Living organisms interact with a variety of inorganic elements using sophisticated mechanisms. Essential elements required for life are incorporated into biological systems, while toxic elements are efficiently removed. Understanding the chemical roles of essential elements and the molecular mechanisms for eliminating harmful ones helps us gain insight into life itself. Some organisms can also biomineralize inorganic elements to create materials with unique properties. Examples include pearls and metallic nanoparticles, which have high industrial value. Studying how these materials are produced can lead to the development of new materials and more cost-effective production methods. To support this research, we develop new analytical techniques for studying these processes. Our laboratory focuses on two main areas: bioinorganic chemistry and analytical chemistry.
1. Elucidation of the mechanisms of biomineralization
Although living organisms are primarily composed of organic matter, they are also known to contain various inorganic substances. Representative examples include human teeth and bones, mollusk shells, and the exoskeletons of shrimp and crabs, which are mainly composed of calcium-containing minerals. These minerals interact with small amounts of organic matrix and with both organic and inorganic components, forming highly organized structures that provide excellent strength and material properties. However, the molecular mechanisms that regulate the formation of these biominerals through organic molecules remain largely unknown. By elucidating these unknown molecular mechanisms, we aim to contribute to the development of novel high-performance materials and to the effective utilization of organisms that produce biominerals.
Biomineralization ＝ Mineral ＋ Organic matrix
Elucidating the molecular mechanisms that control biominerals in vivo from a chemical perspective →「Biomineralogical Chemistry」
Recent Research Topic
Research on the organic matrix that regulates calcium carbonate crystal formation in Akoya pearl shells
The Akoya pearl oyster, known in Japan for pearl cultivation, has a shell composed of various structures primarily made of calcium carbonate. The inner layer of the shell is the lustrous nacre, while the outer layer consists of a prismatic structure. Outside the prismatic layer, the shell is covered by an organic periostracum that has not undergone mineralization. The hinge area of the shell contains a ligament, a flexible structure that allows the shell to open and close.
When the layers of the Akoya pearl oyster shell are magnified and observed under a microscope, they reveal an extremely dense and organized structure. The lustrous appearance of the nacre arises from light interacting with these layered structures, producing reflection and interference effects. Such fine structures are not unique to nacre but are found in many biominerals. These precise architectures are difficult to achieve through simple inorganic chemical reactions alone. It is believed that the small amounts of organic matrix present play a crucial role in controlling their formation. As illustrated in the schematic diagram below, the organic matrix and mineral crystals are organized hierarchically. Insoluble organic matrix frameworks serve as scaffolds for crystal formation, while extracrystalline insoluble organic components mediate interactions between the crystal and the scaffold, regulating nucleation, crystal orientation, and polymorphism. Additionally, organic components within the crystals are thought to control crystal growth and morphology.
So far, we have identified novel matrix proteins from different parts of the Akoya pearl oyster: Pif from the nacreous layer, prismalin-14 from the prism layer, PPP-10 from the periostracum, and LICP from the ligament. Pif consists of two proteins, Pif97 and Pif80, encoded by a single gene, and it is thought that these two proteins are generated through post-translational cleavage at a dibasic site. Pif97 contains a VWA domain, which mediates protein–protein interactions, and a chitin-binding domain, while Pif80 possesses sequences rich in acidic and basic amino acids and interacts with aragonite crystals. For about fifty years, it has been suggested that acidic organic matrix molecules in the nacre interact with aragonite crystals. Pif, however, was the first acidic molecule specifically present in the nacre whose sequence and function in interacting with aragonite crystals were elucidated.
In this way, we have clarified the functions of the organic matrix that interact with calcium carbonate crystals to form the intricate microstructures of Akoya pearl oyster shells. However, many molecular mechanisms remain unknown, including the control of nanocrystal size and morphology, the regulation of crystal defect density, the localization of the organic matrix network, and the maintenance of amorphous states during the initial stages of crystal formation. Currently, we are investigating biomineralization phenomena related to organic–inorganic interactions from a chemical perspective, using Akoya pearl oysters as well as other mollusks and non-molluscan species.
2. Mechanisms of nanoparticle formation using microorganisms
Inorganic materials such as metals and semiconductors exhibit unique properties when their particle sizes are reduced to the nanometer scale, properties that are not observed in the bulk material. This phenomenon, known as the quantum size effect, can result in distinctive coloration and high catalytic activity. Such nanoparticles are widely used not only in chemistry but also in industry and medicine. However, their conventional synthesis often requires high-temperature and high-pressure reactions, which place a significant burden on the environment. In recent years, the production of nanomaterials using microorganisms has been reported. This approach allows nanoparticle formation under mild conditions, making it a promising environmentally friendly method for synthesizing nanoparticles.
Recent Research Topic
Gold nanoparticle synthesis using lactic acid bacteria
Among metallic nanoparticles, gold nanoparticles (AuNPs) are highly versatile. They are gold microcrystals with particle sizes ranging from 1 to 100 nm, and their colloidal solutions exhibit a vivid red color due to surface plasmon resonance (SPR). Historically, AuNPs were used as coloring materials for stained glass, and today they serve as essential optical materials for applications such as immunolabeling in transmission electron microscopy (TEM), detection of single nucleotide polymorphisms (SNPs) in DNA, and diagnostics for diseases like influenza. Conventionally, AuNPs are synthesized by adding citrate to an aqueous solution of gold(III) chloride and heating. In recent years, microbial production methods have been reported, offering a new approach to AuNP synthesis under milder conditions compared to chemical methods. It is believed that the synthesis of AuNPs requires a reducing agent for Au(III) and a dispersant to prevent aggregation of the nanoparticles. In our laboratory, we use the lactic acid bacterium Lactobacillus casei to study the mechanisms of AuNP formation.
Gold nanoparticles (AuNPs) are conventionally synthesized by adding citrate to an aqueous solution of gold(III) chloride and heating. Recently, microbial production methods have been reported, offering a novel approach to AuNP synthesis under milder conditions compared to chemical methods. The synthesis of AuNPs is thought to require a reducing agent for Au(III) and a dispersant to prevent nanoparticle aggregation. In our laboratory, we use the lactic acid bacterium Lactobacillus casei to investigate the mechanisms of AuNP formation.
When L. casei was mixed with an aqueous solution of gold(III) chloride and incubated, the solution turned red. Observation of the sample using transmission electron microscopy (TEM) revealed numerous nanometer-sized dark spots that were not present before incubation. This indicates that incubation led to the formation of electron-dense gold nanoparticles, and their surface plasmon resonance (SPR) is responsible for the red coloration of the solution. We are currently analyzing this solution to identify the reducing agents and dispersants involved in the nanoparticle formation.
3. Development of novel analytical methods for elucidating biological phenomena
The substances present in living organisms are not only chemically diverse but also vary widely in concentration, ranging from trace amounts to abundant components. Analysis of trace components is often hindered by the presence of abundant interfering substances. In other words, conventional analytical methods cannot efficiently measure the target components without modification. In our laboratory, we develop novel analytical techniques tailored to specific research needs, aiming to elucidate biological phenomena through these advanced methods.
Recent Research Topic
Development of metal chelator analysis methods using HPLC-ICP-MS
In biomineralization reactions, organic molecules interact with inorganic elements and metals. To protect themselves from toxic metals such as cadmium and mercury in contaminated soils, or aluminum leaching from acidic soils, and to efficiently acquire essential trace metals like iron, organisms synthesize, accumulate, and secrete various chelators. Analysis of these chelators is essential for understanding the interactions between organisms and metals. In our laboratory, we have established a system combining high-performance liquid chromatography with inductively coupled plasma mass spectrometry (HPLC-ICP-MS) to selectively and sensitively analyze chelators amid the complex mixture of substances present in living organisms.
4. Elucidation of biological detoxification mechanisms against toxic metals
Heavy metals such as cadmium and mercury are toxic to many organisms, often inhibiting their growth. In response, plants and some microorganisms synthesize chelating substances known as “phytochelatins” to detoxify these metals. Similarly, light metals such as aluminum are also known to have harmful effects on various organisms. While these phenomena have been recognized for some time, the precise molecular mechanisms of metal toxicity and the corresponding detoxification processes remain largely unknown. In our laboratory, we aim to elucidate these mechanisms, with the long-term goal of applying this knowledge to the breeding of metal-tolerant plants and to phytoremediation strategies.