Identification and Characterization of the Enzymes Involved in Biosynthesis of FAD and Tetrahydromethanopterin in Methanocaldococcus jannaschii
Methanogens belong to the archaeal domain, are anaerobes and produce methane from CO2 or other simple carbon compounds. Methanogenesis is a key process of the global carbon cycle and methanogens produce about 75-85% of all methane emissions.
Besides the universally occurring coenzymes that are needed in normal metabolic pathways, such as biotin, coenzyme A, thiamine, FAD, PLP, etc.; methanogens need six additional coenzymes that are involved in the methane production pathway: methanofuran, tetrahydromethanopterin, coenzyme F₄₂₀, coenzyme M, coenzyme B, and coenzyme F₄₃₀. Although now it is known that some non-methanogenic archaea and bacteria have several of these coenzymes, they are named methanogenic coenzymes since these six coenzymes were first isolated and identified from methanogens.
We are using Methanocaldococcus jannaschii as a model organism of methanogens to understand and investigate pathways of coenzymes biosynthesis. Our laboratory is involved in establishing the chemical functions of hypothetical proteins that function in targeted biochemical pathways leading to coenzyme production within the euryarchaeon M. jannaschii and identifying their corresponding genes. While there are many coenzymes present in this organism, my focus is on the biosynthetic pathways of tetrahydromethanopterin and FAD.
7,8-Dihydro-D-neopterin 2',3'-cyclic phosphate (H₂N-cP) is the first intermediate in the biosynthesis of the pterin portion of tetrahydromethanopterin (H₄MPT), a C₁ carrier coenzyme. This intermediate is produced from GTP by MptA (MJ0775 gene product), a new class of GTP cyclohydrolase I. An Fe(II)-dependent cyclic phosphodiesterase (MptB, MJ0837 gene product) hydrolyzes the cyclic phosphate of H₂N-cP to a mixture of 7,8-dihydro-D-neopterin 2'-monophosphate and 7,8-dihydro-D-neopterin 3'-monophosphate. MptB requires Fe²⁺ for activity, the same as observed for MptA. Thus the first two enzymes involved in H4MPT biosynthesis in the Archaea are Fe²⁺ dependent.
In the FAD biosynthetic pathway, the conversion of riboflavin first into FMN and then to FAD is catalyzed by a bifunctional enzyme (RibF) that first acts as a kinase converting riboflavin to FMN in the presence of ATP and then acts as a nucleotidyl transferase using a second ATP to convert the FMN to FAD. Identification of the archaeal CTP-dependent riboflavin kinase, RibK (MJ0056 gene product) led us to identify a archaeal monofunctional FAD synthetase, RibL (MJ1179 gene product). RibL is the only air-sensitive FAD synthetase identified.