Bioquímica - Aula 19 - Biossíntese de Lipídios

Summary

In this lecture, Professor Ângelo Cortel explores the biochemical processes involved in lipid biosynthesis, building on the previous discussion of carbohydrate metabolism. He explains how excess nutrients, whether from carbohydrates, proteins, or fats, are converted into energy or stored as lipids. The process begins with acetyl-CoA and malonyl-CoA in the cytoplasm, which undergo a series of enzymatic reactions to form fatty acids. He also covers the catabolism and anabolism of fatty acids, emphasizing the importance of energy storage and the role of unsaturated fatty acids like omega-3 and omega-6 in biological processes such as blood clotting and inflammation.

Takeaways

• 😀 Lipid biosynthesis is the next step after carbohydrate biosynthesis in metabolism, focusing on how excess food is processed and stored as lipids.
• 😀 When food is consumed in excess, molecules like acetyl-CoA accumulate, triggering lipid synthesis processes.
• 😀 Acetyl-CoA is transformed into malonyl-CoA in the cytoplasm, which is essential for the biosynthesis of fatty acids.
• 😀 The biosynthesis of lipids involves a complex set of enzymatic reactions, including acetyl-CoA carboxylase, which helps incorporate CO2 into acetyl-CoA.
• 😀 The process of fatty acid synthesis involves a protein complex called ACP, which carries fatty acid groups and facilitates the biosynthesis of fatty acids.
• 😀 Enzymes in the ACP complex facilitate a series of reactions that elongate fatty acids by adding two carbon atoms at a time, ultimately forming fatty acids like palmitic acid.
• 😀 Energy expenditure in lipid synthesis exceeds that of degradation, as ATP is used both for transporting acetyl-CoA and synthesizing fatty acids.
• 😀 Lipid degradation occurs through beta-oxidation in the mitochondria, breaking down fatty acids into acetyl-CoA molecules, which are then used in the Krebs cycle to produce ATP.
• 😀 Lipid degradation yields more energy than synthesis: for example, the degradation of palmitic acid produces a large net energy gain in the form of ATP, NADH, and FADH2.
• 😀 The body synthesizes polyunsaturated fatty acids (omega-3 and omega-6) that are vital for important compounds like prostaglandins, which help with inflammation and blood coagulation.
• 😀 Omega-6 and omega-3 fatty acids must be obtained through the diet, as the human body cannot synthesize them beyond certain carbon positions (e.g., 9 for omega-6).

Q & A

What happens when we ingest more food than our body needs for energy?

When we consume more food than needed, the excess nutrients are processed and stored. For instance, carbohydrates are converted into pyruvate in the cytosol, and proteins are broken down into amino acids and their carbon skeletons. These components can then enter metabolic pathways to form acetyl-CoA, which accumulates and can eventually be used to synthesize lipids.

What role does acetyl-CoA play in lipid biosynthesis?

Acetyl-CoA is a key molecule in lipid biosynthesis. It is the precursor to fatty acids. When excess nutrients are available, acetyl-CoA accumulates in the cytosol, where it participates in reactions that lead to the formation of fatty acids, a process known as lipogenesis.

How does acetyl-CoA accumulate in the cell when there is sufficient energy?

Acetyl-CoA accumulates in the cell when there is sufficient ATP, meaning energy needs are met. As a result, the cycle of the Krebs cycle slows down, and citrates accumulate in the mitochondria. These citrates are transported into the cytosol, where they are broken down into acetyl-CoA, which can be used for fatty acid synthesis.

What is the process of acetyl-CoA transformation into malonyl-CoA?

In the cytoplasm, acetyl-CoA is carboxylated to form malonyl-CoA. This reaction involves the enzyme acetyl-CoA carboxylase, which uses CO2 and energy to add a carbon to acetyl-CoA, resulting in malonyl-CoA. This is a crucial step in fatty acid synthesis.

What enzymes are involved in the fatty acid biosynthesis pathway?

Fatty acid biosynthesis involves a complex of enzymes, including acetyl-CoA-ACP transferase, malonyl-CoA-ACP transferase, beta-keto-ACP synthase, beta-hydroxy-ACP dehydratase, enoil reductase, and others. These enzymes work in a coordinated manner to elongate the fatty acid chain by adding two carbon units at a time.

What is the significance of the enzyme ACP in lipid biosynthesis?

ACP, or acyl carrier protein, plays a central role in lipid biosynthesis. It carries the growing fatty acid chain through a series of enzymatic reactions. It allows the transfer of acyl groups (such as acetyl and malonyl groups) and facilitates the elongation of fatty acids.

How are fatty acids elongated during the biosynthesis process?

Fatty acids are elongated by adding two-carbon units derived from malonyl-CoA to the growing fatty acid chain. The process involves several steps, including condensation, reduction, dehydration, and another reduction, ultimately forming longer chains, typically up to 16 carbon atoms in the cytoplasm.

What happens to fatty acids beyond 16 carbon atoms?

Fatty acids longer than 16 carbon atoms are further elongated in the endoplasmic reticulum (ER), not the cytoplasm. Additionally, the process of desaturation, or the introduction of double bonds, can occur in the ER to produce polyunsaturated fatty acids.

What is the difference between omega-3 and omega-6 fatty acids?

Omega-3 and omega-6 fatty acids are both polyunsaturated fatty acids. The difference lies in the position of the first double bond in their carbon chain. Omega-3 fatty acids have a double bond starting at the third carbon from the end, while omega-6 fatty acids have it starting at the sixth carbon. These fatty acids are essential and must be obtained from the diet.

Why is the intake of omega-3 and omega-6 fatty acids important?

Omega-3 and omega-6 fatty acids are crucial for the formation of eicosanoids like prostaglandins and thromboxanes, which play important roles in inflammation, blood clotting, and various cellular functions. They are also vital for maintaining cell membrane integrity and supporting cardiovascular health.