With low levels of the pyruvate dehydrogenase complex PDC , the rate of cycling through the Krebs cycle is dramatically reduced. This results in a decrease in the total amount of energy that is produced by the cells of the body. PDC deficiency results in a neurodegenerative disease that ranges in severity, depending on the levels of the PDC enzyme.
It may cause developmental defects, muscle spasms, and death. Treatments can include diet modification, vitamin supplementation, and gene therapy; however, damage to the central nervous system usually cannot be reversed.
PKU affects about 1 in every 15, births in the United States. People afflicted with PKU lack sufficient activity of the enzyme phenylalanine hydroxylase and are therefore unable to break down phenylalanine into tyrosine adequately. Because of this, levels of phenylalanine rise to toxic levels in the body, which results in damage to the central nervous system and brain. Symptoms include delayed neurological development, hyperactivity, mental retardation, seizures, skin rash, tremors, and uncontrolled movements of the arms and legs.
Pregnant women with PKU are at a high risk for exposing the fetus to too much phenylalanine, which can cross the placenta and affect fetal development. The earlier a modified diet is begun, the less severe the symptoms will be. The person must closely follow a strict diet that is low in phenylalanine to avoid symptoms and damage.
Phenylalanine is found in high concentrations in artificial sweeteners, including aspartame. Therefore, these sweeteners must be avoided. Some animal products and certain starches are also high in phenylalanine, and intake of these foods should be carefully monitored. Digestion of proteins begins in the stomach, where HCl and pepsin begin the process of breaking down proteins into their constituent amino acids.
As the chyme enters the small intestine, it mixes with bicarbonate and digestive enzymes. The bicarbonate neutralizes the acidic HCl, and the digestive enzymes break down the proteins into smaller peptides and amino acids.
Digestive hormones secretin and CCK are released from the small intestine to aid in digestive processes, and digestive proenzymes are released from the pancreas trypsinogen and chymotrypsinogen. Enterokinase, an enzyme located in the wall of the small intestine, activates trypsin, which in turn activates chymotrypsin. These enzymes liberate the individual amino acids that are then transported via sodium-amino acid transporters across the intestinal wall into the cell.
The amino acids are then transported into the bloodstream for dispersal to the liver and cells throughout the body to be used to create new proteins. When in excess, the amino acids are processed and stored as glucose or ketones. The nitrogen waste that is liberated in this process is converted to urea in the urea acid cycle and eliminated in the urine.
In times of starvation, amino acids can be used as an energy source and processed through the Krebs cycle. Answer the question s below to see how well you understand the topics covered in the previous section. Skip to main content. Module 8: Metabolism and Nutrition. Search for:. Protein Metabolism Learning Objectives By the end of this section, you will be able to: Describe how the body digests proteins Explain how the urea cycle prevents toxic concentrations of nitrogen Differentiate between glucogenic and ketogenic amino acids Explain how protein can be used for energy.
Critical Thinking Questions Amino acids are not stored in the body. Most branch chain amino acids pass through the liver. Amino acids are unique because they contain nitrogen.
Several things can happen to the nitrogen. First, it can remain on the molecule and be incorporated into the product that cell is making, for example, a polypeptide. The nitrogen may be transaminated, in other words, the amine group NH 2 is transferred to another carbon skeleton to form a new amino acid.
An example would be the transfer of the amine from the non-essential amino acid, alanine, to alpha-ketoglutaric acid to make glutamic acid, another non-essential amino acid.
The water-soluble vitamin B 6 is needed for this process. The amine group may be removed from the amino acid in a process known as deamination. This process is used for the excretion of the nitrogen, and the carbon skeleton is used to produce energy. Again, vitamin B 6 is needed for this process. The nitrogen removed from amino acids is excreted via several different routes.
The most familiar path is urine where most of the nitrogen is in the form of urea. Nitrogen is also excreted in the feces, skin, hair, and nails. In skin, hair, and nails the nitrogen is bound to protein as this is the building block of each.
Just as some plastics can be recycled to make new products, amino acids are recycled to make new proteins. All cells in the body continually break down proteins and build new ones, a process referred to as protein turnover. Every day over grams of protein in your body are dismantled and grams of new protein are built. Amino acids are used not only to build proteins, but also to build other biological molecules containing nitrogen, such as DNA and RNA, and to some extent to produce energy.
It is critical to maintaining amino acid levels within this cellular pool by consuming high-quality proteins in the diet, or the amino acids needed for building new proteins will be obtained by increasing protein destruction from other tissues within the body, especially muscle. This amino acid pool is less than one percent of total body protein content. Thus, the body does not store protein as it does with carbohydrates as glycogen in the muscles and liver and lipids as triglycerides in adipose tissue.
The amino acids in this pool need to be replenished because amino acids are outsourced to make new proteins, energy, and other biological molecules. Skills to Develop Discuss how proteins are digested and absorbed by our bodies. From the Mouth to the Stomach Unless you are eating it raw, the first step in egg digestion or any other protein food involves chewing.
From the Stomach to the Small Intestine The stomach empties the chyme containing the broken down egg pieces into the small intestine, where the majority of protein digestion occurs. When the blood sugar level falls below that range, which may happen between meals, the body has at least three ways of reacting:. Other hormones can raise the blood sugar level, including epinephrine also called adrenaline and cortisol released by the adrenal glands and growth hormone released by the pituitary gland.
Author: Healthwise Staff. Medical Review: E. This information does not replace the advice of a doctor. Healthwise, Incorporated, disclaims any warranty or liability for your use of this information. Your use of this information means that you agree to the Terms of Use. Learn how we develop our content. To learn more about Healthwise, visit Healthwise.
Healthwise, Healthwise for every health decision, and the Healthwise logo are trademarks of Healthwise, Incorporated. Updated visitor guidelines. Top of the page. Carbohydrates, Proteins, Fats, and Blood Sugar. Topic Overview The body uses three main nutrients to function— carbohydrate , protein , and fat.
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