with O2 Factors effect on the binding with O2 Mutation & disease,liguofu,“Breathing” : Molecular motions,The binding of O2 to heme in myoglobin depends on its “breathing”,In myoglobin,2. Binding with O2,liguofu,NO CO,Also,In free heme,or,In myoglobin heme,or,liguofu,Qua

4、ntitative description of Hb binding O2,liguofu,Quantitative description of Hb binding O2,(Hill equation),(Hill plot),liguofu,Quantitative description of Mb binding O2,Why?,liguofu,Quantitative description of interaction (1),Protein + nLigand PLn,liguofu,Quantitative description of interaction (2),If

5、 kd is constant,liguofu,Quantitative description of interaction (3),If kd is not constant,liguofu,Hb undergoes a structural change on binding O2 (1),Tense state is more stable and thus the predominant conformation of deoxyHb,Relaxed state is the predominant conformation of higher affinity to O2,Bind

6、ing O2,Val E11,liguofu,In the T state, the porphyrin is slightly puckered, 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 chan

7、ge on binding O2 (2),liguofu,Allosteric protein is one in which the binding 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 “

8、other shapes,” or conformations, induced by the binding of ligands referred to as modulators. Modulator is a molecule that binds to an allosteric protein and affects its binding properties. Homotropic: the normal ligand and modulator are identical. Heterotropic: the normal ligand and modulator are n

9、ot identical.,Hemoglobin binds oxygen cooperatively,liguofu,Two model for cooperative binding,Concerted Model MWC Model,Sequential Model,liguofu,4.1 Hemoglobin,Structure Binding with O2 Factors effect on the binding with O2 Mutation & disease,liguofu,Hemoglobin also transports H+,Bohr effect,H+ bind

10、s to: NH of His146 in b 4 N-terminal NH2 Others: Arg, Lys ,liguofu,O2 binding to Hb is regulated by BPG (1),liguofu,O2 binding to Hb is regulated by BPG (2),liguofu,Binding of BPG to deoxy-Hb stabilizes the T-state, thus lowers the O2 affinity of Hb.,Only one BPG binds to each deoxy-Hb tetramer,The

11、BPG binding pocket exists in the T-state, but disappears in the R-state of Hb.,T-state (deoxy-Hb),R-state (oxy-Hb),Positively charged groups that BPG interacts with,In fetus hemoglobin (a2g2), the conversion of b-His143 to g-ser143 results in the decrease of the affinity of BPG,liguofu,4.1 Hemoglobi

12、n,Structure Binding with O2 Factors effect on the binding with O2 Mutation & disease,liguofu,Distribution of mutations in human hemoglobin,GluVal,Sickle cell anemia,liguofu,liguofu,Sickle cell anemia is caused by a single amino acid replacement on the b subunit: Glu6Val,liguofu,GluVal,liguofu,liguof

13、u,Electron micrograph of deoxy-Hb S fibers spilling out of a ruptured erythrocyte.,The sickled cells are fragile. Their breakdown leads to an anemia that leaves the victim susceptible to infections and diseases.,liguofu,The elongated cells tend to block capillaries, causing inflammation and consider

14、able pain,liguofu,By Linus Pauling in 1949.,1903,1987,Normal,Trait,Patient,(Normal),(defective),liguofu,The HbS allele confers a small but significant resistance to malaria in heterozygous individuals,The HbS allele (i.e., sickle cell trait) was found common in certain parts of Africa. Plasmodium (a

15、 protozoan causing malaria) increases the acidity of erythrocyte, favoring the formation of deoxy-HbS (the Bohr effect), thus probably allowing the spleen to preferentially remove the infected red cells.,liguofu,4 Protein Function,4.1 Hemoglobin 4.2 Muscle contraction 4.3 Conclusion,liguofu,Muscle s

16、tructure,liguofu,Muscle structure,liguofu,Muscle structure,liguofu,Muscle contraction,liguofu,Muscle contraction,liguofu,Actin molecules,Tropomyosin molecules,Troponin molecules,Cross bridges (head groups of myosin molecules),Actin,Tropomyosin,Troponin,Myosin molecule,head groups,Split by papain,Thi

17、n filament,Thin filament,Thick filament,liguofu,Myosin (1),325 nm,liguofu,Myosin (2),liguofu,Myosin (3)：S1 structure,liguofu,Myosin (3)：S1 structure,liguofu,G actin F actin (1),liguofu,G actin F actin (2),liguofu,G actin F actin (3),liguofu,Interaction between myosin and F-actin,liguofu,Step 1,Step

18、2,The “power stroke” model of muscle contraction,(ATP-consuming Motor),Two conformations of the cross-bridge were detected in insect muscle (1965),liguofu,Step 3,Step 4,Step 1,liguofu,Tropomyosin: lying along the groove in the F-actin helix. Troponin: TnC, TnI, TnT TnC: the Ca2+binding subunit TnI:

19、the inhibitory subunit TnT: the Tm-binding subunit Tm & 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.,liguofu,Tropomyosin：coiled-coil,liguofu,Troponin (1),TnC: red TnI: light blu