Structural and Biophysical Analysis of the Phytochelatin-Synthase-Like Enzyme from Nostoc sp. Shows That Its Protease Activity is Sensitive to the Redox State of the Substrate

Complexation and immobilization of arsenic in maize using green synthesized silicon nanoparticles (SiNPs)

Arsenic (As) is a heavy metal that is toxic to both plants and animals. Silicon nanoparticles (SiNPs) can alleviate the detrimental effects of heavy metals on plants, but the underlying mechanisms remain unclear. The study aims to synthesize SiNPs and reveal how they promote plant health in Arsenic-polluted soil. 0 and 100% v/v SiNPs were applied to soil, and Arsenic 0 and 3.2 g/ml were applied twice. Maize growth was monitored until maturity. Small, irregular, spherical, smooth, and non-agglomerated SiNPs with a peak absorbance of 400 nm were synthesized from Pycreus polystachyos. The SiNPs (100%) assisted in the development of a deep, prolific root structure that aided hydraulic conductance and gave mechanical support to the maize plant under As stress. Thus, there was a 40-50% increase in growth, tripled yield weights, and accelerated flowering, fruiting, and senescence. SiNPs caused immobilization (As(III)=SiNPs) of As in the soil and induced root exudates Phytochelatins (PCs) (desGly-PC2 and Oxidized Glutathione) which may lead to formation of SiNPs=As(III)-PCs complexes and sequestration of As in the plant biomass. Moreover, SiNPs may alleviate Arsenic stress by serving as co-enzymes that activate the antioxidant-defensive mechanisms of the shoot and root. Thus, above 70%, most reactive ROS (OH) were scavenged, which was evident in the reduced MDA content that strengthened the plasma membrane to support selective ion absorption of SiNPs in place of Arsenic. We conclude that SiNPs can alleviate As stress through sequestration with PCs, improve root hydraulic conductance, antioxidant activity, and membrane stability in maize plants, and could be a potential tool to promote heavy metal stress resilience in the field.

Serine and Cysteine Peptidases: So Similar, Yet Different. How the Active-Site Electrostatics Facilitates Different Reaction Mechanisms

The catalytic mechanisms of serine and cysteine peptidases are similar: the proton of the nucleophile (serine or cysteine) is transferred to the catalytic histidine, and the nucleophile attacks the substrate for cleavage. However, they differ in an important aspect: cysteine peptidases form a stable ion-pair intermediate in a stepwise mechanism, while serine peptidases follow a concerted mechanism. While it is known that a positive electrostatic potential at the active site of cysteine peptidases stabilizes the cysteine anion in the ion-pair state, the physical basis of the concerted mechanism of serine peptidases is poorly understood. In this work, we use continuum electrostatic analysis and quantum mechanical/molecular mechanical (QM/MM) simulations to demonstrate that a destabilization of an anionic serine by a negative electrostatic potential in combination with a compact active site geometry facilitates a concerted mechanism in serine peptidases. Moreover, we show that an anionic serine would destabilize the protein significantly compared to an anionic cysteine in cysteine peptidases, which underlines the necessity of a concerted mechanism for serine peptidases. On the basis of our calculations on an inactive serine mutant of a natural cysteine peptidase, we show that the energy barrier for the catalytic mechanism can be substantially decreased by introducing a negative electrostatic potential and by reducing the relevant distances indicating that these parameters are essential for the activity of serine peptidases. Our work demonstrates that the concerted mechanism of serine peptidases represents an evolutionary innovative way to perform catalysis without the energetically expensive need to stabilize the anionic serine. In contrast in cysteine peptidases, the anionic cysteine is energetically easily accessible and it is a very efficient nucleophile, making these peptidases mechanistically simple. However, a cysteine is highly oxygen sensitive, which is problematic in an aerobic environment. On the basis of the analysis in this work, we suggest that serine peptidases represent an oxygen-insensitive alternative to cysteine peptidases.