Hydrogenase is a specialized enzyme that plays a fundamental role in the metabolism of molecular hydrogen (H2) within living organisms. As a metalloenzyme, it facilitates chemical reactions that involve the reversible conversion of hydrogen gas. These enzymes are biological catalysts, meaning they accelerate specific biochemical reactions without being consumed. Their function centers on managing hydrogen, a simple yet highly reactive molecule, allowing organisms to either utilize or produce it as needed for their metabolic activities.
Catalytic Action
Hydrogenases primarily catalyze the reversible oxidation of molecular hydrogen (H2), converting it into protons (H+) and electrons, or conversely, reducing protons and electrons to form H2 gas. These reactions are represented by the general formula H2 ↔ 2H+ + 2e-.
The enzyme’s active site, often deeply embedded within its structure, is where this chemical transformation occurs. For hydrogen oxidation, the hydrogen molecule is heterolytically cleaved, meaning it splits into a proton and a hydride ion, which then interact with the metal centers in the enzyme’s active site. This process is significant in energy conversion because it allows organisms to either gain energy by breaking down H2 or store energy by creating H2.
Presence in Nature
Hydrogenases are widely distributed across various life forms, predominantly found in microorganisms such as bacteria and archaea, but also in some eukaryotes like algae. In anaerobic conditions, hydrogenases are important for metabolic processes, allowing microbes to gain energy from hydrogen uptake or to dispose of excess electrons through hydrogen evolution.
They play roles in processes like nitrogen fixation, where bacteria use hydrogenases to recycle hydrogen produced during nitrogen conversion. During fermentation, bacteria may produce hydrogen gas as a byproduct. Photosynthetic organisms, including some algae, can also use hydrogenases to produce hydrogen gas, particularly under specific light and nutrient conditions. These enzymes are also involved in chemosynthesis, where microbes derive energy from the oxidation of inorganic compounds, including hydrogen.
Major Classifications
Hydrogenases are categorized into three main types based on the specific metal content found at their active sites: [NiFe]-hydrogenases, [FeFe]-hydrogenases, and iron-only hydrogenases. [NiFe]-hydrogenases contain both nickel and iron at their active center and are generally found in bacteria and archaea. These enzymes typically consist of large and small subunits, with the large subunit housing the nickel-iron active site and the small subunit containing iron-sulfur clusters for electron transfer.
[FeFe]-hydrogenases possess an iron-iron active site and are commonly found in bacteria and some unicellular eukaryotes. These are often known for their high catalytic rates in hydrogen production, though many are highly sensitive to oxygen. The active site of both [NiFe] and [FeFe] hydrogenases can feature unusual ligands such as carbon monoxide (CO) and cyanide (CN-) bound to the iron atoms. Iron-only hydrogenases also play a role in hydrogen metabolism.
Potential Uses
Understanding hydrogenases offers promising avenues for various technological advancements, particularly in sustainable energy production. Researchers are exploring their use in biological hydrogen production, aiming to create clean fuel for applications like fuel cells. Algae and bacteria, equipped with hydrogenases, can be engineered to produce hydrogen gas, a renewable energy source.
These enzymes are also being investigated for bioremediation efforts, where their ability to metabolize hydrogen or facilitate electron transfer can be harnessed to break down pollutants. The soluble [NiFe] hydrogenase from Ralstonia eutropha H16, for example, is considered a candidate for H2-based biofuel applications due to its preference for H2 oxidation and relative tolerance to oxygen. Furthermore, hydrogenases can inspire the design of biosensors to detect hydrogen or other related molecules.