Bioquímica Básica-Aula 16: Catabolismo de Proteínas e Aminoácidos

Summary

In this 16th lesson of a basic biochemistry course, the instructor Luana explains the catabolism of proteins and amino acids. The lesson covers how amino acids are degraded and used for energy, especially in scenarios where carbohydrate availability is low. Key topics include the role of the urea and citric acid cycles, enzymatic processes like deamination and transamination, and how these metabolic pathways interact, particularly in the liver. The instructor emphasizes the importance of understanding how these processes contribute to protein turnover and energy generation, especially in animals.

Takeaways

• 🧬 Protein catabolism is the process of breaking down protein molecules to renew body proteins and amino acids.
• 🦁 Carnivores can derive up to 90% of their energy from proteins, while herbivores derive less than 10%.
• 🔄 Amino acids are not stored in the body and must be degraded when in excess or when carbohydrates are scarce.
• ⚙️ Catabolism of amino acids primarily occurs in the liver through deamination, which removes the amino group to form glutamate.
• 🔋 Alpha-ketoglutarate is a key product of amino acid breakdown, entering the Krebs cycle to help generate energy.
• 💡 The urea cycle and the citric acid (Krebs) cycle are interconnected, allowing for the removal of toxic ammonia and the generation of energy.
• 🧪 Digestive enzymes like pepsin, trypsin, and chymotrypsin help break down proteins into amino acids during digestion.
• 💪 Muscle tissue can use amino acids for energy, especially during anaerobic conditions, converting them to alanine, which is transported to the liver.
• ☠️ Ammonia is toxic, so it must be converted to a less toxic substance like urea in mammals or uric acid in reptiles and birds.
• 🔗 Various amino acids, such as alanine and glutamate, can enter the Krebs cycle through different intermediates like pyruvate or alpha-ketoglutarate, linking amino acid catabolism with energy production.

Q & A

What is the primary purpose of protein and amino acid catabolism?

The primary purpose of protein and amino acid catabolism is to degrade molecules, primarily proteins, to facilitate the renewal of cells and generate energy when necessary.

How does protein catabolism contribute to energy generation, especially in carnivores?

In carnivores, up to 90% of their metabolic energy needs can be fulfilled through protein catabolism. This process involves breaking down amino acids to generate energy, particularly when carbohydrates are scarce.

What are the three general scenarios in which amino acid catabolism occurs?

Amino acid catabolism occurs in three general scenarios: 1) During protein turnover and cell renewal, 2) When amino acids are ingested in excess, and 3) During carbohydrate scarcity, when proteins and amino acids are broken down for energy.

Why can't amino acids be stored in the body?

Amino acids cannot be stored because they must either be used for protein synthesis, converted into energy, or excreted. Any excess amino acids undergo catabolism as they cannot be stockpiled for future use.

Where does the primary process of amino acid catabolism take place in the body?

The primary process of amino acid catabolism takes place in the liver, where amino acids undergo deamination to remove the amino group, leading to the formation of substances like glutamate and alpha-ketoglutarate.

What is the role of transaminases (aminotransferases) in amino acid catabolism?

Transaminases (or aminotransferases) are enzymes that catalyze the transfer of amino groups from amino acids to alpha-ketoglutarate, forming glutamate and alpha-keto acids. These enzymes are critical for amino acid breakdown and are found in both the cytosol and mitochondria.

How does the body deal with the toxic accumulation of ammonia during amino acid breakdown?

The body detoxifies ammonia by converting it into urea through the urea cycle in the liver. This process ensures that ammonia, which is highly toxic, is safely eliminated from the body.

What is the relationship between the urea cycle and the citric acid (Krebs) cycle?

The urea cycle and the citric acid (Krebs) cycle are interconnected through a process known as the 'Krebs bicycle.' Components like fumarate, generated during the urea cycle, can enter the citric acid cycle to help balance energy expenditure between the two processes.

How do different organisms handle the excretion of ammonia, and why does it vary?

Organisms excrete ammonia differently based on their environment. Aquatic animals, like fish, diffuse ammonia directly into the water, while terrestrial animals convert ammonia into less toxic substances, like urea or uric acid, to minimize water loss and avoid toxicity.

What happens to amino acids after they are absorbed in the intestines?

After digestion, amino acids are absorbed into the bloodstream via capillaries in the intestinal walls and transported to the liver, where they undergo catabolism or are used for protein synthesis.