In the analysis of energy transformation and biosynthetic processes in organisms, great advances have been made during the last three decades. The fundamental energy-liberating reactions which are essential for maintaining the diverse and sometimes complex cellular mechanisms are relatively simple.
These reactions are examples of electron or hydrogen transport. To trigger biochemical reactions, energy-yielding reactions to energy-consuming steps might be simply coupled. The special form of chemical energy which is employed by biological systems for these coupling reactions is stored in the pyrophosphate bond of ATP (Fig. 10.101).
According to Krebs the fundamental step of energy transformation in living matter terminates in the synthesis of ATP at the liberation of free energy during metabolism. In these energy transformations oxidation, dehydration, decarboxylation and transamination are a few basic processes. For protein, fat and carbohydrate metabolism, acetyl CoA, α-ketoglutaric acid and oxaloacetic acid are key substances that are formed prior to any significant energy liberation.
Acetyl CoA, α-ketoglutaric acid and oxaloacetic acid are broken down in the TCA cycle that liberates more than two – thirds of the cellular energy. The reactions of numerous food materials may be summarised in Fig. 10.102.
Interrelation:
There are much informations about the chemical mechanism of metabolism in general. There are also certain common reversible reactions through which one type of food may be converted to another type and vice versa. The metabolic reactions of different foodstuffs produce intermediates which lead to final oxidation in a oxidative cycle known as TCA cycle.
This cycle integrates carbohydrate, fat and protein metabolism and shows the chemical reactions of interrelationship of path formation of carbohydrate to protein and fat and vice versa. The reactions of this cycle provide the mechanism of oxidation of fatty acids through the oxidation of acetyl CoA and that of carbohydrate through the oxidation of pyruvic acid and acetyl CoA. The final stages of amino acids oxidation are also carried out through this cycle.
There are some amino acids, e.g., leucine, tyrosine and phenylalanine which go to form acetoacetic acid, and fumaric acid and acetyl CoA. These are oxidised through this cycle or acetyl CoA may be used for the synthesis of fatty acids, cholestserol and other acetylation reactions.
There are other amino acids, e.g., alanine, cystine, cysteine and serine which go to form pyruvic acid, and finally they are either oxidised in this cycle or converted into glycogen or glucose through reversible glycolytic path or oxidised to acetyl CoA which go to form fatty acids, cholesterol, etc.
Some amino acids, e.g., aspartic and glutamic acids after transamination reaction, enter directly in the TCA cycle at oxaloacetate and α-ketoglutaric stages respectively from where they are either oxidised in the TCA cycle or go to form carbohydrate or fatty acids, etc., through this cycle.
Other amino acids, arginine, histidine, ornithine, proline, etc., also enter at the α-ketoglutaric acid stage through glutamic acid and serve the same purpose. These amino acids are also synthesised through this cycle, e.g., pyruvate to alanine, oxaloacetate to aspartate and α-ketoglutarate to glutamic acid respectively.
Protein-sparing actions of carbohydrate indicate that protein may be formed from carbohydrate through this cycle. Increased amount of ATP is formed due to intake of protein diet indicating large amount of carbohydrate being formed from protein and oxidised through TCA cycle.