CARBON DIOXIDE 



presence of pyruvic acid, carbon dioxide, and hydrogen, all three are 

 utilized to form succinic acid (9,10,28). 



The reduction of fumarate to succinate is strongly exergonic 

 and provides ample energy to drive carbon dioxide fixation by this 

 system to completion (see Table III). 



Reductive Carhoxylation 



This type of carbon dioxide fixation has been discussed recently 

 by Lipmann (17) and will only be briefly considered here. The fixa- 

 tion is a consequence of the reversal of the oxidative decarboxylation of 

 a-keto acids catalyzed by specific enzymes. These reactions involve 

 inorganic phosphate, and lead to thdf formation of an anhydride of the 

 next lower fatty acid and phosphoric acid with liberation of either formic 

 acid, or carbon dioxide and hydrogen; the hydrogen may either ap- 

 pear as molecular hydrogen or reduce a hydrogen acceptor (see re- 

 actions Ilia and 1 1 lb). The anhydride bond formed has a high energy 

 content and is generally referred to as an energy-rich phosphate bond. 

 There are other types of energy-rich phosphate bonds of biological im- 

 portance, such as enol phosphate, guanidine phosphate, and pyro- 

 phosphate bonds. The pyrophosphate type of bond has a special 

 significance because, by the action of specific enzymes, it can give rise 

 to any of the other phosphate bonds. Since the reactions involved in 

 these conversions are reversible, it follows not only that any energy- 

 rich phosphate bond can generate pyrophosphate bonds, but also 

 that the various types of bonds can be converted into one another 

 through the intermediate formation of pyrophosphate linkages. 



The biologically important pyrophosphate group is the one 

 present in adenosine polyphosphates, that is, adenosine triphosphate 

 (abbreviated ATP), and adenosine diphosphate (abbreviated ADP). 



N--CNH2 



HG C— N 



I \^Tj OH OH OH OH OH 



I /^^ II III 



N— C— N CH— CH— CH— CH— CH2— O— P— O— P— O— P— OH 



I I II II II 



I o 1 000 



Adenosine triphosphate 



By enzymic hydrolysis, ATP is dephosphorylated to ADP, and this, 

 in turn, to adenosine monophosphate (adenine ribose 5-phosphate or 



