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What is the reverse journey in biosynthesis: How come animals are unable to convert fats back into carbohydrates?
In biochemistry, fatty acid synthesis involves the conversion of acetyl-CoA and reducing equivalent NADPH into fatty acids. This process is primarily dependent on fatty acid synthase and occurs in the cytoplasm of the cell. In this process, most of the acetyl-CoA comes from carbohydrates and is formed via the glycolysis pathway. However, there are obvious differences between the synthesis of fat and its breakdown; in particular, animals cannot convert fat back into carbohydrates, which has triggered many people's exploration and thinking.
The main form of energy storage in animals is fat, while glycogen in their bodies is only a temporary source of energy.
The way animals store energy is interesting: a young adult has about 15 to 20 kg of fat stored, which reflects the metabolic needs of the organism and its energy-saving strategy. In contrast, the human body's glycogen reserves are pitifully small, only about 400 grams, and most of this glycogen is located in the muscles and cannot be used by the whole body. Worse still, the liver's glycogen stores are depleted after an afternoon of starvation. So, in order to continuously supply blood sugar, the liver uses certain precursors to synthesize glucose. These precursors include amino acids and a few native precursors of sugar, none of which can come from fatty acids.
The decomposition of fatty acids takes place in the mitochondria, while the synthesis of fatty acids takes place in the cytoplasm. These two processes are independent of each other and also inhibit each other.
In the mitochondria, fatty acids are converted to acetyl-CoA via β-oxidation. However, these acetyl-CoA cannot directly enter the process of carbohydrate synthesis. During this process, if acetyl-CoA enters the synthesis process, it will undergo a condensation reaction with oxaloacetate and enter the tricarboxylic acid cycle. Each cycle results in the release of carbon dioxide and energy, while oxaloacetate provides a substrate for new reactions. From this reverse journey, we can see that animals do not have the ability to convert fat into carbohydrates.
Only plants have the mechanism to convert acetyl-CoA into oxaloacetate to form closed carbohydrates.
In humans, the enzyme responsible for converting acetyl-CoA to maleyl-CoA is acetyl-CoA carboxylase, a process that is the first step in fatty acid synthesis. However, once acetyl-CoA is converted, it enters the fatty acid synthesis pathway, and the regulation of this process involves enzyme phosphorylation and holo-osteric regulation. When there is sufficient fatty acid in the body, positive feedback regulation will occur to prevent excessive fatty acid accumulation.
Remarkably, animals are unable to break these synthesized fatty acids back into glucose. This reveals a basic biosynthetic principle, namely that the synthesis of fat requires a lot of energy, while at the same time, the decomposition of fatty acids releases energy. Such chemical pathways limit the options animals have for energy storage and utilization.
These mechanisms and limitations also explain why plant-based oils and fats are so important in the diet for the physiological health and growth of animals. Because it is these high-nutrition fats that provide the key ingredients needed for animal growth.
Behind all this, we need to think again: Why don't animals have the ability to convert fat back into carbohydrates, and how does this affect our dietary choices?
