Food Chemistry and Fish in Nutrition ( 2+1)

6.3.1.2. Energy production under anaerobic and aerobic condition

a. Aerobic condition

Under aerobic condition, pyruvate is taken up into mitochondria, and after conversion to acety-CoA is oxidized to CO₂ by the citric acid cycle. The reducing equivalents from the NADH+H+ formed in glycolysis are taken up into mitochondria for oxidation.

Two triosephosphates are produced from each molecule of hexose metabolized. Dihydroxyacetone phosphate can be isomerized by isomerase to glyceraldehyde 3 phosphate which is then converted to 1, 3 diphosphoglyceric acid by glyceraldehyde 3 phosphate dehydrogenase system requiring NAD⁺ and inorganic phosphate.

ATP Production:                                                                    ATP

Conversion of 1, 3 diphosphoglycerateto 3 phosphoglycerate : 2

Conversion of phosphoenol pyruvate to enol pyruvate             : 2

Action of glyceraldehyde 3 phosphatedehydrogenase

and ETC          (2x3)                                                                   : 6

                                                                                                --------

                              Total                                                             :10

                                                                                                --------

  ATP molecules  used  up in the initial reactions                     :  2

                                                                                                ---------

Net ATP production                                                    :  8

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b. Anaerobic condition

If anaerobic conditions prevail, the reoxidation of NADH by transfer of reducing equivalents through the respiratory chain to oxygen is prevented. Then, pyruvate is reduced by the NADH+H⁺ to lactate, the reaction being catalyzed by lactate dehydrogenase. The reoxidation of NADH via lactate formation allows glycolysis to proceed in the absence of oxygen by regenerating sufficient NAD⁺ for another cycle of the reaction catalyzed by glyceraldehyde-3 phosphate dehydrogenase.

Lactate dehydrogenase

Pyruvate + NADH + H⁺ ↔ NAD⁺ + Lactate

During fermentation, pyruvate is reduced by NADH to ethyl alcohol being catalysed by alcohol dehydrogenase.

  Alcohol dehydrogenase

Pyruvate + NADH + H⁺ ↔ NAD⁺ + Ethyl alcohol

ATP production:                                                                      ATP

     

  Conversion of two triose phosphates to lactic acid (or ethanol): 4.

  ATP molecules used up in the initial reactions                         :  2

                                                                                                 ----------               

Net ATP production                                                      : 2

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