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Proposed pathway for the assimilation of C1 compounds by isocitrate lyase-negative type II methylotrophs by using the serine cycle for formaldehyde fixation. The ethylmalonyl-CoA pathway described here (excluding the condensation of acetyl-CoA and glyoxylate) is integrated in the serine cycle (upper part) and is involved in assimilation of acetyl-CoA and regeneration of glyoxylate during growth on C 1 compounds. It is assumed that during growth on C 2 compounds the ethylmalonyl-CoA pathway (Fig. 4) is used exclusively. Dotted lines indicate more than one reaction step.

Proposed pathway for the assimilation of C1 compounds by isocitrate lyase-negative type II methylotrophs by using the serine cycle for formaldehyde fixation. The ethylmalonyl-CoA pathway described here (excluding the condensation of acetyl-CoA and glyoxylate) is integrated in the serine cycle (upper part) and is involved in assimilation of acetyl-CoA and regeneration of glyoxylate during growth on C 1 compounds. It is assumed that during growth on C 2 compounds the ethylmalonyl-CoA pathway (Fig. 4) is used exclusively. Dotted lines indicate more than one reaction step.

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Fifty years ago, Kornberg and Krebs established the glyoxylate cycle as the pathway for the synthesis of cell constituents from C2-units. However, since then, many bacteria have been described that do not contain isocitrate lyase, the key enzyme of this pathway. Here, a pathway termed the ethylmalonyl-CoA pathway operating in such organisms is desc...

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... of acetyl-CoA to glyoxylate (6,19). Here, we have shown high crotonyl-CoA carboxylase/reductase activity in cell extracts of methanol-grown M. extorquens. The presence of the ethylmalonyl-CoA pathway (excluding the condensation of acetyl-CoA and glyoxylate) in M. extorquens simplifies the scheme for isocitrate lyase-negative C 1 - assimilation (Fig. 5) compared with former proposals (12). Genes of the ethylmalonyl-CoA pathway, including ccr, were affected in mutants of M. extorquens unable to form glyoxylate from acetyl-CoA. However, different catalytic roles were ini- tially assigned to the corresponding gene products (12,16,20). It has been reported that marine species of the ...
Context 2
... 1 -compounds by these aerobic phototrophs has not been explicitly shown. All genes implicated in the herein-described ethylmalonyl-CoA pathway, including ccr, now known to encode the key enzyme crotonyl-CoA carboxylase/ reductase, are conserved in sequenced representatives of this phylogenetic group (13), suggesting that the extended serine cycle (Fig. 5) may be involved in metabolism of further ecologically important compounds such as ...

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