Methenyl groups

Tetrahydrofolate can carry one-carbon fragments attached to N-5 (formyl, formimino, or methyl groups), N-10 (formyl group), or bridging N-5 to N-10 (methylene or methenyl groups). 5-Formyl-tetrahydrofolate is more stable than folate and is therefore used pharma-... 

Furthermore, as (10) illustrates, multiplicity of the singly-connected R groups is indicated in the superscripts only. A subscript after the R would imply a repeat of the same R group, such as in an ethenyl vs. a methenyl group, etc. 

The one-carbon group carried by tetrahydrofolate is bonded to its N-5 or N-10 nitrogen atom (denoted as TV and TV i ) or to both. This unit can exist in three oxidation states (Table 24.2). The most-reduced form carries a methyl group, whereas the intermediate form carries a methylene group. More-oxidized forms carry a formyl, formimino, or methenyl group. The fully oxidized one-carbon unit, CO2, is carried by biotin rather than by tetrahydrofolate. 

The radical resulting from transfer to the methyl group is stabilized by delocalization and, in practice, most branching occurs at the side-groups branching from chain methenyl groups occurs to a small extent. (See also Section 5.4.2.)... 

In birds and amphibia, the green biliverdin IX is excreted in mammals, a soluble enzyme called biliverdin reductase reduces the methenyl bridge between pyrrole III and pyrrole IV to a methylene group to produce bilirubin, a yellow pigment (Figure 32-12). 

One-carbon units in different oxidation states are required in the pathways producing purines, thymidine, and many other compounds. When a biochemical reaction requires a methyl group (methylation), S-adenos dmethionme (SAM) is generally the methyl donor. If a one-carbon unit in another oxidation state is required (methylene, methenyl, formyl), tetrahydrofolate (THF) typically serves as its donor. 

Tetrahydrofolic acid then functions as a carrier of one-carbon groups for amino acid and nucleotide metabolism. The basic ring system is able to transfer methyl, methylene, methenyl, or formyl groups, and it utilizes slightly different reagents as appropriate. These are shown here for convenience, we have left out the benzoic acid-glutamic acid portion of the structure. These compounds are all interrelated, but we are not going to delve any deeper into the actual biochemical relationships. 

Fatty acids containing one or more cyclopropane rings are present in many bacteria (p. 381).124 125 The extra carbon of the cyclopropane ring is added from S-adenosylmethionine (AdoMet) at the site of a cis double bond in a fatty acyl group of a phosphatidyle-thanolamine molecule in a membrane (Eq. 21-4).126/1263 The same type of intermediate carbocation can yield either a cyclopropane fatty acid (Eq. 21-4, step a) or a methenyl fatty acid (Eq. 21-4, step b). The latter can be reduced to a branched fatty acid. This is an alternative way of introducing methyl branches that is used by some bacteria.127... 

Tetrahydrofolate, a carrier of activated one-carbon units, plays an important role in the metabolism of amino acids and nucleotides. This coenzyme carries one-carbon units at three oxidation states, which are interconvertible most reduced—methyl intermediate—methylene and most oxidized—formyl, formimino, and methenyl. The major donor of activated methyl groups is -adenosylmethionine, which is synthesized by the transfer of an adenosyl group from ATP to the sulfur atom of methionine. -Adenosylhomocysteine is formed when the activated methyl group is transferred to an acceptor. It is hydrolyzed to adenosine and homocysteine, the latter of which is then methylated to methionine to complete the activated methyl cycle. 

Reduction of CO2 to CH4 is a multi-step process involving intermediates at formyl-(methenyl-), methylene-, and methyl-stages of oxidation (Fig. 1)[2,184]. Formyl-MF is the first stable intermediate in the C02-reduction pathway [88]. Purified formyl-MF dehydrogenase can oxidize the formyl group to CO2, using methylviologen as electron acceptor [95]. It has been hypothesized that the first step traps CO2 as a carbamate, MF-COO (Reaction 9) [184]. However, formation of MF-COO" has not been demonstrated. In water, furfuralamine carbamate is spontaneously decarboxylated (Reaction 10) a similar situation is expected with MF-COO. ... 

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