64 METABOLISM 



producing gas from dextrose also did so from formate. The production of molecular 

 hydrogen from formic acid was shown by Stephenson and Stickland (1932) to be due 

 to hydrogenlyase, which catalyses the reaction 



HCOOH -> Ha + COo, 

 a reaction that Woods (1936) showed to be reversible. Strains of Bad. coli that produce 

 no gas have no formic hydrogenlyase (Ordal and Halvorsen 1939). Formate is not neces- 

 sarily the intermediary in hydrogen production, for though, CZ. tetanomorphum produces 

 hydrogen from dextrose and pyruvate, it will not do so from formate (Woods and Clifton 

 1937). As we shaU see in the section on protein metabolism, the hydrogen may also 

 be produced by the hydrolytic deamination of amino-acids. 



So far we have considered examples of anaerobic breakdown of carbohydrates. 

 These substances are incompletely oxidized and the partly oxidized substrates 

 often act as hydrogen acceptors promoting the further oxidation of substances 

 occurring in another chain of metabolic events. The utilization of molecular 

 oxygen results in a more complete oxidation and the liberation for cell synthesis 

 of more free energy than can be obtained by anaerobic glycolysis. 



Many bacteria are unable to utilize certain carbohydrate substances unless 

 oxygen, either molecular or combined, is present. Thus Bad. coli, supplied with 

 organic acids such as lactic, fumaric, succinic or pyruvic as sole sources of carbon, 

 will not grow except in the presence of air (Stephenson and Whetham 1924). 

 With the addition of nitrate, anaerobic growth takes place, oxidation of the organic 

 acid being achieved at the expense of the nitrate, which is reduced to nitrite. 

 The activation of the nitrate to become, in terms of Wieland's hypothesis, a hydrogen 

 acceptor, is due to a specific enzyme which Bact. coli happens to possess in common 

 with other bacteria capable of both aerobic and anaerobic dissimilation. 



The participation of oxygen as a hydrogen acceptor in glycolysis does not 

 usually take place without the intervention either of carriers, or catalysts, or both. 

 The cytochrome-cytochrome oxidase systems, involving heemin compounds, 

 appear to be the most important in this respect, though certain bacteria contain 

 respiratory pigments which enable them to utilize oxygen after the cytochrome 

 systems have been poisoned by cyanide. As an example, we may cite the pigment 

 pyocyanin found in Pseudomonas jpyocyanea, which Friedheim and Michaelis (1931) 

 showed to be autoxidizable. Green, Stickland and Tarr (1934) demonstrated 

 its ability to act as a hydrogen carrier between pairs of dehydrogenase systems 

 studied in the test-tube. Added to suspensions of pigment-free strains of Pseudo- 

 monas pyocyanea pyocyanin strongly stimulates the oxygen uptake (Friedheim 

 1931). The stimulation may in part be due to the addition of an effective carrier 

 between dehydrogenase and cytochrome systems, but in certain circumstances 

 (Friedheim 1934) the pyocyanin carries hydrogen directly to molecular oxygen. 

 The violet pigment of Chromobacterium violaceum acts in an analogous manner. 

 Chr. violaceum frequently gives rise to non-pigmented variants, which, suspended 

 in a buffer solution, take up oxygen at a moderate rate. A solution of the pigment 

 of Chr. violaceum takes up no oxygen, but the addition of the dissolved pigment 

 to the suspension results in a two- to three-fold increase in oxygen uptake (Fried- 

 heim 1932). 



Many organisms contain enzyme systems for both aerobic and anaerobic 

 glycolysis ; the predominance of one system or the other in the cell at a given 

 moment depends on the presence or absence of molecular oxygen in the environ- 

 ment. The mere presence of oxygen, however, does not necessarily impose aerobic 



