Macromolecular crowding effects on the kinetics of opposing reactions catalyzed by alcohol dehydrogenase

Deeper Insights into Mixed Crowding through Enzyme Activity, Dynamics, and Crowder Diffusion

Given the fact that the cellular interior is crowded by many different kinds of macromolecules, it is important that in vitro studies be carried out in the presence of mixed crowder systems. In this regard, we have used binary crowders formed by the combination of some of the commonly used crowding agents, namely, Ficoll 70, Dextran 70, Dextran 40, and PEG 8000 (PEG 8), to study how these affect enzyme activity, dynamics, and crowder diffusion. The enzyme chosen is AK3L1, an isoform of adenylate kinase. To investigate its dynamics, we have carried out three single point mutations (A74C, A132C, and A209C) with the cysteine residues being labeled with a coumarin-based solvatochromic probe [CPM: (7-diethylamino-3-(4-maleimido-phenyl)-4-methylcoumarin)]. Both enzyme activity and dynamics decreased in the binary mixtures as compared with the sum of the individual crowders, suggesting a reduction in excluded volume (in the mixture). To gain deeper insights into the binary mixtures, fluorescence correlation spectroscopy studies were carried out using fluorescein isothiocyanate-labeled Dextran 70 and tetramethylrhodamine-labeled AK3L1 as the diffusion probes. Diffusion in binary mixtures was observed to be much more constrained (relative to the sum of the individual crowders) for the labeled enzyme as compared to the labeled crowder showing different environments being faced by the two species. This was further confirmed during imaging of the phase-separated droplets formed in the binary mixtures having PEG as one of the crowding agents. The interior of these droplets was found to be rich in crowders and densely packed, as shown by confocal and digital holographic microscopy images, with the enzymes predominantly residing outside these droplets, that is, in the relatively less crowded regions. Taken together, our data provide important insights into various aspects of the simplest form of mixed crowding, that is, composed of just two components, and also hint at the enhanced complexity that the cellular interior presents toward having a detailed and comprehensive understanding of the same.

Bridging Soft Interaction and Excluded Volume in Crowded Milieu through Subtle Protein Dynamics

The impact of macromolecular crowding on biological macromolecules has been elucidated through the excluded volume phenomenon and soft interactions. However, it has often been difficult to provide a clear demarcation between the two regions. Here, using temperature-dependent dynamics (local and global) of the multidomain protein human serum albumin (HSA) in the presence of commonly used synthetic crowders (Dextran 40, PEG 8, Ficoll 70, and Dextran 70), we have shown the presence of a transition that serves as a bridge between the soft and hard regimes. The bridging region is independent of the crowder identity and displays no apparent correlation with the critical overlap concentration of the polymeric crowding agents. Moreover, the dynamics of domains I and II and the protein gating motion respond differently, thereby bringing to the fore the asymmetry underlying the crowder influence on HSA. In addition, solvent-coupled and decoupled protein motions indicate the heterogeneity of the dynamic landscape in the crowded milieu. We also propose an intriguing correlation between protein stability and dynamics, with increased global stability being accompanied by eased local domain motion.

Tunable In Vivo Colocalization of Enzymes within P22 Capsid-Based Nanoreactors

Virus-like particles (VLPs) derived from bacteriophage P22 have been explored as biomimetic catalytic compartments. In vivo colocalization of enzymes within P22 VLPs uses sequential fusion to the scaffold protein, resulting in equimolar concentrations of enzyme monomers. However, control over enzyme stoichiometry, which has been shown to influence pathway flux, is key to realizing the full potential of P22 VLPs as artificial metabolons. We present a tunable strategy for stoichiometric control over in vivo co-encapsulation of P22 cargo proteins, verified for fluorescent protein cargo by Förster resonance energy transfer. This was then applied to a two-enzyme reaction cascade. l-homoalanine, an unnatural amino acid and chiral precursor to several drugs, can be synthesized from the readily available l-threonine by the sequential activity of threonine dehydratase and glutamate dehydrogenase. We found that the loading density of both enzymes influences their activity, with higher activity found at lower loading density implying an impact of molecular crowding on enzyme activity. Conversely, increasing overall loading density by increasing the amount of threonine dehydratase can increase activity from the rate-limiting glutamate dehydrogenase. This work demonstrates the in vivo colocalization of multiple heterologous cargo proteins in a P22-based nanoreactor and shows that controlled stoichiometry of individual enzymes in an enzymatic cascade is required for the optimal design of nanoscale biocatalytic compartments.

Molecular crowding elicits the acceleration of enzymatic crosslinking of macromolecular substrates

Cytoplasm contains high concentrations of biomacromolecules. Protein behavior under such crowded conditions is reportedly different from that in an aqueous buffer solution, mainly owing to the effect of volume exclusion caused by the presence of macromolecules. Using a crosslinking reaction catalyzed by microbial transglutaminase (MTG) as a model, we herein systematically determined how the substrate size affects enzymatic activity in both dilute and crowded solutions of dextran. We first observed a threefold reduction in MTG-mediated crosslinking of a pair of small peptide substrates in 15 wt% dextran solution. In contrast, when proteinaceous substrates were involved, the crosslinking rates in 15 wt% dextran solutions accelerated markedly to levels comparable with the level in the absence of dextran. Our results provide new insights into the action of enzymes with regard to macromolecular substrates under crowded conditions, of which the potential utility was demonstrated by the formation of highly crosslinked protein polymers.

Parameter Reliability and Understanding Enzyme Function

Knowledge of the Michaelis-Menten parameters and their meaning in different circumstances is an essential prerequisite to understanding enzyme function and behaviour. The published literature contains an abundance of values reported for many enzymes. The problem concerns assessing the appropriateness and validity of such material for the purpose to which it is to be applied. This review considers the evaluation of such data with particular emphasis on the assessment of its fitness for purpose.