Question

Gluten is a mixture of proteins abundant in wheat, barley, and rye. Although these grains are staples in the Western diet, almost 1 percent of Americans have celiac disease- a disorder of the intestines caused by an abnormal immune response after eating gluten. This immune response damages the fingerlike villi of the small intestine, which can lead to intestinal upset and malnourishment. Currently, the only treatment is to maintain a strict gluten- free diet. One of the peptides that can be recovered after gluten digestion is 33 residues long; 13 of the 33 residues are proline. How many times would you expect proline to appear in this peptide if it were made up of a completely random assortment of the 20 most common amino acids?

Short answer

You would expect proline to appear 1.65 times in a 33-residue long peptide if it were made up of a completely random assortment of the 20 most common amino acids.

Step-by-step solution

Calculate the probability of proline occurrence

The peptide has a random assortment of the 20 most common amino acids, so the probability of a single residue being proline is \(\frac{1}{20}\), as there is an equal chance of each amino acid occurring.

Calculate expected number of prolines in the peptide

To calculate the expected number of occurrences of proline in the 33-residue long peptide, multiply the probability of proline (calculated in the previous step) by the total length of the peptide (33 residues). The expected number of prolines can be calculated as follows: Expected number of prolines = (Probability of proline) * (Total length of the peptide) = \(\frac{1}{20}\) * 33 = \(\frac{33}{20}\) = 1.65 Thus, you would expect proline to appear 1.65 times in a 33-residue long peptide if it were made up of a completely random assortment of the 20 most common amino acids.

Key concepts

Gluten

Gluten is a protein complex found primarily in wheat, barley, and rye. It plays a significant role in the texture of dough, making it elastic and helping baked goods rise. For most people, gluten is harmless. However, for those with celiac disease, consuming gluten triggers adverse health effects.
Gluten includes two main proteins, gliadin and glutenin. The digestion of gluten results in various peptides, including some that are rich in proline residues. Proline's unique structure makes it resistant to full digestion, resulting in longer peptide chains in the gut. These peptides can be problematic because they are often the ones eliciting immune responses in individuals with celiac disease.
It's important to understand the role of gluten in food products and the reason why some individuals need to avoid it entirely to manage their health.

Amino Acids

Amino acids are essential building blocks of proteins. There are 20 common amino acids that combine in various sequences to form proteins, like those found in gluten. Each amino acid has an amino group ( ext{-NH}_2 ext{)}, a carboxyl group ( ext{-COOH} ext{)}, and a unique side chain that defines its properties.
In the context of gluten peptides, one of the amino acids, proline, plays a special role. Proline's distinctive cyclic structure imparts rigidity to proteins and often hinders their digestion.
Understanding amino acids helps in grasping why certain peptides, such as those in gluten, resist breakdown in the digestive system and contribute to conditions like celiac disease.

Immune Response

In individuals with celiac disease, ingestion of gluten peptides leads to an abnormal immune response. The body identifies these peptides as foreign, triggering inflammatory reactions in the small intestine.
This response involves the immune system producing antibodies against components of gluten or tissue transglutaminase, an enzyme that modifies gluten protein. This reaction eventually leads to the destruction of intestinal structures.
This immune response is not typical and is distinct to those with celiac disease, where even tiny amounts of gluten can lead to significant intestinal damage.

Intestinal Villi

Intestinal villi are small, fingerlike projections lining the small intestine. They play a critical role in nutrient absorption by increasing the surface area of the gut.
In celiac disease, the immune response triggered by gluten leads to damage and blunting of the villi. This damage results in a reduced ability to absorb nutrients, leading to symptoms such as malnutrition, weight loss, and diarrhea.
The healing and regeneration of these villi are possible, but only through strict adherence to a gluten-free diet. Understanding the function of intestinal villi highlights the importance of maintaining gut health, especially for individuals with celiac disease.