Minimalistic Cellulosome of the Butanologenic Bacterium Clostridium saccharoperbutylacetonicum.

Clostridium saccharoperbutylacetonicum is a mesophilic, anaerobic, butanol-producing bacterium, originally isolated from soil. It was recently reported that C. saccharoperbutylacetonicum possesses multiple cellulosomal elements and would potentially form the smallest cellulosome known in nature. Its genome contains only eight dockerin-bearing enzymes, and its unique scaffoldin bears two cohesins (Cohs), three X2 modules, and two carbohydrate-binding modules (CBMs). In this study, all of the cellulosome-related modules were cloned, expressed, and purified. The recombinant cohesins, dockerins, and CBMs were tested for binding activity using enzyme-linked immunosorbent assay (ELISA)-based techniques. All the enzymes were tested for their comparative enzymatic activity on seven different cellulosic and hemicellulosic substrates, thus revealing four cellulases, a xylanase, a mannanase, a xyloglucanase, and a lichenase. All dockerin-containing enzymes interacted similarly with the second cohesin (Coh2) module, whereas Coh1 was more restricted in its interaction pattern. In addition, the polysaccharide-binding properties of the CBMs within the scaffoldin were examined by two complementary assays, affinity electrophoresis and affinity pulldown. The scaffoldin of C. saccharoperbutylacetonicum exhibited high affinity for cellulosic and hemicellulosic substrates, specifically to microcrystalline cellulose and xyloglucan. Evidence that supports substrate-dependent in vivo secretion of cellulosomes is presented. The results of our analyses contribute to a better understanding of simple cellulosome systems by identifying the key players in this minimalistic system and the binding pattern of its cohesin-dockerin interaction. The knowledge gained by our study will assist further exploration of similar minimalistic cellulosomes and will contribute to the significance of specific sets of defined cellulosomal enzymes in the degradation of cellulosic biomass.IMPORTANCE Cellulosome-producing bacteria are considered among the most important bacteria in both mesophilic and thermophilic environments, owing to their capacity to deconstruct recalcitrant plant-derived polysaccharides (and notably cellulose) into soluble saccharides for subsequent processing. In many ecosystems, the cellulosome-producing bacteria are particularly effective "first responders." The massive amounts of sugars produced are potentially amenable in industrial settings to further fermentation by appropriate microbes to biofuels, notably ethanol and butanol. Among the solvent-producing bacteria, Clostridium saccharoperbutylacetonicum has the smallest cellulosome system known thus far. The importance of investigating the building blocks of such a small, multifunctional nanomachine is crucial to understanding the fundamental activities of this efficient enzymatic complex.

Adenosine/guanosine-3',5'-bis-phosphates as biocompatible and selective Zn2+-ion chelators. Characterization and comparison with adenosine/guanosine-5'-di-phosphate.

Although involved in various physiological functions, nucleoside bis-phosphate analogues and their metal-ion complexes have been scarcely studied. Hence, here, we explored the solution conformation of 2'-deoxyadenosine- and 2'-deoxyguanosine-3',5'-bisphosphates, 3 and 4, d(pNp), as well as their Zn(2+)/Mg(2+) binding sites and binding-modes (i.e. inner- vs. outer-sphere coordination), acidity constants, stability constants of their Zn(2+)/Mg(2+) complexes, and their species distribution. Analogues 3 and 4, in solution, adopted a predominant Southern ribose conformer (ca. 84%), gg conformation around C4'-C5' and C5'-O5' bonds, and glycosidic angle in the anti-region (213-270°). (1)H- and (31)P-NMR experiments indicated that Zn(2+)/Mg(2+) ions coordinated to P5' and P3' groups of 3 and 4 but not to N7 nitrogen atom. Analogues 3 and 4 formed ca. 100-fold more stable complexes with Zn(2+)vs. Mg(2+)-ions. Complexes of 3 and 4 with Mg(2+) at physiological pH were formed in minute amounts (11% and 8%, respectively) vs. Zn(2+) complexes (46% and 44%). Stability constants of Zn(2+)/Mg(2+) complexes of analogues 3 and 4 (log KML(M) = 4.65-4.75/2.63-2.79, respectively) were similar to those of the corresponding complexes of ADP and GDP (log KML(M) = 4.72-5.10/2.95-3.16, respectively). Based on the above findings, we hypothesized that the unexpectedly low log K values of Zn(2+)-d(pNp) as compared to Zn(2+)-NDP complexes, are possibly due to formation of outer-sphere coordination in the Zn(2+)-d(pNp) complex vs. inner-sphere in the NDP-Zn(2+) complex, in addition to loss of chelation to N7 nitrogen atom in Zn(2+)-d(pNp). Indeed, explicit solvent molecular dynamics simulations of 1 and 3 for 100 ns supported this hypothesis.

Site-directed decapsulation of bolaamphiphilic vesicles with enzymatic cleavable surface groups.

Stable nano-sized vesicles with a monolayer encapsulating membrane were prepared from novel bolaamphiphiles with choline ester head groups. The head groups were covalently bound to the alkyl chain of the bolaamphiphiles either via the nitrogen atom of the choline moiety, or via the choline ester's methyl group. Both types of bolaamphiphiles competed with acetylthiocholine for binding to acetylcholine esterase (AChE), yet, only the choline ester head groups bound to the alkyl chain via the nitrogen atom of the choline moiety were hydrolyzed by the enzyme. Likewise, only vesicles composed of bolaamphiphiles with head groups that were hydrolyzed by AChE released their encapsulated material upon exposure to the enzyme. Injection of carboxyfluorescein (CF)-loaded vesicles with cleavable choline ester head groups into mice resulted in the accumulation of CF in tissues that express high AChE activity, including the brain. By comparison, when vesicles with choline ester head groups that are not hydrolyzed by AChE were injected into mice, there was no accumulation of CF in tissues that highly express the enzyme. These results imply that bolaamphiphilic vesicles with surface groups that are substrates to enzymes which are highly expressed in target organs may potentially be used as a drug delivery system with controlled site-directed drug release.