Principles of symmetrical organization for the pyruvate dehydrogenase complex.

The experimentally observed phenomenon of non-equimolarity for enzyme components, assembled into multienzyme complexes of the 2-oxo acid dehydrogenases family, is structurally interpreted to predict the only possible stable symmetrical distribution of peripheral components on the complex core. To obey the equivalent neighboring, that is necessary for unique self-assembled structures, we should deduce discrete conformational states for core subunits, those with different affinity for peripheral components. Two kinetically different types of substrate-intermediate pathways through the lipoyl network of the mammalian pyruvate dehydrogenase complex follow from this structural theory. The theory predicts unusual kinetic behavior for the multienzyme complex.

[Structure-activity relationships in multienzyme complexes]. / Strukturno-funktsional'nye otnosheniia v mul'tifermentnykh kompleksakh.

Structures and molecular kinetic models of function of 2-oxo acid dehydrogenase complexes were analyzed. It was suggested that identical protein subunits in the multienzyme complexes as in the structure capable of self-assembly have identical contacts with the neighbors and identical environment. By sharing the enzyme aggregate subunits into distinct conformational classes the peripheral components were demonstrated to be arranged on the core so that the entire complex would have a definite symmetry. The number of the conformational classes is specified by the architecture of the core and considerations of symmetry. The results have allowed us to consider the mechanisms of functioning of the complexes. Specific examples are discussed.

[Analysis of stationary kinetics of translation elongation within the framework stereospecific stabilization hypothesis of codon- anticodon complexes in a ribosome. I. Kinetic schemes of factorless elongation]. / Analiz statsionarnoi kinetiki élongatsii transliatsii v ramkakh gipotezy o stereospetsificheskoi stabilizatsii kodon-antikodonovykh kompleksov na ribosomes. I. Kineticheskie skhemy besfaktornoi élongatsii.

Dependences of steady-state rates of polypeptide elongation on concentrations of substrate (aminoacyl-tRNA) and product (deacylated tRNA) in the absence of elongation factors and GTP are theoretically analyzed in context of stereospecific stabilization of the codon-anticodon complexes at a ribosome. General kinetic scheme and different ribosome isomerization stages are examined. The effect of isomerization stage allows to identify reaction stage experimentally. Regulation of the direct reaction by product and regulation of the reverse reaction by substrate are possible. Under certain conditions elongation system may show kinetic cooperativity.

[Analysis of stationary kinetics of translation elongation within the framework of the stereospecific stabilization hypothesis of codon- anticodon complexes in a ribosome. II. Kinetic schemes in the presence of protein elongation factors and GTP]. / Analiz statsionarnoi kinetiki élongatsii transliatsii v ramkakh gipotezy o stereospetsificheskoi stabilizatsii kodon-antikodonovykh kompleksov na ribosome. II. Kineticheskie skhemy v prisutstvii belkovykh faktorov élongatsii i GTP.

Kinetics of the factor-dependent polypeptide elongation is theoretically studied in context of stereospecific stabilization of the codon-anticodon complexes at a ribosome. Kinetic schemes for the different ribosome isomerization stages are examined. The dependence of steady-state elongation rate on elongation factor concentration for each of the schemes is unique, allowing to identify isomerization stages experimentally.

[A model of an enzyme with mixed cooperation: 3 states of aspartate transcarbamylase]. / Model' fermenta so smeshannoi kooperativnost'iu: tri sostoianiia aspartat-transkarbamilazy.

Allosteric enzyme models on the basis of the known properties of aspartate transcarbamylase (ATCase) from Escherichia coli are suggested. In the first model molecules are supposed to equilibrate between two states. In contrast to the classical Monod-Wyman-Changeux model the symmetry of enzyme molecules changes during the conformational transition. It is shown that the number of binding sites of the enzyme defined from the Scatchard plots is sufficiently dependent on values of parameters of enzyme reaction. This fact results from the mixed (both positive and negative) cooperative effects. However the complex kinetic of ATCase is not completely simulated by this model. Therefore the model is complicated by taking into account the inactive third state of the enzyme. Thus the complex kinetic behaviour of ATCase is explained. The models may be also used for other enzymes.

[Observation of non-equilibrium phase transitions in a closed enzyme system. Intermittent regulation of the activity of lactate dehydrogenase and similar enzymes]. / O nabliudenii neravnovesnykh fazovykh perekhodov v zakrytoi fermentnoi sisteme. Skachkoobraznaia reguliatsiia aktivnosti laktatdegidrogenazy i skhodnykh fermentov.

Kinetic models of closed enzymic systems which allow the existence of multi-steady-states were considered. In the basis of these models lies the assumption about the coexistence in solution of different substrate forms and their non-enzymic transitions one to another. If the forms of substrate have different affinity to the enzyme and, moreover, one of the forms inhibits the enzyme, then the enzymic activity in certain conditions may change discretely. The transition of the system incidentally from one steady state to another is similar to the first order phase transition. It is shown that for the enzymic reaction catalyzed by lactate dehydrogenase, the conditions may exist for the non-equilibrium phase transition. It is shown that the existence of the metastability region in the system causes the unusual shifts of the discrete transition from one experiment to another. A model of an allosteric enzyme that describes the discrete behaviour similar to the phase transition is also considered.