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The secrets of the liver and adipose tissue: How do they convert carbohydrates into fat?
In our bodies, fat synthesis is a crucial process that involves the conversion of carbohydrates by the liver and adipose tissue. This not only affects our weight, but also our health. This article takes a closer look at this mysterious process and how it happens within cells.
Fatty acid synthesis is the process of creating fatty acids from acetyl-CoA and NADPH. This process is facilitated by enzymes called fatty acid synthases and occurs primarily in the cytoplasm of cells.
Under normal circumstances, 85% to 90% of synthetic fatty acids come from dietary carbohydrates. These carbohydrates are converted into pyruvate through the glycolysis pathway, and then further converted into acetyl-CoA, opening the way for fat synthesis. Of course, the liver plays a central role in this process, but so does adipose tissue.
When we eat carbohydrate-rich foods, the body releases insulin, which helps convert excess sugar into fat for storage. Among them, acetyl-CoA is transported to the cytoplasm through multiple steps, where fatty acid synthesis is performed.
During the synthesis of fatty acids, acetyl-CoA is first carboxylated to malonate-CoA, which is one of the most critical steps in fatty acid synthesis.
To understand how the liver is able to easily convert carbohydrates into fat, we need to first understand how acetate and fatty acid synthases work. Acetyl-CoA first needs to pass through the activities of the citric acid cycle and combine with oxaloacetate to form citric acid, which is then cleaved into acetyl-CoA and oxaloacetate when entering the cytoplasm. Oxaloacetate can be further involved in glycogenogenesis or returned to the mitochondria.
The reaction of fatty acid synthesis is catalyzed by fatty acid synthase, a huge dimeric protein that contains a variety of enzymatic activities.
Many factors need to be adjusted during the synthesis process, including the production of NADPH. NADPH is a reducing agent for fatty acid synthesis, and its source can be obtained from oxidative pyruvate decarboxylase or the pentose phosphate pathway. These broad biochemical pathways cooperatively regulate the entire process of fat synthesis.
How animals handle this transition
Unlike plants, animals cannot resynthesize carbohydrates from fatty acids. The animal body's primary energy storage form is fat, while its glycogen reserves are relatively small. This means that in the fasted state, the animal's liver needs to rely on other sources, such as glucodiacids, to maintain blood sugar levels.
When animals consume fat, they convert it into acetyl-CoA and enter the cycle of energy metabolism.
During this process, fatty acids cannot return to glucose through the reverse step because this involves an irreversible reaction. Fat therefore plays an important role in the process of energy conversion and storage.
Regulatory mechanism
The synthesis process is also subject to complex regulation, especially the activity of acetyl-CoA carboxylase. When the concentration of palm oil coenzyme A in the cells is high, it inhibits its activity, while citric acid can activate its activity. This process helps prevent excessive accumulation of fatty acids in cells.
When insulin rises, it will promote fat synthesis, which is very obvious after eating. However, when exercising or fasting, fat synthesis will be inhibited through other pathways.
For example, epinephrine and glucagon in this case promote the oxidation of fat, showing the mutual constraints between synthesis and breakdown. This also allows us to be more flexible in handling fat under different circumstances.
Future Discussion
With in-depth research on the mechanism of fatty acid synthesis, scientists have gradually realized that there are more variables involved, especially in the generation of unsaturated fatty acids, and different metabolic pathways and gene regulation pathways are also constantly being explored. Some bacteria can synthesize unsaturated fatty acids through anaerobic and aerobic methods, which provides us with a wider range of biosynthetic inspirations.
However, the complexity of this process also makes us think about how we can better manage our diet and health and make the body's carbohydrate conversion process more efficient?
