NS230 Unit 7 Protein Turnover Analysis Example

The Nitrogen That Escapes the Loop: A Unit 7 Protein Turnover Analysis From the Amino Acid Pool to Urea

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Purdue University Global

NS230: Macronutrient Metabolism

Unit 7 Protein Turnover Analysis

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Quantities are rounded textbook estimates for an adult; the biochemistry and nitrogen metabolism sources cited are real.

Turnover: The Loop

The body's proteins are constantly broken down and rebuilt. An adult turns over roughly 250 to 300 grams of body protein a day, several times more than the 60 to 100 grams most adults eat. The amino acids released by breakdown join those absorbed from food in a small free amino acid pool, and most are reused to make new proteins (Wu, 2016).

Turnover is not waste. It lets the body replace damaged proteins, change the amounts of enzymes as conditions change and supply amino acids to tissues that need them. But the loop is not perfectly closed. Some amino acids leave the pool for good, burned as fuel or remade into glucose or fat, and their nitrogen then has to be removed and excreted. In an adult in nitrogen balance, the nitrogen excreted each day, mostly as urea, roughly equals the nitrogen eaten.

Step 1: Removing Nitrogen by Transamination

The first step in disposing of an amino acid's nitrogen is usually transamination. Aminotransferases, with vitamin B6 in the form of pyridoxal phosphate as cofactor, move the amino group from an amino acid to alpha-ketoglutarate, forming glutamate and leaving the amino acid's carbon skeleton as a keto acid. For example, alanine aminotransferase converts alanine and alpha-ketoglutarate to pyruvate and glutamate.

Transamination does not dispose of nitrogen; it collects it. Most amino groups end up on glutamate.

Step 2: Releasing Nitrogen by Deamination

In the liver, glutamate dehydrogenase removes the amino group from glutamate as free ammonia, regenerating alpha-ketoglutarate and producing NADH or NADPH. Ammonia is toxic, particularly to the brain, so it must be converted quickly to a safe form. In the liver, it enters the urea cycle (Berg et al., 2019).

Step 3: Carrying Nitrogen From Other Tissues

Muscle and other tissues also break down amino acids, especially during fasting and exercise, but they cannot make urea. They send their nitrogen to the liver in two safe carriers.

Alanine. Muscle transfers amino groups to pyruvate, forming alanine, which travels in the blood to the liver. There, alanine is transaminated back to pyruvate, which can be used to make glucose, and its nitrogen enters the urea cycle. This exchange, glucose to muscle and alanine to liver, is the glucose-alanine cycle (Felig, 1973).

Glutamine. Many tissues combine ammonia with glutamate to form glutamine, using glutamine synthetase and ATP. Glutamine carries two nitrogen atoms safely in the blood to the liver, kidney and intestine, where glutaminase releases the ammonia. The kidney uses some of this ammonia to excrete acid.

Step 4: The Urea Cycle

In the liver, ammonia and carbon dioxide combine to form carbamoyl phosphate, using 2 ATP, in a reaction catalyzed by carbamoyl phosphate synthetase I in the mitochondria. This enzyme requires N-acetylglutamate as an activator, which links the cycle's speed to the supply of amino acids. Carbamoyl phosphate joins ornithine to form citrulline, which leaves the mitochondrion. In the cytosol, citrulline joins aspartate, which supplies the second nitrogen, to form argininosuccinate, using ATP. Argininosuccinate is split into arginine and fumarate. Arginase then cleaves arginine into urea and ornithine, which returns to the mitochondrion to begin the cycle again (Morris, 2002).

Each urea molecule carries two nitrogen atoms, one from ammonia and one from aspartate. The cycle costs the equivalent of four high-energy phosphate bonds per urea. Urea travels in the blood to the kidneys and is excreted in urine.

Urea production is not a measure of how much protein the body breaks down, which is large and mostly recycled, but of how much nitrogen escapes the recycling loop, which is why a person in balance excretes about as much nitrogen as they eat.

Why the Body Uses Two Carriers

Alanine and glutamine serve somewhat different roles. Alanine carries nitrogen together with a carbon skeleton the liver can turn into glucose, so it links muscle protein breakdown directly to glucose supply during fasting. Glutamine carries two nitrogens and serves as fuel for the intestine and immune cells, and its nitrogen supports the kidney's handling of acid. Having two carriers lets the body route nitrogen to different organs depending on need: mostly to the liver for urea during fasting, and to the kidney when the body must excrete acid.

Measuring Turnover

Turnover cannot be seen by measuring urine nitrogen alone, which reflects only net loss. Researchers measure it by giving amino acids labeled with stable isotopes and tracking how quickly the label appears in body proteins and in breakdown products. These methods show that turnover rates differ by tissue: the intestine and liver turn over their proteins quickly, in days, while muscle turns over more slowly but, because of its large mass, accounts for a large share of the body's total.

Regulation by Diet

The urea cycle adapts to protein intake. On a high-protein diet, the liver increases the amounts of urea cycle enzymes over several days, and more N-acetylglutamate is made, speeding the first step. On a low-protein diet, the enzymes decrease and less nitrogen is lost (Morris, 2002). During prolonged fasting, muscle protein breakdown supplies amino acids for gluconeogenesis, and urea excretion initially rises; as fasting continues and the brain shifts to ketone bodies, protein breakdown and urea excretion fall, sparing muscle (Cahill, 2006).

A Worked Example

An adult eats 90 grams of protein a day, about 14 grams of nitrogen, since protein is roughly 16 percent nitrogen. In balance, about 14 grams of nitrogen leave the body daily, most as urea in urine and smaller amounts in feces, skin and other routes. Meanwhile, about 250 grams of body protein are broken down and largely rebuilt. The nitrogen in urine reflects the 90 grams eaten, not the 250 grams turned over.

Why Vitamin B6 Matters Here

Every aminotransferase needs pyridoxal phosphate, the active form of vitamin B6, to carry the amino group between molecules. A diet low in vitamin B6 can impair transamination, and with it the body's ability to move nitrogen and to make nonessential amino acids. This is one of several points where a micronutrient is essential to macronutrient metabolism, a link the course's micronutrient units develop.

Clinical Relevance

When the liver cannot run the urea cycle, as in severe liver disease or inherited enzyme deficiencies, ammonia rises in the blood and can cause confusion and, in severe cases, coma. Treatment in such cases may include limiting protein intake under medical supervision.

References

Berg, J. M., Tymoczko, J. L., Gatto, G. J., Jr., & Stryer, L. (2019). Biochemistry (9th ed.). W. H. Freeman.

Cahill, G. F., Jr. (2006). Fuel metabolism in starvation. Annual Review of Nutrition, 26, 1-22. https://doi.org/10.1146/annurev.nutr.26.061505.111258

Felig, P. (1973). The glucose-alanine cycle. Metabolism, 22(2), 179-207. https://doi.org/10.1016/0026-0495(73)90269-2

Morris, S. M., Jr. (2002). Regulation of enzymes of the urea cycle and arginine metabolism. Annual Review of Nutrition, 22, 87-105. https://doi.org/10.1146/annurev.nutr.22.110801.140547

Wu, G. (2016). Dietary protein intake and human health. Food & Function, 7(3), 1251-1265. https://doi.org/10.1039/C5FO01530H