Hemocyanins Relationships in Their Structure, Function and Assembly

Hemocyanins are high molecular weight oxygen-carrying proteinsthat occur in the molluscs and arthropods. The oxygen-bindingsite in these proteins is a pair of copper atoms bound directlyto ammo acid side chains. The biscopper sites of these proteinsbind single molecules of dioxygen or carbon monoxide. In arthropodsthere are two copper atoms per approximately 70 000 daltonsof protein. This corresponds to the molecular weight of theminimum polypeptide chain. In molluscs however there are twocopper atoms per 50,000 daltons of protein. This does not correspondto the minimum polypeptide chain although it does correspondto the minimal functional unit. The minimal polypeptide chainin molluscan hemocyanins is approximately 400,000 daltons andappears to be composed of eight or more 50 000 dalton unitslinked together like a string of pearls. In the molluscs, thenative hemocyanins found in the hemolymph generally occur asgiant cylindrical molecules 350 Å in diameter, 380 Ålong, with molecular weights of 9 000,000. These large moleculesare composed of approximately 20 polypeptide chains. In thehemolymph of the arthropods depending upon the species, hemocyaminsoccur as hexamers dodecamers 24-mers and 48-mers. The molecularweight of the 48-mers is about 3 600,000. The respiratory functionsof hemocyamns show a wide range of allosteric properties. Thelarge molecules commonly show cooperativity in oxygen binding.The number of interacting subunits is in some cases dependentupon external conditions of pH and ionic composition. The oxygenaffinity is usually sensitive to pH and to low molecular weightcofactors like chloride, calcium and magnesium ions. The intrinsicoxygen binding properties of an organism s hemocyanin and itsallosteric control by modulators allow organisms possessingthese giant molecules to adapt to their environmental conditions.     

阿根廷淡水大米草沼泽中野生豚鼠的种群统计和微生境的利用

本文研究了秋冬季节淡水大米草(Spartina densiflora)沼泽中野生豚鼠(Cavia a perea)的丰度、繁殖和微生境利用,以及其对当地植被和棉鼠类(Sigmodontine)啮齿动物的影响。野生豚鼠喜好S.densiflora覆盖度高的生境。繁殖个体(成体)主要利用矮草为主的斑块,幼体则主要利用禾本科植物为主的斑块。结果说明,野生豚鼠微生境的利用受捕食风险和食物种类的影响。在淡水沼泽中,野生豚鼠的丰度、繁殖、体重和微生境利用没有季节性变化,它们对植物的取食和活动跑道的建造对植被结构和同域的啮齿类动物没有负面影响。通过对具有中度季节性变化的淡水生境中的豚鼠种群和具有高度季节变化的草地和路边中的豚鼠种群进行比较,表明野生豚鼠的种群动态以及豚鼠种群对植被和与其共生的啮齿动物群落的影响都受到冬季植被盖度的限制。     

Anionic Control of Function in Vertebrate Hemoglobins

Point mutations in the amino acid sequence of normal human hemoglobinhave provided a powerful means of probing structure-functionrelationships in this respiratory protein. Through studies ofspecific hemoglobin variants it has been possible to gain abetter understanding of how electrostatic interactions exercisecontrol over the functional properties of hemoglobin. Humanhemoglobin variants of particular interest in this respect arethose with alterations of amino or carboxyl terminal residuesand alterations at or near the binding site for the physiologicallyimportant cofactor 2,3-diphosphoglycerate. In deoxy hemoglobinit has been established that salt bridges formed by the terminalresidues constrain the tetramer in a low affinity conformation.From the information presently available, it appears that thecharge cluster of the 2,3 diphosphoglycerate binding site isan important part of the innerspring mechanism that tends todestabilize the deoxy conformation. When anions bind to theseresidues the repulsive interactions between the positively chargedresidues of this region are decreased. This provides a directmeans by which anionic interactions with hemoglobin can shiftthe conformational equilibrium toward the low affinity state.Accordingly anion and pH effects are decreased in a number ofhemoglobin variants whose substitutions reduce the positivecharge density in the region of the binding site for polyphosphates.The presence of the charge cluster provides for a fine tuningof hemoglobin s functional properties that is responsive tothe concentration of metabolic effectors in vivo. The degreeto which this is possible varies in the vertebrate hemoglobinswhich have been examined. In human hemoglobin eight positivelycharged residues contribute to the charge cluster and anionicmodulation of oxygen affinity is effective. Susceptibility toanionic modulation is decreased in hemoglobins where the chargecluster is less developed and is completely absent in some vertebratehemoglobins. Anionic modulation, which occurs via an effecton the equilibrium between conformational states of high andlow oxygen affinity is possible even in systems which do notshow cooperative interactions in oxygen binding. This is wellestablished by studies on isolated chains of human hemoglobinand by studies on enzymatically modified tetramers of Amphiumahemoglobin.