In the yeast Saccharomyces cerevisiae, the first committed step of the lysine biosynthetic pathway is catalysed by two homocitrate synthases encoded by LYS20 and LYS21. We undertook a study of the duplicate homocitrate synthases to analyse whether their retention and presumable specialization have affected the efficiency of lysine biosynthesis in yeast. Our results show that during growth on ethanol, homocitrate is mainly synthesized through Lys21p, while under fermentative metabolism, Lys20p and Lys21p play redundant roles. Furthermore, results presented in this paper indicate that, in contrast to that which had been found for Lys20p, lysine is a strong allosteric inhibitor of Lys21p (K i 0.053 mM), which, in addition, induces positive cooperativity for a-ketoglutarate (a-KG) binding. Differential lysine inhibition and modulation by a-KG of the two isozymes, and the regulation of the intracellular amount of the two isoforms, give rise to an exquisite regulatory system, which balances the rate at which a-KG is diverted to lysine biosynthesis or to other metabolic pathways. It can thus be concluded that retention and further biochemical specialization of the LYS20-and LYS21-encoded enzymes with partially overlapping roles contributed to the acquisition of facultative metabolism.
In the yeast Saccharomyces cerevisiae, the paralogous genes ALT1 and ALT2 have been proposed to encode alanine aminotransferase isozymes. Although in other microorganisms this enzyme constitutes the main pathway for alanine biosynthesis, its role in S. cerevisiae had remained unclear. Results presented in this paper show that under respiratory conditions, Alt1p constitutes the sole pathway for alanine biosynthesis and catabolism, constituting the first example of an alanine aminotransferase that simultaneously carries out both functions. Conversely, under fermentative conditions, it plays a catabolic role and alanine is mainly synthesized through an alternative pathway. It can thus be concluded that ALT1 has functions in alanine biosynthesis and utilization or only alanine utilization under respiratory and fermentative conditions, respectively. ALT2 expression was repressed under all tested conditions, suggesting that Alt2p biosynthesis is strictly controlled and only allowed to express under peculiar physiological conditions.
An equivalent discrete model is developed for time domain dynamic analysis of uniform high-rise shear wall-frame buildings with fixed base and carrying any number of tuned mass dampers (TMDs). The equivalent model consists of a flexural cantilever beam and a shear cantilever beam connected in parallel by a finite number of axially rigid members that allow the consideration of intermediate modes of lateral deformation. The proposed model was validated by a building whose lateral resisting system consists of a combination of shear walls and braced frames. The results showed the effectiveness of TMDs to reduce the peak displacements, interstory drift ratio, and accelerations when the building is subjected to a seismic load. The root mean square accelerations due to along-wind loads also decrease if TMDs are attached to the building. KEYWORDScoupled two-beam model, earthquake engineering, passive control devices, tall buildings, tuned mass dampers, wind engineering | INTRODUCTIONThe lateral deformation of certain types of buildings can be modeled by shear beams and flexural beams. However, there are many buildings for which these two extreme modes of lateral deformation do not adequately represent their dynamic behavior. [1][2][3][4][5][6][7] Miranda, [8] Miranda and Reyes, [9] Miranda and Taghavi, [10] Reinoso and Miranda, [11] Miranda and Akkar, [12] and Cruz et al. [13] considered intermediate modes of lateral deformation in seismic response of buildings through a two-beam model that couples the bending and shear stiffnesses in parallel. Van Oosterhout [14] used the same coupled two-beam model to evaluate the wind-induced acceleration in tall buildings through an analysis in the frequency domain.Dym et al. [15] and Rahgozar et al. [16] considered intermediate modes of lateral deformation in buildings through a Timoshenko beam model that accounts for shear deformation and rotatory inertia by adding them to an Euler-Bernoulli beam. This model reflects a series coupling of the beam's bending and shear stiffnesses, although the effect of rotational inertia is not a significant factor in tall buildings. [15] The literature features several empirical formulas that allow the estimation of the lowest natural frequency of a tall building as a function of its height. [17][18][19][20][21] Dym and Williams [22] concluded that a coupled shear-flexural model in parallel seems the better model for estimating the frequencies of shear wall-frame buildings because it provides predictions that are consistent with the observed data. On the other hand, a Timoshenko beam model cannot exhibit the correct dependence between the frequencies and the height of the building because it reflects a series coupling of the beam's bending and shear stiffnesses. [22] This shows that a coupled shear-flexural model estimates better the frequencies of tall buildings than a Timoshenko beam model, particularly in shear wall-frame buildings and tube-and-core constructions with the parallel nature of the two-beam model in which transverse displacements du...
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