Gas molecules are well known as substrates/products of enzymes in a variety of biological reactions including respiration, denitrification, photosynthesis, methanogenesis, and several other metabolism/catabolism systems. It has become apparent that they also act as signalling molecules to regulate their biological activities in all living organisms. In the late 1980s, nitric oxide (NO) was shown to act as the mediator of endothelium-derived relaxing factor via the activation of an NO receptor (soluble guanylate cyclase) by a nitrosyl–haem complex. The Nobel Prize in Physiology or Medicine 1998 was awarded jointly to Robert F. Furchgott, Louis J. Ignarro and Ferid Murad “for their discoveries concerning nitric oxide as a signalling molecule in the cardiovascular system”.

In the late 1990s carbon monoxide (CO) was confirmed to act as a physiological effector of a bacterial transcriptional regulator CooA employing haem to sense CO and before that it was reported that haem acts as a molecular oxygen (O₂) sensor in FixL, a histidine kinase in the FixL/FixJ two-component system responsible for the regulation of O₂-dependent gene expression. In 2000, haem-based O₂-sensors HemATs were identified in aerotaxis regulatory systems, which are chemotaxis signal transducer proteins directly sensing O₂. Though it had been well known that molecular oxygen controls the metabolic switch between respiration and fermentation in facultative anaerobes such as Escherichia coli, the molecular mechanism of the metabolic switch was unknown. In the 1990s and later, FNR was identified as a master switch whose function is regulated by O₂. A protein-bound iron–sulfur cluster acts as the O₂ sensor. Recently, it was verified that iron–sulfur clusters act as NO sensors as well.

Though the first report suggesting that ethylene functions as a plant hormone dates back to 1896, developments of research on ethylene signalling had to wait for the introduction of molecular biology techniques and the model plant, Arabidopsis thaliana. An ethylene receptor was cloned in 1988 for the first time and 16 genes related to ethylene signalling have been identified in Arabidopsis thaliana. Cu(i) is proposed as the active site for sensing ethylene in ethylene receptors.

The numbers of gas-sensor proteins identified and characterized are increasing partly because of the expansion of genomic data and development of experimental techniques including X-ray crystallography and certain spectroscopies. Metal-containing prosthetic groups are good probes for spectroscopic measurements that enables elucidation of the structural and functional relationships of active sites in gas-sensor proteins at atomic/molecular levels. This book focuses on recent developments in research on gas sensor systems. Haem-, iron–sulfur cluster- and nonhaem iron-based gas-sensor proteins are surveyed in Chapters 2–6 and mammalian O₂ and plant ethylene signalling systems in Chapters 7 and 8, respectively.

I would like to thank the authors of each of the chapters in this book for their efforts in preparing the comprehensive and up-to-date chapters. Finally, I would like to express my gratitude to Anthony G. Wedd for his encouragement, advice, and help in preparing this book.

Shigetoshi Aono