Control Of Catalytic Activity Research Articles

Electrochemical promotion of Rh catalyst in gas-phase reduction of NO by propylene

The concept of nonfaradaic electrochem. modification of catalytic activity (NEMCA) was applied for the in situ control of catalytic activity of a rhodium film deposited on YSZ (yttria stabilized zirconia) solid electrolyte towards redn. of 1000 ppm NO by 1000 ppm C3H6 in presence of excess (5000 ppm) O2 at 300 Deg. A temporary heating at this feed compn. results in a long-lasting deactivation of the catalyst under open circuit conditions due to partial oxidn. of the rhodium surface. Pos. current application (5 mA) over both the active and the deactivated catalysts gives rise to an enhancement of N2 and CO2 prodn., the latter exceeding several hundred times the faradaic rate. While active rhodium exhibits a reversible behavior, electrochem. promotion on the deactivated catalyst is composed of a reversible and an irreversible part. The reversible promotion results from the steady-state accumulation of current-generated active species at the gas exposed catalyst surface whereas the irreversible effect is due to the progressive redn. of the catalyst resulting in an increased recovery rate of lost catalytic activity. The results are encouraging with respect to application of rhodium for the catalytic removal of NO from auto-exhaust gases under lean-burn conditions. [on SciFinder (R)]

Nitrate reductase in higher plants: A case study for transduction of environmental stimuli into control of catalytic activity

In higher plants, cytosolic NAD(P)H‐nitrate reductase (NR) is rapidly modulated by environmental conditions such as light, CO2, or oxygen availability. In leaves, NR is activated by photosynthesis, reaching an activation state of 60–80%. In the dark, or after stomatal closure, leaf NR is inactivated down to 20 or 40% of its maximum activity. In roots, hypoxia or anoxia activate NR, whereas high oxygen supply inactivates NR. Spinach leaf NR is inactivated by phosphorylation of serine 543 and subsequent Mg2+‐dependent binding of 14‐3‐3 proteins at, or close to, this phosphorylation site. At least three different protein kinases (NR‐PK) have been identified in spinach leaves that are able to phosphorylate NR on serine 543. Two of them show up as calmodulin‐like domain protein kinases (CDPKs), and one as a SNF1‐like protein kinase. Dephosphorylation of serine 543 is catalyzed by a Mg2+‐dependent protein phosphatase and by a type 2A protein phosphatase (NR‐PP), which is regulated by a trimer/dimer interconversion. The NR‐PKs, NR‐PPs, and 14‐3‐3s are present even in NR‐depleted plant tissues. Artificial activation of NR in vivo is achieved by cellular acidification, by respiratory inhibitors, or by mannose feeding. As for anoxia, these treatments seem to act, at least in part, via cytosolic acidification, mediated by low cytosolic ATP levels. Activation is also achieved by ionophore‐induced release of divalent cations from the cytosol. In addition, cytosolic AMP and phosphate esters seem to regulate NR‐PK and NR‐PP activities, thereby adapting NR activity within minutes to the changing environment.

The Enzyme Activity Allosteric Regulation Model Based on the Composite Nature of Catalytic and Regulatory Sites Concept

A new kinetic model of enzymatic catalysis is proposed, which postulates that enzyme solutions are equilibrium systems of oligomers differing in the number of subunits and in the mode of their assembly. It is suggested that the catalytic and regulatory sites of allosteric enzymes are of composite nature and appear as a result of subunits joining. Two possible joining modes are postulated at each oligomerization step. Catalytic site may arise on oligomer formed only by one of these modes. Effector acts by fastening together components of certain oligomeric form and increases the life time of this form. It leads to a shift of oligomer equilibrium and increases a proportion of effector-binding oligomers. Effectors-activators bind the oligomers carrying composite catalytic sites and effectors-inhibitors bind the oligomers, which do not carry active catalytic sites. Thus, catalytic activity control in such system is explained by effector-induced changes of a catalytic sites number, but not of a catalytic site activity caused by changes of subunit's tertiary structure. The postulates of the model do not contradict available experimental data and lead to a new type of general rate equation, which allows to describe and understand the specific kinetic behavior of allosteric enzymes as well as Michaelis type enzymes. All known rate equations of allosteric The equation was tested by modeling the kinetics of human erythrocyte phosphofructokinase. It enabled to reproduce quantitatively the 66 kinetic curves experimentally obtained for this enzyme under different reaction conditions.

Complement factor D, a novel serine protease.

Factor D is unique among serine proteases in that it requires neither enzymatic cleavage for expression of proteolytic activity nor inactivation by a serpin for its control. Regulation of factor D activity is instead attained by a novel mechanism that depends on reversible conformational changes for expression and control of catalytic activity. These conformational changes are believed to be induced by the single natural substrate, C3bB, and to result in realignment of the catalytic triad, the specificity pocket, and the nonspecific substrate binding site, all of which have atypical conformations. Mutational studies have defined structural determinants responsible for these unique structural features of factor D and for the resultant low reactivity with synthetic esters.

Catalytic Pyrolysis of Coal in a Powder-Particle Fluidized Bed. Control of catalytic activity by adjusting the sulfur content.

粉粒流動層を用いた石炭の接触熱分解において, より高い軽質芳香族炭化水素収率を得ることを目的として, 触媒中の硫黄分の影響を検討した.CoMo/Al2O3, NiMo/Al2O3 とそれらの硫黄分添加改質触媒を用いて, 太平洋炭の熱分解を行い, 生成物とその収率に対する触媒種類と触媒中の硫黄含有量の影響を調べた.触媒の硫黄含有量の調節により本熱分解プロセスに適する触媒が調整できる可能性がある.CoMo/Al2O3触媒 (S : 2.26wt%) では, 触媒静止層高0.08m, 水素雰囲気ガス速度0.24m・s-1と熱分解温度903Kの実験条件で, 7.2wt%, daf (BTX収率5.3wt%, daf, ナフタレン収率1.9wt%, daf) の軽質芳香族炭化水素収率が得られた.この値は前報告のものと等しいが, 触媒使用量と水素流量は少なくなった.

有機溶媒中の酵素反応 触媒活性と選択性の制御

有機溶媒中の酵素反応 触媒活性と選択性の制御