H2 | Hydrogenases for biological hydrogen production
Biological H2 production offers distinctive advantages for environmental protection over existing physico-chemical methods. This study focuses specifically on hydrogenases, a class of enzymes that serves to effectively catalyze H2 formation from protons or oxidation to protons. It reviews the classification schemes (i.e. [NiFe]-, [FeFe]-, and [Fe]-hydrogenases) and properties of these enzymes, which are essential to understand the mechanisms for H2 production, the control of cell metabolism, and subsequent increases in H2 production. There are five kinds of biological hydrogen production methods, categorized based upon the light energy requirement, and feedstock sources. The genetic engineering work on hydrogenase to enhance H2 production is reviewed here. Further discussions in this study include nitrogenase, an enzyme that normally catalyzes the reduction of N2 to ammonia but is also able to produce H2 under photo-heterotrophic conditions, as well as other applicable fields of hydrogenase other than H2 production.
Introduction
Growing concerns with severe global warming and fossil fuel depletion have prompted extensive research in pursuit of alternative and clean energy sources. Among the various candidates, hydrogen gas (H2) is regarded as the most promising future energy carrier as it has higher energy content by 2.75 times compared to hydrocarbon fuels (gasoline) and produces only water upon combustion. In addition, H2 can be directly used in a fuel cell, generating electricity with high efficiency (Momirlan and Veziroglu, 2002). The pioneering notion of a “Hydrogen Energy System” drew inspiration from the French science-fiction novel “The Mysterious Island” by Jules Verne (1874), where the idea of using H2 as an energy carrier first appeared (Mason, 2007). To date however, over 90% of the production of H2 remains based upon steam reforming of hydrocarbons and coal gasification, which starts from fossil fuels and requires high temperature and pressure conditions. Meanwhile, biological methods have great potential for the production of H2 in an environmentally friendly way.
H2 is the most common element in the universe, and it is expected that the Earth’s early atmosphere was reducing one predominated by H2. At present, the atmosphere has turned to an oxidizing one, which was carried out mostly by biological processes that still continue (Vignais and Billoud, 2007). Many bacteria obtain energy by the oxidation of H2 assisted by a number of complex mechanisms, and O2 is released by the oxidation of water via photosynthesis. Meanwhile through a less well appreciated process, various species evolve H2 under anaerobic conditions. Actually, this is a proximate and everyday process for individuals: the bacteria in our digestive tract produce H2 (Cammack et al., 2001). However, most of this produced H2 is undetectable, because it is immediately recycled by other bacteria.
Life depends on numerous series of chemical reactions, yet many of these reactions proceed too slowly on their own. Hence, nature has designed catalysts to greatly accelerate the rates of biochemical reactions, which we now refer to as enzymes (Copeland, 2000). The key enzyme involved in catalyzing H2 formation from protons or oxidation to protons is hydrogenase. The reaction (2H + 2e− ↔ H2) is reversible, and its direction depends on the redox potential of the components that are able to interact with hydrogenase (Vignais and Colbeau, 2004). In addition, nitrogenase, an enzyme that normally catalyzes the reduction of N2 to ammonia, is able to reduce protons to H2 as a byproduct under photo-heterotrophic conditions (McKinlay and Harwood, 2010).
A study of hydrogenase is essential for understanding the H2 production mechanism, controlling cell metabolism, and finally increasing H2 production. Since 1931, when hydrogenase was first described by Stephenson and Stickland, extensive research has been conducted in this area (Mertens and Liese, 2004). The sequences of ca. 450 hydrogenases are now available, and they are well categorized by their specific characteristics and active site models have been developed (Meyer, 2007).
In the present work, various aspects of hydrogenases (classification and properties), biological H2 production (BHP) systems, and genetic engineering work on hydrogenase for enhanced H2 production are reviewed.
