- Amino acids are organic compounds that combine to form
proteins. When proteins are digested, amino acids are left. The human body
requires a number of amino acids to grow and breakdown food.
Amino acids are classified into two groups:
1-Essential amino acids
- An essential amino acid for an organism is an amino acid
that cannot be synthesized by the organism from other available resources,
and therefore must be supplied as part of its diet.Eight amino acids are
generally regarded as essential for humans: tryptophan, lysine, methionine,
phenylalanine, threonine, valine, leucine, and isoleucine. Two others,
histidine and arginine are essential only in children.
-
- Essential amino acids include:
-
Arginine-Histidine-soleucine-Leucine-Lysine-Methionine-Phenylalanine-Threonine-Tryptophan
-Valine
2-Nonessential amino acids- Nonessential amino acids are made by the body from the
essential amino acids. They include aspartic acid, glutamic acid, and
glycine.
Hereditary Metabolic Disorders Related to
Amino Acids
Amino acids are the building blocks of proteins and have many functions in
the body. Hereditary disorders of amino acid processing can be the result
of defects either in the breakdown of amino acids or in the body's ability
to get the amino acids into cells. Because these disorders produce
symptoms early in life, newborns are routinely screened for several common
ones:
Phenylketonuria
Phenylketonuria (PKU) is a disorder that causes a buildup of the amino
acid phenylalanine, which is an essential amino acid that cannot be
synthesized in the body but is present in food. Excess phenylalanine is
normally converted to tyrosine, another amino acid, and eliminated from
the body. Without the enzyme that converts it to tyrosine, phenylalanine
builds up in the blood and is toxic to the brain, causing mental
retardation.
Most affected newborns are detected during routine screening tests.
Newborns with PKU rarely have symptoms right away, although sometimes an
infant is sleepy or eats poorly. If not treated, affected infants
progressively develop mental retardation over the first few years of life,
which eventually becomes severe. Other symptoms include seizures, nausea
and vomiting, an eczema-like rash, lighter skin and hair than their family
members, aggressive or self-injurious behavior, hyperactivity, and
sometimes psychiatric symptoms. Untreated children often give off a
"mousy" body and urine odor as a result of a by-product of phenylalanine (phenylacetic
acid) in their urine and sweat.
Maple Syrup Urine Disease
Children with maple syrup urine disease are unable to metabolize certain
amino acids. By-products of these amino acids build up, causing neurologic
changes, including seizures and mental retardation. These by-products also
cause body fluids, such as urine and sweat, to smell like maple syrup.
This disease is most common among Mennonite families.
There are many forms of maple syrup urine disease; symptoms vary in
severity. In the most severe form, infants develop neurologic
abnormalities, including seizures and coma, during the first week of life
and can die within days to weeks. In the milder forms, children initially
appear normal but develop vomiting, staggering, confusion, coma, and the
odor of maple syrup particularly during physical stress, such as infection
or surgery.
Homocystinuria
Children with homocystinuria are unable to metabolize the amino acid
homocysteine, which, along with certain toxic by-products, builds up to
cause a variety of symptoms. Symptoms may be mild or severe, depending on
the particular enzyme defect.
Infants with this disorder are normal at birth. The first symptoms,
including dislocation of the lens of the eye, causing severely decreased
vision, usually begin after 3 years of age. Most children have skeletal
abnormalities, including osteoporosis; the child is usually tall and thin
with a curved spine, elongated limbs, and long, spiderlike fingers.
Psychiatric and behavioral disorders and mental retardation are common.
Homocystinuria makes the blood more likely to spontaneously clot,
resulting in strokes, high blood pressure, and many other serious
problems.
Tyrosinemia
Children with tyrosinemia are unable to completely metabolize the amino
acid tyrosine. By-products of this amino acid build up, causing a variety
of symptoms. In some states, the disorder is detected on the newborn
screening tests.
There are two main types of tyrosinemia, I and II:
Type I tyrosinemia is most common in children
of French-Canadian or Scandinavian descent. Children with this disorder
typically become ill sometime within the first year of life with
dysfunction of the liver, kidneys, and nerves, resulting in irritability,
rickets, or even liver failure and death. Restriction of tyrosine in the
diet is of little help. An experimental drug, which blocks production of
toxic metabolites, may help children with type I tyrosinemia. Often,
children with type I tyrosinemia require a liver transplant.
Type II tyrosinemia is less common. Affected
children sometimes have mental retardation and frequently develop sores on
the skin and eyes. Unlike type I tyrosinemia, restriction of tyrosine in
the diet can prevent problems from developing.
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