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Nadja Hellmann

Publications and source records attributed to Nadja Hellmann.

7 recordsLinked to original sources
PubMed2006

Evidence that clustered phosphocholine head groups serve as sites for binding and assembly of an oligomeric protein pore.

High susceptibility of rabbit erythrocytes toward the pore-forming action of staphylococcal alpha-toxin correlates with the presence of saturable, high affinity binding sites. All efforts to identify a protein or glycolipid receptor have failed, and the fact that liposomes composed solely of phosphatidylcholine are efficiently permeabilized adds to the enigma. A novel concept is advanced here to explain the puzzle. We propose that low affinity binding moieties can assume the role of high affinity binding sites due to their spatial arrangement in the membrane. Evidence is presented that phosphocholine head groups of sphingomyelin, clustered in sphingomyelin-cholesterol microdomains, serve this function for alpha-toxin. Clustering is required so that oligomerization, which is prerequisite for stable attachment of the toxin to the membrane, can efficiently occur. Outside these clusters, binding to phosphocholine is too transient for toxin monomers to find each other. The principle of membrane targeting in the absence of any genuine, high affinity receptor may also underlie the assembly of other lipid-inserted oligomers including cytotoxic peptides, protein toxins, and immune effector molecules.

Animals↗
PubMed2005

Allosteric models for multimeric proteins: oxygen-linked effector binding in hemocyanin.

In many crustaceans, changing concentrations of several low molecular weight compounds modulates hemocyanin oxygen binding, resulting in lower or higher oxygen affinities of the pigment. The nonphysiological effector caffeine and the physiological modulator urate, the latter accumulating in the hemolymph of the lobster Homarus vulgaris during hypoxia, increase hemocyanin oxygen affinity and decrease cooperativity of oxygen binding. To derive a model that describes the mechanism of allosteric interaction between hemocyanin and oxygen in the presence of urate or caffeine, studies of oxygen, urate, and caffeine binding to hemocyanin were performed. Exposure of lobster hemocyanin to various pH values between 7.25 and 8.15 resulted in a decrease of p50. In this pH interval, p50 decreases from 95 to 11 Torr without effectors and from 49 to 6 Torr and from 34 to 5 Torr in the presence of 1 mM urate or caffeine, respectively. Thus, the allosteric effects induced by protons and urate or caffeine are coupled. In contrast, isothermal titration calorimetry did not reveal any differences in binding enthalpy (DeltaH degrees ) for urate or caffeine under either normoxic or hypoxic conditions at different pH values. Despite these apparently conflicting results, they can be explained by the nested MWC model if two different types of modulator binding sites are assumed, an allosteric and a nonallosteric type of site. Simulations of in vivo conditions with this model indicate that the naturally occurring modulator urate is physiologically relevant in H. vulgaris only during hypoxic conditions, i.e., either during environmental oxygen limitation or extensive exercise.

Allosteric Regulation↗
PubMed2005

Quaternary structure and functional properties of Penaeus monodon hemocyanin.

The hemocyanin of the tiger shrimp, Penaeus monodon, was investigated with respect to stability and oxygen binding. While hexamers occur as a major component, dodecamers and traces of higher aggregates are also found. Both the hexamers and dodecamers were found to be extremely stable against dissociation at high pH, independently of the presence of calcium ions, in contrast to the known crustacean hemocyanins. This could be caused by only a few additional noncovalent interactions between amino acids located at the subunit-subunit interfaces. Based on X-ray structures and sequence alignments of related hemocyanins, the particular amino acids are identified. At all pH values, the p50 and Bohr coefficients of the hexamers are twice as high as those of dodecamers. While the oxygen binding of hexamers from crustaceans can normally be described by a simple two-state model, an additional conformational state is needed to describe the oxygen-binding behaviour of Penaeus monodon hemocyanin within the pH range of 7.0 to 8.5. The dodecamers bind oxygen according to the nested Monod-Whyman-Changeaux (MWC) model, as observed for the same aggregation states of other hemocyanins. The oxygen-binding properties of both the hexameric and dodecameric hemocyanins guarantee an efficient supply of the animal with oxygen, with respect to the ratio between their concentrations. It seems that under normoxic conditions, hexamers play the major role. Under hypoxic conditions, the hexamers are expected not to be completely loaded with oxygen. Here, the dodecamers are supposed to be responsible for the oxygen supply.

Allosteric Regulation↗
PubMed2004

Bohr-effect and buffering capacity of hemocyanin from the tarantula E. californicum.

A previous report showed that binding of oxygen to the 24-meric hemocyanin from E. californicum does not correlate linearly with the release of protons as known from hemoglobin. However, this unusual complex phenomenological observation could not be explained at that time. Here, I present a full analysis of the thermodynamic coupling between protons and oxygen for the 24-meric tarantula hemocyanin in Ringer-solution based on the Nested-MWC-model. A strategy is presented which allows to reduce the number of free parameters when fitting the model to the data by including explicitly the equilibrium constants for binding of protons to the different conformations. The results show that the Nested-MWC-model presents a good description of the cooperative and allosteric properties of spider hemocyanin also under physiological conditions and additional constraints imposed by proton-binding data. The analysis allowed to estimate the average number of allosteric proton-binding sites per subunit and the corresponding pK for each conformation. Furthermore, an estimate of the number and affinity of proton buffering binding sites could be given. Approximately 80% of all proton-binding sites are non-allosteric buffering binding sites. The results obtained in this study allow to predict the relative contribution of the four different conformations under conditions found in vivo. The conformational distribution indicates that the conformation with the highest proton affinity (tR) might be important for the proton transport in the hemolymph.

Animals↗
PubMed2003

Nested allosteric interactions in extracellular hemoglobin of the leech Macrobdella decora.

Hemoglobin from the leech Macrobdella decora belongs to the class of giant extracellular hexagonal bilayer globin structures found in annelid and vestimentiferan worms. These complexes consist of 144 heme-bearing subunits, exhibit a characteristic quaternary structure (2 x (6 x (3 x 4))), and contain tetramers as basic substructures that express cooperative oxygen binding and thus provide a structural basis for a hierarchy in allosteric interactions. A thorough analysis of the isolated tetramer indicates that it functions as a trimer of cooperatively interacting subunits and a non-cooperative monomer rather than as four interacting subunits. A thermodynamic analysis of the whole molecule favors the application of a nested Monod-Wyman-Changeux model with six cooperatively interacting 12-mer allosteric units. In contrast to the isolated tetramers, all subunits of the tetramers seem to be coupled cooperatively within the oligomerized 144-mer. Thus, besides hemocyanins and GroEL, the hexagonal bilayer hemoglobins represent another class of proteins in which the hierarchical quaternary structure provides the basis for nested interaction in their functional properties.

Animals↗
PubMed2002

Nested MWC model describes hydrolysis of GroEL without assuming negative cooperativity in binding.

Folding assistance and ATPase activity of GroEL are based on the existence of different conformations. In order to characterise these conformations, published data on steady state ATPase activity in the absence of GroES were reanalysed simultaneously in terms of the Nested MWC model. This model is a hierarchical extension of the symmetry-model of Monod et al. [J. Mol. Biol. 12 (1965) 88]. An unique set of GroEL specific parameters was obtained. This set was supported by comparison of predictions arising from this set of values with experimental data for hydrolysis of ATP in the presence of ADP and ATPgammaS, binding of ATPgammaS and ADP to GroEL in the absence of ATP, and binding of ATP as monitored by fluorescence labelling. Thus, for the first time, multiple data sets for the interaction of nucleotides with GroEL are described quantitatively by an allosteric model. A noteworthy feature of our model is that no negative cooperativity in ATP binding occurs in accordance to experimental observations. Furthermore, the model also includes the existence of a conformation with very high ATPase activity. Such a conformation might be of importance at a certain stage in the folding cycle.

Adenosine Diphosphate↗