Using Steady-State Kinetics to Quantitate Substrate Selectivity and Specificity: A Case Study with Two Human Transaminases

We examined the ability of two human cytosolic transaminases, aspartate aminotransferase (GOT1) and alanine aminotransferase (GPT), to transform their preferred substrates whilst discriminating against similar metabolites. This offers an opportunity to survey our current understanding of enzyme selectivity and specificity in a biological context. Substrate selectivity can be quantitated based on the ratio of the k_cat/K_M values for two alternative substrates (the ‘discrimination index’). After assessing the advantages, implications and limits of this index, we analyzed the reactions of GOT1 and GPT with alternative substrates that are metabolically available and show limited structural differences with respect to the preferred substrates. The transaminases’ observed selectivities were remarkably high. In particular, GOT1 reacted ~10⁶-fold less efficiently when the side-chain carboxylate of the ’physiological’ substrates (aspartate and glutamate) was replaced by an amido group (asparagine and glutamine). This represents a current empirical limit of discrimination associated with this chemical difference. The structural basis of GOT1 selectivity was addressed through substrate docking simulations, which highlighted the importance of electrostatic interactions and proper substrate positioning in the active site. We briefly discuss the biological implications of these results and the possibility of using k_cat/K_M values to derive a global measure of enzyme specificity.     

New Histamine-Related Five-Membered N-Heterocycle Derivatives as Carbonic Anhydrase I Activators

A series of histamine (HST)-related compounds were synthesized and tested for their activating properties on five physiologically relevant human Carbonic Anhydrase (hCA) isoforms (I, II, Va, VII and XIII). The imidazole ring of HST was replaced with different 5-membered heterocycles and the length of the aliphatic chain was varied. For the most interesting compounds some modifications on the terminal amino group were also performed. The most sensitive isoform to activation was hCA I (K_A values in the low micromolar range), but surprisingly none of the new compounds displayed activity on hCA II. Some derivatives (1, 3a and 22) displayed an interesting selectivity for activating hCA I over hCA II, Va, VII and XIII.     

Cationic oligomerization of isoprene and structure of its oligomers

The results obtained in a study of isoprene cationic oligomerization in the presence of some oxyacids (H₂SO₄, CH₃SO₃H, Nafion^® H) are reported. Oligomers up to pentamers are obtained which were examined through gas chromatography/mass spectrometry analysis either of the reaction products or of the hydrogenated products. The structure of the dimer fraction was investigated and revealed the prevalent presence of α-pinene and p-cymene together with different cyclic unsaturated hydrocarbons. Only small quantities of linear dimers were found. Possible reaction mechanisms are presented.     

Stationary layered solutions in {\mathbb{R}}^2 for a class of non autonomous Allen-Cahn equations

We consider a class of non autonomous Allen-Cahn equations where is a multiple-well potential and is a periodic, positive, non-constant function. We look for solutions to (0.1) having uniform limits as corresponding to minima of W. We show, via variational methods, that under a nondegeneracy condition on the set of heteroclinic solutions of the associated ordinary differential equation the equation (0.1) has solutions which depends on both the variables x andy. In contrast, when a is constant such nondegeneracy condition is not satisfied and all such solutions are known to depend only on x. Received April 16, 1999 / Accepted October 1, 1999 / Published online June 28, 2000     

Second-order moving average and scaling of stochastic time series

Long-range correlation properties of stochastic time series y(i) have been investigated by introducing the function σ² _MA = [y(i) - (i)]², where (i) is the moving average of y(i), defined as 1/n y(i - k), n the moving average window and N_max is the dimension of the stochastic series. It is shown that, using an appropriate computational procedure, the function σ _MA varies as n^H where H is the Hurst exponent of the series. A comparison of the power-law exponents obtained using respectively the function σ _MA and the Detrended Fluctuation Analysis has been also carried out. Interesting features denoting the existence of a relationship between the scaling properties of the noisy process and the moving average filtering technique have been evidenced. Received 31 December 2001     

Some trends in the infrared spectra of matrix isolated tetrahedral ternary oxides

The splitting of the T₂ stretching vibration into B₂ and E modes on passing from T_d to D_2d symmetry is discussed for M₂XO₄ molecules (M = Li, Na, K, Cs, In and T1. X = Cr, Mo, W and S). Correlations between XO stretching force constants, OXO bond angles and cation polarizabilities are made.     

Picosecond and femtosecond pulsed laser ablation and deposition of quasicrystals

A Nd:glass laser with pulse duration of 250 fs and 1.3 ps has been used to evaporate a Al₆₅Cu₂₃Fe₁₂ quasicrystalline target. The gaseous phase obtained from the ablation process has been characterised by several techniques such as emission spectroscopy, quadrupole mass spectrometry and ICCD imaging, used to study the plume composition, energy and morphology. The results show that the ablation processes in the short-pulse regimes are very different to the nanosecond one. In particular the plume angular distribution shows a characteristic high cosine exponent and the composition is completely stoichiometric and independent from the laser fluence. Furthermore the mass spectra indicate the presence of clusters, both neutral and ionised and the emission from the target suggest a rapid thermalisation leading to the melting of the surface. To clarify the ablation process some films have been deposited, on oriented silicon, at different experimental conditions and analysed by scanning electron microscopy, atomic force microscopy, energy dispersive X-ray analysis and X-ray diffraction. The analyses show the presence of nanostructured films retaining the target stoichiometry but consisting of different crystalline and non crystalline phases. In particular the nanostructure supports the hypothesis of the melting of the target during the ablation and a mechanism of material ejection is proposed for both picosecond and femtosecond regimes.     

Diffusion of cosmic-ray nuclei in the atmosphere: Spectra and composition

Summary The diffusion equations of primary cosmic-ray nuclei in the atmosphere have been solved, assuming different shapes of the prmary energy spectra, and deriving a new simple analytical form of the solutions. Variations in the upper atmosphere of the spectra and charge ratios of the different groups of nuclei are derived.

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Riassunto Sono state risolte le equazioni di diffusione dei nuclei primari della radiazione cosmica nell'atmosfera, consderando differenti forme degli spettri di energia primari, e ottenendo una nuova e semplice forma analitica delle soluzioni. Si presentano le variazioni, nell'alta atmosfera, degli spettri e dei rapporti di carica dei diversi gruppi di nuclei.

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Резюме Решаются диффузионные уравнения для распространения ядер первичного космического излучения в атмосфере, предполагая различные формы спектров первичных энергий, и получается новая простая аналитическая форма решений. Для верхних слоев атмосферы выводятся изменения спектров и зарядовых отношений для различных групп ядер.

     

The ground-level cosmic-ray event of September 29, 1989 as recorded by the rome detectors

Summary An extraordinary increase in cosmic-ray intensity has been recorded on September 29, 1989 by the Rome detectors (rigidity threshold ∼6.2 GV). We show the time evolution of the event as recorded at Rome in the nucleonic component and in the total ionizing and muon components; as well as at Campo Imperatore (2300 m a.s.l. near Rome) in the nucleonic component by a special monitor designed for the detection of solar-neutron events. The one-minute time resolution of the Campo Imperatore data offers the possibility to follow in detail the development of the event for most of the time. The possible contribution of solar neutrons to the early phase of the Campo Imperatore event has been not clarified so far, further results may be obtained by the analysis of the multiplicity data registered in the event.