Proteins Health Article

Common diseases and disorders

The metabolic pathways in the body for protein metabolism and energy metabolism are interrelated. Certain metabolic conditions distort this relationship, namely diabetes, kidney failure, fever, cancer, and liver cirrhosis.

Inborn errors of metabolism (also called human hereditary biochemical disorders) have genetic origins; these errors interfere with the synthesis of proteins, carbohydrates, fats, enzymes, and many other substances in the body. Abnormalities in the breakdown, storage, or production of proteins, fats and carbohydrates, or in the energy cycles of cells are typically the manifestation of this disorder. Disease and death may result from the absence or excess of normal or abnormal metabolites. For example, phenylketonuria (PKU) is a hereditary condition in which phenylalanine (an amino acid) is not properly metabolized. PKU may cause severe mental retardation. Some inborn errors of metabolism require dietary and/or nutrient modification depending on the specific metabolic error. A low-phenylalanine diet is normal treatment for PKU.

Celiac disease, also known as nontropical sprue, gluten enteropathy, or celiac sprue, is an inherited disorder resulting in malabsorption because of an allergic reaction after consumption of a protein called gluten. This intolerance causes patients with celiac disease to suffer weight loss, diarrhea, malnutrition, and bloating. By eliminating foods containing gluten from the diet, further damage to the intestines can be prevented, symptoms are relieved, and malabsorption of nutrients is corrected. Gluten is found in wheat, rye, barley, and oats. Registered dietitians and physicians can assist the patient with the diet modifications needed for each disease.

Other conditions that may occur due to protein metabolism or absorption abnormalities include:

• muscle wasting and atrophy, which may occur when there is decreased protein absorption and metabolism due to causes such as malabsorption syndrome
• edema (fluid retention in the body's tissues) due to decreased protein absorption
• malnutrition and weight loss due to decreased fat, carbohydrate, and protein absorption

Protein-calorie malnutrition (or protein-energy malnutrition) is a condition associated most closely with weight loss, starvation, or illness and is common in cancer patients. It occurs when a lack of protein and calories are consumed to sustain body composition. When inadequate calories are consumed, the body's functionality declines, which may lead to illness and perhaps death. Exhaustion, weakness, decreased resistance to infection, and a progressive wasting of body muscle and fat stores occur.

Certain conditions may require protein restrictions; for example, acute liver or kidney failure, and uremia (increased urea in the blood).

Powder and tablet forms of amino acids have become popular as health supplements. But their prolonged excessive use can upset the natural amino acid balance and lead to kidney, liver, and nervous system damage. Do not take these supplements without first consulting a registered dietitian or physician.

KEY TERMS

Alpha helix—A type of secondary structure in which a single peptide chain arranges itself in a three-dimensional spiral.

Beta sheet—A type of secondary structure in which several peptide chains arrange themselves alongside each other.

Cachexia—A condition in which the body weight "wastes" away, characterized by a constant loss of weight, muscle, and fat.

Creatine—A substance found in skeletal muscles; it is produced by other amino acids.

Domain—A relatively compact region of a protein, separated from other domains by short stretches in which the protein chain is more or less extended; different domains often carry out distinct parts of the protein's overall function.

Enzymes—Enzymes are protein catalysts that increase the speed of chemical reactions in the cell without themselves being changed.

Hormones—Hormones are messengers that travel to tissues or organs, where they may stimulate or inhibit specific metabolic pathways.

Messenger ribonucleic acid (mRNA)—A molecule of RNA that carries the genetic information for producing one or more proteins; mRNA is produced by copying one strand of DNA, but is able to move from the nucleus to the cytoplasm (where protein synthesis takes place).

Peptide bond—A chemical bond between the carboxyl group of one amino acid and the amino nitrogen atom of another.

Phenylketonuria (PKU)—A rare hereditary condition in which phenylalanine (an amino acid) is not properly metabolized. PKU may cause severe mental retardation.

Polypeptide—A group of amino acids joined by peptide bonds; proteins are large polypeptides, but no agreement exists regarding how large they must be to justify the name.

Primary structure—The linear sequence of amino acids making up a protein.

Quaternary structure—The number and type of protein chains normally associated with each other in the body.

Protein-calorie malnutrition—A lack of protein and calories are consumed to sustain the body composition, resulting in weight loss and muscle wasting.

Ribosome—A very large assemblage of RNA and protein that, using instructions from mRNA, synthesizes new protein molecules.

Secondary structure—Certain highly regular three-dimensional arrangements of amino acids within a protein.

Tertiary structure—A protein molecule's overall three-dimensional shape.

Transfer ribonucleic acid (tRNA)—A small RNA molecule, specific for a single amino acid, that transports that amino acid to the proper spot on the ribosome for assembly into the growing protein chain.

Wasting—When inadequate calories are consumed, it can lead to "wasting" or depletion of body mass. Wasting results in weight loss in such tissues as skeletal muscle and adipose tissue (fat).