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18,8 Quaternary Structure of Proteins

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18,8 Quaternary Structure of Proteins 570 CHAPTERt8 Amino Acids,Peptides, and Proteins 18,8Quaternary structure of proteins AIMS: Todefine the termssubunit dnd quaternarystructure. Io describethe quoternorystructure of hemoglobin.To distinguishomong oxyhemoglobin,deoxyhemoglobin, ond methemoglobin. Someproteins consist of more than one pollpeptide chain. Theseindiuid- ual chains are calledsubunits of the protein. Proteins composedof subunits In some proteins, polypeptide are said to haue quaternary structure. Many proteins have structures that chains aggregateto form contain subunits. Proteins consistingof dimers (two subunits), tetramers quaternary structures. (four subunits), and hexamers (six subunits) are fairly common. The pro- teins that comprise the individual subunits may be identical, or they may be different. Like the secondary and tertiary structures, the quaternary structure of a protein is determined by its primary structure. The pollpep- tide chains of subunits are held in place by the same forces that determine tertiary structure-hydrogen bonds, salt bridges, and sometimes disulfide bridges-except the forces are betweenthe polypeptide chains of the sub- units instead of within them. Hydrophobic aliphatic and aromatic side chains of subunits can aggregateto exclude water. Hemoglobin-the globular oxygen-transport protein of blood-is an example of a protein that has a quaternary structure. Max Perutz, also of the Medical ResearchCouncil laboratories,determined the structure of horse blood hemoglobin in 1959.Hemoglobin is a larger molecule than myoglo- bin. The hemoglobin molecule has a molar mass of 64,500.It contains about 5000 individual atoms, excluding hydrogens, in 574 amino acid residues. The quaternary structure of hemoglobin consistsof four peptide sub- units. TWo of the subunits are identical and are called the alpha subunits. The remaining two subunits, called the beta subunits, are identical to each other but different from the alpha subunits. Figure 18.13shows the four subunits of hemoglobin interlocked in a compact globular structure held togetherby ionic and hydrogenbonds betweenthe amino acid side chains of the polypeptide subunits. A chromoprotein has a color be- A heme prosthetic group containing an iron(Il) ion capableof carrying causeof a colored prosthetic one oxygenmolecule is associatedwith eachof the four protein subunits of group. Hemoglobin is a chromo- hemoglobin. Globin is the protein from which the heme groups haue been protein that getsits red color from remoued. The hemoglobin complex with oxygen is oxyhemoglobin; that the heme group. Plants have their without oxygenzs deoxyhemoglobin. Oxyhemoglobin, the major hemoglo- green color becauseofchloro- bin of arterial blood, is bright red; deoxyhemoglobinof venous blood is phyll, a porphyrin ring with a purplish. Long exposureto oxygenwill conuert the heme iron of hemoglobin structure similar to the heme group. to the iron(III), or ferric state @d+), rc giue methemoglobin. The bro',,rm color of dried blood resultsfrom the conversionof hemoglobin to methe- moglobin. Oxygen does not form complexes with methemoglobin. In methemoglobinemia, a hereditary blood disease,the heme iron of hemoglo- bin, in either the alpha or the beta chains, existsin the iron(III) state.People with iron(III) ions in both the alpha- and the beta-chain heme groups would be unable to live becausetheir blood could not transport oxygen. The folding of the polypeptide chains of the alpha and beta chains of the hemoglobin subunits is very similar. Moreover, the folding of I8.9 HemoglobinFunction 571 Br Figure18.15 Thequaternary structure of hemo- globin,showing the arrangement of alphaand beta chains. myoglobin is very similar to the folding of the subunits of hemoglobin. This surprised biochemists, since the primary structures of sperm whaie myo- globin and horse hemoglobin are very dissimilar. It is now clear that the ter- tiary structures devised by nature for myoglobin and the hemoglobin sub- units are crucial to oxygen transport by these proteins. 18.9Hemoglobin function AIM: To describethe mechonismof oxygentransport by hemoglobin. Nature has gone to considerabletrouble to construct complicatedproteins such as myoglobin and hemoglobin for the apparently simple task of bind- The environment of the heme ing oxygen. In oxygen transport, however, as in many biological processes, groups gives myoglobin and a delicatebalance must be maintained. The binding between iron(Il) ions hemoglobin the ability to carry and oxygen must be strong enough that the oxygen can be stored or trans- oxygen. ported yet weak enough to provide a means for releasingthe oxygen where it is needed. Free iron(Il) ions in water form very unstable complexes with molecu- lar oxygen. Complexes of oxygen with iron(Il) ions contained in heme groups are only slightly more stable. In myoglobin and hemoglobin, how- ever,the heme groups are tucked into hydrophobic folds in the globin pep- tide chains. This hydrophobic environment fosters formation of relatively stable complexes between molecular oxygen and heme iron. It is the hydrophobic environment of the heme group that makes myoglobin and 572 CHAPTERl8 Amino Acids,Peptides, and proteins More than 100 different types of hemoglobin useful oxygen carriers.Some molecules and ions form more protein moleculesare found in the stable complexes with the heme iron of hemoglobin than oxygen. carbon blood. The normal concentration monoxide (co) reactswith the heme iron of hemoglobin to a complex of protein in blood plasma ranges form called carboryhemoglobin. This complex is about 200 times more stable from 7.0to 8.0g/ml. Bloodpro- than oxyhemoglobin. Breathing even low levels of carbon monoxide teins help maintain osmotic pres- raises the amount of carboxyhemoglobin sure,play a part in blood and decreasesthe amount of oxyhemo- globin in the coagulation,transport lipids, and blood. The result is carbon monoxide poisoning (discussedin help fight disease. Sec.11.11). Cyanide ions (CN ) bind more tightly than either oxygenor car- bon monoxide to the heme iron of hemoglobin to form ryanohemogrobin. Compoundsthat produce cyanideions are deadlypoisons. The fact that the hemoglobin molecule consists of four subunits increases the efficiency of oxygen transport. once the flrst oxygen of the hemoglobin molecule is complexed, the second, third, and fourth follow with increasing ease.This property guaranteesthat hemoglobin that is oxy- genated will always carry a full load of oxygen. Myoglobin, which is involved only in oxygen storage and not transport, performs its function satisfactorilywith only a single peptide chain. 18.10Sickle cell onemia AIM: To explain the moleculor bosisfor sickle cell onemia. sickle cell anemia you Focus affects about 3 in every 1000African-Americans. may recall from the introduction to chapter 1 that Linus pauling discoveredthat Sickle cell anemia sterns from a the hemoglobin of people who suffer from sickle cell anemia (Hbs) is dif- single substitution in the ferent from the hemoglobin of normal adults (HbA).Nowthatwe arefamil- primarystructure of iar with protein structure, we can understand the molecular difference hemoglobin. between Hbs and HbA, and the reasonthis difference causesthe slrnptorns of sicklecell anemia.HbA and HbS differ by only one amino acid residue.A negativelycharged glutamic acid residue ut th" sixth position of the beta chain of normal HbA is replaced by an unchargedvaine residue in Hbs. The difference in electric charge causesHbs, especially in its deoxygenated form, to be much less soluble than HbA. precipitation of HbS in the red blood cell causesthese cells to sickleand sometimesburst. (Fora compari- son of normal and sickled red blood cells, see the figure on page 2.) The sickled cells as well as clumped HbS and cellular debris from burst cells block the flow of blood to capillaries,triggering the painful clinical episodes of sicklecell anemia. The sickle cell trait is exhibited by about I in every 10 African-Ameri- cans. In people who carry the sickle cell trait, half the hemoglobin is HbA and half is Hbs. Thesepeople often lead normal lives.Exposure to low oxy- gen levels, however, as in the mountains or on an airplane flight, can be dangerous. There is no cure for sickle cell anemia; treatment of its symp- loms may involve hyperbaric oxygenation-administration of oxygen at higher than atmospheric pressure. The idea behind hyperbaric oxygena- tion is to keep the number of deoxygenatedred blood cells as low as possible to prevent sickling and subsequent clumping. Hydroxyurea (H2NCONHOH),formerly used as an anticancer drug, has been found to reduce the painful episodes associatedwith sickle cell anemia by 50 percent. I8.10 SickleCell Anemia 57t -: ...-: V\4ryis sicklecell anemia found almost exclusivelyin people of African descent?Although patients with sickle cell anemia often die very young, personswith sickle cell trait have a high resistanceto malaria, a disease prevalent in certain parts of Africa. Apparently, ancestralAfricans with . |'\- sickle cell trait had a distinct survival advantageover those who lacked it. Sicklecell trait and sicklecell anemia are passedfrom generationto gener- ation, accountingfor the high incidence of the two conditions today. The electrophoresisof hemoglobin obtained from red blood cells is a simple and effectivetechnique for screeningindMduals for sicklecell trait or sicklecell anemia.Electrophoresis ls a methodfor the separationof ions accordingto their charge,as describedin A CloserLook Electrophoresis.At pH 8,4,HbA
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