24.4 Protein Metabolism

Protein Digestion and Metabolism

Process of Protein Digestion

Protein digestion breaks large protein molecules into individual amino acids so the body can absorb and use them. This process spans from the stomach to the small intestine, with different enzymes handling each stage.

• Stomach: Hydrochloric acid (HCl) activates pepsinogen into pepsin, which cleaves proteins into smaller polypeptides. The acidic environment also denatures (unfolds) proteins, making them easier for enzymes to access.
• Small intestine (duodenum): Pancreatic enzymes take over. Trypsin, chymotrypsin, and carboxypeptidase break polypeptides into shorter peptides and some free amino acids. Each enzyme cuts at different points along the peptide chain.
• Brush border: Enzymes on the surface of intestinal epithelial cells, including aminopeptidases and dipeptidases, finish the job by splitting remaining small peptides into individual amino acids.

Once freed, amino acids are absorbed into intestinal cells via active transport and facilitated diffusion. They then travel to the liver through the hepatic portal vein.

In the liver, amino acids can follow several paths:

• Protein synthesis (e.g., producing albumin and clotting factors)
• Gluconeogenesis (conversion to glucose)
• Ketogenesis (conversion to ketone bodies)
• Energy production through deamination followed by entry into the citric acid cycle

Role of the Urea Cycle

When amino acids are broken down, the removal of their amino group ($-NH_2$) produces ammonia ($NH_3$). Ammonia is highly toxic to the nervous system, so the body must convert it to something safer. That's the job of the urea cycle, which takes place primarily in liver hepatocytes.

The urea cycle converts ammonia into urea ($NH_2CONH_2$), a much less toxic, water-soluble compound. Urea then travels through the bloodstream to the kidneys, where it's filtered out and excreted in urine. This is the body's main route for eliminating excess nitrogen.

Steps of the urea cycle:

1. Ammonia combines with $CO_2$ and ATP to form carbamoyl phosphate, catalyzed by carbamoyl phosphate synthetase I (this occurs in the mitochondrial matrix).
2. Carbamoyl phosphate combines with ornithine to form citrulline, catalyzed by ornithine transcarbamoylase (still in the mitochondria).
3. Citrulline moves to the cytoplasm and combines with aspartate to form argininosuccinate, catalyzed by argininosuccinate synthetase.
4. Argininosuccinate is cleaved into fumarate and arginine by argininosuccinate lyase.
5. Arginase hydrolyzes arginine to produce urea and regenerate ornithine, which re-enters the cycle.

Notice that ornithine is both the starting and ending molecule, which is why it's called a cycle.

Glucogenic vs. Ketogenic Amino Acids

Not all amino acids are metabolized the same way. Their carbon skeletons determine what the body can convert them into.

• Glucogenic amino acids (e.g., alanine, glycine, serine) can be converted to glucose or glycogen through gluconeogenesis in the liver and kidneys. These are critical for maintaining blood glucose levels during fasting or prolonged exercise, when glycogen stores run low.
• Ketogenic amino acids (leucine and lysine are the only two that are exclusively ketogenic) are converted to ketone bodies such as acetoacetate and $\beta$-hydroxybutyrate. They cannot be converted to glucose. Ketone bodies serve as an alternative fuel for the brain and heart during prolonged fasting or on very low-carbohydrate diets.
• Both glucogenic and ketogenic: Some amino acids, including phenylalanine, tyrosine, isoleucine, and tryptophan, can feed into either pathway because their carbon skeletons yield both glucose precursors and ketone body precursors.

Proteins as an Alternative Energy Source

The body preferentially burns carbohydrates and fats for energy, reserving protein as a last resort to spare lean body mass. However, when carbohydrate intake is low (fasting, low-carb diets) or during prolonged exercise, the body will break down muscle proteins for fuel.

Here's how that process works:

1. Muscle proteins are broken down into amino acids.
2. Amino acids undergo deamination, which removes the amino group ($-NH_2$). That amino group is converted to urea via the urea cycle.
3. The remaining carbon skeleton enters metabolic pathways:
   • Glucogenic amino acids are converted to glucose through gluconeogenesis, helping fuel the brain and red blood cells (which depend on glucose).
   • Ketogenic amino acids are converted to ketone bodies, providing alternative fuel for the brain and heart.

Prolonged reliance on protein for energy has consequences. Excessive protein breakdown can lead to muscle wasting (cachexia, often seen in cancer or starvation) and impaired immune function due to decreased antibody production.

Protein Metabolism and Homeostasis

Several concepts tie protein metabolism together at the whole-body level:

• Protein synthesis is how cells build new proteins for growth, tissue repair, enzyme production, and maintenance. It requires all necessary amino acids to be available simultaneously.
• Essential amino acids are the nine amino acids the body cannot synthesize on its own. They must come from dietary sources (meat, eggs, legumes combined with grains, etc.). Without them, protein synthesis stalls.
• Transamination is a reaction where an amino group from one amino acid is transferred to a keto acid, creating a different amino acid. This allows the body to interconvert certain nonessential amino acids as needed. Enzymes called aminotransferases (or transaminases) catalyze these reactions, and they require vitamin $B_6$ as a coenzyme.
• Nitrogen balance compares nitrogen intake (from dietary protein) to nitrogen excretion (mainly as urea in urine). A positive nitrogen balance means more nitrogen is taken in than lost, which occurs during growth, pregnancy, or recovery from injury. A negative nitrogen balance means more is lost than consumed, indicating protein breakdown exceeds synthesis, as seen in starvation or severe illness.
• Protein turnover is the continuous cycle of protein synthesis and degradation happening in every cell. This process allows the body to replace damaged proteins, adapt to changing demands, and recycle amino acids.