Quaternary structure changes in aspartate transcarbamylase studied by X-ray solution scattering. Signal transmission following effector binding.

The sites of energy transduction within the human hemoglobin molecule for the regulation of oxygen affinity have been determined by an extensive study of the molecule's energetic response to structural alteration at individual amino acid residues. For 22 mutant and chemically modified hemoglobins we have determined the total free energy used by the tetrameric molecule for alteration of oxygen affinity at the four binding steps. The results imply that the regulation of oxygen binding affinity is due to energy changes which are mostly localized at the alpha 1 beta 2 interface. They also indicate a high degree of "internal cooperativity" within this contact region--i.e., the structural perturbations at individual residue sites are energetically coupled. Cooperativity in ligand binding is thus a reflection of cooperativity at a deeper level--that of the protein-protein interactions within the alpha 1 beta 2 interfacial domain.

Probing the energetics of proteins through structural perturbation: sites of regulatory energy in human hemoglobin.

Homotropic effects in aspartate transcarbamoylase. What happens when the enzyme binds a single molecule of the bisubstrate analog N-phosphonacetyl-L-aspartate?

Allosteric Activation of L-Lactate Dehydrogenase Analyzed by Hybrid Enzymes with Effector-sensitive and -insensitive Subunits

The thermal denaturation of Escherichia coli aspartate transcarbamoylase (c6r6) in the absence and presence of various ligands has been studied by means of high-sensitivity differential scanning calorimetry (DSC). As previously reported [Vickers, K.P., Donovan, J.W., & Schachman, H.K. (1978) J. Biol. Chem. 253, 8493-8498], the denaturational endotherm consists of two peaks, the lower of which is due to denaturation of the three regulatory, r2, subunits while the upper involves the two catalytic, c3, subunits. The temperature of maximal excess apparent specific heat, tm, of the lower peak is raised from the value of 51.4 degrees C for the isolated subunit to 66.8 degrees C as a result of subunit interactions, whereas tm for the c3 peak is essentially the same in the isolated subunit and in the holoenzyme, indicating that the denatured r2 subunits do not interact with the c3 subunits. The total specific denaturational enthalpy for c6r6, 4.83 +/- 0.16 cal g-1, is significantly larger than the weighted mean, 4.08 cal g-1, of the enthalpies for c3 and r2. The fact that no endotherm is observed when previously scanned protein is rescanned indicates that the denaturation is irreversible, as is also the case with the r2 and c3 subunits. Empirical justification for analyzing the data in terms of equilibrium thermodynamics is cited. The observed DSC curves can be expressed within experimental uncertainty as the sum of five sequential two-state steps. The value of t 1/2, the temperature of half-completion, for each step increases with increasing protein concentration, indicating that some dissociation of the protein takes place during denaturation.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential scanning calorimetric study of the thermal denaturation of aspartate transcarbamoylase of Escherichia coli.

Communication between subunits in aspartate transcarbamoylase. Effect of active site ligands on the tertiary structure of regulatory chains.

Seasonal hydrological change shaping the relationship between dissolved organic matter and land use in the middle reaches of the Yangtze river

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Communication between subunits in aspartate transcarbamoylase. Effect of active site ligands on the tertiary structure of regulatory chains.

Seasonal hydrological change shaping the relationship between dissolved organic matter and land use in the middle reaches of the Yangtze river