| Popis: |
The 3-hydroxypropionate cycle has been proposed to operate as the autotrophic CO 2 fixation pathway in the phototrophic bacterium Chloroflexus aurantiacus . In this pathway, acetyl coenzyme A (acetyl-CoA) and two bicarbonate molecules are converted to malate. Acetyl-CoA is regenerated from malyl-CoA by l -malyl-CoA lyase. The enzyme forming malyl-CoA, succinyl-CoA: l -malate coenzyme A transferase, was purified. Based on the N-terminal amino acid sequence of its two subunits, the corresponding genes were identified on a gene cluster which also contains the gene for l -malyl-CoA lyase, the subsequent enzyme in the pathway. Both enzymes were severalfold up-regulated under autotrophic conditions, which is in line with their proposed function in CO 2 fixation. The two CoA transferase genes were cloned and heterologously expressed in Escherichia coli , and the recombinant enzyme was purified and studied. Succinyl-CoA: l -malate CoA transferase forms a large (αβ) n complex consisting of 46- and 44-kDa subunits and catalyzes the reversible reaction succinyl-CoA + l -malate → succinate + l -malyl-CoA. It is specific for succinyl-CoA as the CoA donor but accepts l -citramalate instead of l -malate as the CoA acceptor; the corresponding d -stereoisomers are not accepted. The enzyme is a member of the class III of the CoA transferase family. The demonstration of the missing CoA transferase closes the last gap in the proposed 3-hydroxypropionate cycle. |
