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1、UNIT I Protein Structure and Function第一单元第一单元 蛋白质的结构和功能蛋白质的结构和功能Chapter 3 Globular Proteins第第3章章 球状蛋球状蛋白白 OBJECTIVE AND REQUIREMENT After learning this chapter, students should have a good mastery ofnstructure and function of myoglobin nstructure and function of hemoglobin nallosteric effects nBohr
2、effects neffect of 2,3-BPG on oxygen affinity of HbAfter learning this chapter, students should have a fair knowledge of noxygen dissociation curve of hemoglobin nbiochemical mechanism of hemoglobin S disease After learning this chapter, students should have some acquaintance withn Structure of heme
3、 n Minor hemoglobins n Organization of the globin gene I. OVERVIEWBy arranging the basic structural elments in different combinations, widely diverse proteins can be constructed that are capable of various functions.This chapt. will illustrate the relationship betw. structure & function by the globu
4、lar hemeproteins (e.g. Mb & Hb). II. GLOBULAR HEMEPROTEINSA group of proteins that contain heme (e.g., peroxidase, cytochromes, Mb, Hb) . Hb and Mb are the most abundant hemeproteins in humans.As a prosthetic group of hemeproteins, hemes are very important for their structure & function. II-A. Struc
5、ture of hemeFig.A. Cytochrome C Fig.B. Heme structure (protopophyrin+Fe2+) II-B. Structure & function of myoglobinPresent in heart & skeletal muscle.As an oxygen reservoir & carrier.17.8kDa,153AAs single chain, the globular heme-protein.Composite sequence of Mb cDNA & deduced Mb primary structure.II
6、-B-1. -Helical content of myoglobinFig. Model of Mb showing helix A to H II-B-2. Location of polar & nonpolar AAsCharged AAs on the surface of Mb, where forming H-bond with each other and with water .Nonpolar AAs in the interior, where forming hydropho- bic interaction betw. AAs for structure stabil
7、ization.II-B-3. Binding of the hemeIn the crevice, the proximal His(F8) binds directly to the iron of heme, & the distal(E7) doesnt directly interact with heme. II-C. Structure & function of hemoglobinIts location; Hb in adultsThe structural & functional characteristics;Compare with MbII-C-1. Quater
8、nary structure of hemoglobinA tetramer(2 identical dimers); 22 in adultsStable factors: hydrophobic interactions(primary/ interior/surface) , salt bridges & H-bonds The oxy-Hb conformation (R form, or high affinity binding of O2 ) different from the deoxy-Hb (T form, or low affinity binding of O2 )T
9、he salt bridges(8 pairs) of subunits & betw. subunits in HbII-D. Binding of oxygen to Mb & HbOne heme can bind one O2. One Hb contains four hemes, & Mb only one.There is difference in the degree of saturation of the O2-binding sites on Mb or Hb.II-D-1. Oxygen dissociation curveDefinition P50: pO2 ne
10、eded to achieve half-saturation of the binding site Oxygen affinity & P50 The curve analysis (shape; right & left shift) The hyperbonic & the sigmoidalCompare of Mb & Hb Cooperative binding of oxygen by 4 units of Hb (Heme-Heme interaction)II-E. Allsteric effectsDefinition: an effect that a small mo
11、lecule, called an effector, noncovalently binds to a protein and alters pr. conformation and pr. activity. The allosteric effectors for O2 binding of Hb: O2, H+, CO2, 2,3-bisphosphoglycerate(2,3-BPG)Oxygen affinity of Mb is not influenced by allsteric effectors.(monomeric, not oligomeric)II-E-1. Hem
12、e-heme interactionOnce the first heme of Hb binds to oxygen, the specific structural changes can transmitted to other hemes in the Hb & promote their oxygen binding. The last net effect is approximately 300 times greater than the initial. Loading O2unloading O2Loading CO2unloading CO2II-E-2. Bohr ef
13、fectThe phenomenon that CO2 & H+ promote the release of O2 from Hb. Conversely, Bohr effect decreases O2 affinity of Hb, & enable the curve to right shift. Sourcs of the H+ that lower the pH organic acids produced in the catabolism CO2 + H2O H+ + HCO3-Mechanism of the Bohr effect HbO2 + H+ HbH + O2
14、CO2 also causes an increase H+ due to the action of carbonic anhydrase which catalyzes the reaction. II-E-3. Effect of 2,3-BPG on O2 affinityThe most abundant organic phosphate & an important regulator of the Hb-O2 affinity in the RBC. mutasephosphatasekinaseOr 2, 3 - Bisphosphoglycerate (BPG)II-E-3
15、-a. Effect of 2,3-BPG on O2 affinity2.3-BPG decreases Hb-O2 affinity by binding to deoxy-Hb but not to oxy-Hb.(preferential binding)HbO2 + 2,3-BPG Hb-2,3-BPG + O2Keep deoxy-Hb(T-form)s stabilizationII-E-3-b. Binding site of 2,3-BPGII-E-3-c. Shift of the O2 dissociation curveII-E-3-d. Response of 2,3
16、-BPG levels to chronic hypoxia or anaemia2,3-BPG in the RBC increases in response to chronic hypoxia or anaemia.It helps to release of O2 from HbO2.II-E-3-e. Role of 2,3-BPG in transfused bloodStoring blood in acid-citrate-dextrose leads to a decrease of 2,3-BPGThe RBCs are able to restore their dep
17、leted supplies of 2,3-BPG only in 24-48hrs. The descrease in 2,3-BPG can be prevented by adding inosine being converted to 2,3-BPG.II-E-4. Binding of CO2Some CO2 is carried as the following: Hb-NH2 + CO2 Hb-NH-COO- + H+CO2-binding stablized the T or deoxy form of Hb. II-E-5. Binding of COTightly bin
18、dCO binds to one or more of 4heme sites, causing the remaining heme sites to bind O2 with high affinity.the curve left shift, hyperbola shapeII-F. Minor hemoglobinII-F-1. Fetal Hb(Hb F)22(belongs to-F)Major Hb in the fetus & newborn.Synthesis place: liverFrom the liver to the bone marrow, finally to
19、 RBC. Fetal Hb (22) has a higher affinity for O2 than maternal (adult) Hb .Hb FHb AO2placentamaternalfetal(positively charged AA than Hb A)(2,3-BPG binding )II-F-2. Hb A222(belongs to-F)Adult Hb, Minor(2-5%).Appearing about 12weeks after birthlower affinity for O2 than HbAII-F-3. HbA1CNonenzymically
20、 glycosy-lated Hb A(22-gluc.)Glucose groups attached to the NH2 of the N-end Val of chains.Normal adult: HbA1C 3-9%; diabetes : HbA1C Higher affinity for O2 than HbATwo separate gene clusters, the -gene family & the -gene family, are responsible for the code of different subunits of HbThere are temp
21、oral & spacial specificity in their expression.Their organization shown in Fig III. ORGANIZATION OF THE GLOBIN GENESIV. HEMOGLOBINOPATHIESIt is a kind of genetic defect that results in abnormal structure, insufficient quantities of the globin chains of the Hb, or, rarely, both.There are 2 main group
22、s, the structural variants (e.g. HbS) and disorders of synthesis, the thalassemias. IV-A. Sickle cell disease(Hb S disease) The most common inherited blood disorder in the US, & rarely seen in China.A homozygous (HbSS), recessive disorder (HbAS).consequence: change in protein structure and solubilit
23、y change in RBC shape and its function RBC to clog blood vessels or appear in “crisis”IV-A-1. AA substitution in the Hb S chains Single specific amino acid changes, that is,Glu at position 6 of -chains is replaced with Val.Negative charge Fig.3.19. Digram of HbA, S & C after electrophoresis HbA(6Glu
24、)HbS(6Val)HbC(6Lys)IV-A-2. Sicking causes tissue anoxia IV-A-3. Variables that increase sickling pO2 , pCO2 , pH(H+ ), 2,3-BPG.These variables can increase the proportion of Hb S in the deoxy-Hb(that is, reduces the HbS-O2 affinity).IV-A-4. TreatmentNon-special treatment in totalThe general treatmen
25、ts involve adequate hydration, analgesics, antibioticsbe careful of transfusions.Hydroxyurea increases circulating levels of HbF, which decreases RBC sickling.IV-A-5. Possible selective advantage of the heterozygous state The heteozygous HbS(that is, HbA/HbS) can resist to malaria among black Africa
26、ns.The homozygous (HbS/HbS) easily suffer from severe anaemia, despite their resistance of homozygous to malaria is stronger than the heteozygous.Normal black Africans(HbA/HbA) are susceptible to malaria. (the parasite causing malaria spends an obliga-tory part of its life cycle in RBC)IV-B. Hb C di
27、sease Patients homozygous for HbC generally have a relatively mild, chronic hemolytic anaemia, & no specific therapy is required.Similar to HbS IV-C. Hb SC disease Patients with HbSC (HbS/HbC) are heterozygous, both of their beta genes are abnormal.Different from HbS & HbC diseases, the patients wit
28、h HbSC generally have painful crises, and this crises often follows childbirth or surgery & may be fatal.IV-D. Methemoglobinemias Oxidation of heme( Fe2+ Fe3+ ) in Hb molecule.Causation: certain drugs(such as nitrates) ;endogenous products(such as reactive O2);some golobin mutations; the lack of NAD
29、H-Cytb5 reductase(NADH-HbM reductase) Met-Hb(HbM) is not bind O2, & results in hypoxia. IV-E. Thalassemmias A groups of hereditary hemolytic diseases in which an inbalance of globin-chain synthesis.This inbalance may occur in the DNA level or the protein level. There are abnormalities of either - or
30、-chains. IV-E-1. -Thalassemmias -globin decreased or absent. -globin synthesis is normal , & -globin cannot form stable tetramers. RBCs die young. Accumulation of HbF(22) & Hb Barts(4 )IV-E-2. - Thalassemmias -globin decreased or absent . -globin synthesis is normal , & -globin cannot form stable te
31、tramers.If 3 of 4 - genes defective , RBCs will die young. Accumulation of Hb H(4) & Hb Barts(4 )V. CHAPTER SUMMARY HbA(composing, T & R form)The O2 dissociation curve(characters, cooperative interaction among the subunits, influence of pO2, pCO2, & 2,3-BPG. )Hemoglobinopathies(HbS,HbC, thallasemia)Thanks!
