Abstract
This chapter focuses on enzymology and the occurrence of nitric oxide in the biological nitrogen cycle. This chapter begins with a discussion on the global nitrogen cycle. Nitrogen enters the global nitrogen cycle through the reduction of N2 to ammonia by nitrogen-fixing bacteria. The ammonia is then converted into the proteins, nucleic acids, and aminosugars of the bacteria and the symbiotic plants with which the bacteria are associated. Nitrogen thus enters the biosphere where most of it remains at redox level 3ˉ. The oxidation of amino acids, aminosugars, and nucleotides by bacteria and animals ultimately regenerates ammonia, either directly or through the prior production of urea or uric acid. This chapter explains in detail concepts related to denitrifying bacteria, and describes evidence for and against nitric oxide as an intermediate in denitrification pathway. An overview of nitric oxide reductase, nitric oxide-consuming enzyme of denitrification pathway is also presented in this chapter. The chapter elaborates in detail about nitric oxide production by heterotrophic nitrifiers. The chapter concludes with a discussion on the toxicity of nitric oxide toward bacteria.