General information on Phenylketonuria
Phenylketonuria, generally referred to as PKU, is an inherited disorder in humans arising from an inability to effectively metabolize an amino acid known as phenylalanine derived in protein foods. This is a rare condition found among geographic regions and ethnic groups to varying extents in different parts of the world.
Being an inherited disorder, it is mostly detected in early childhood. The condition is identified also by the names Deficiency Disease, Folling Disease, Folling’s Disease, Phenylalanine Hydroxylase Deficiency Disease, PH Deficiency and PKU. (“Medicine.net.com”). PKU is known to exist among newborns in the range of 1 in 10,000 to 1 in 15,000. (“Genetics Home Reference”).
A normal individual acquires the capability to break down phenylalanine amino acid using an enzyme known as phenylalanine hydroxylase (PAH) that gives instructions to break down amino acid phenylalanine to other compounds. If genetic mutations (“Genetics Home Reference”) reduce the activity of PAH, phenylalanine from the diet is not effectively metabolized. The most severe form of the disorder is known as “Classic PKU” in which the PAH activity is severely reduced or absent. Other conditions are “Variant PKU” or “non-PKU hyperphenylalaninemia.”(Medicine.net.com)
PKU has inherited from either or both the parents through gene transfers. In a patient affected by PKU, the amino acid phenylalanine, instead of being eliminated from the body through the normal metabolic process, accumulates in the blood. This affects the brain eventually leading to mental retardation, seizures and related complications.
A newborn child with PKU needs to be nurtured with special diets that are balanced and tolerant, to be able to grow normally. Depending on the severity of the condition, special diets have to be continued during adolescence and maybe throughout life. Regular monitoring of blood is also essential to maintain good health.
Identification of Amino acids by Titration method
Amino acid is characterized by a basic amino (NH2) group and an acidic Carboxylic (COOH) group. When dissolved in water the compound exists largely as zwitterions with positive and negative charges. When the solution is treated with an acid, the H+ ions in the acid will add to the COO- to form COOH. Similarly, when the zwitterions are treated with an alkali, will result in the loss of proton attached to the NH3 group to form NH2 (“Equilibrium curves of Amino acids”). The pH-dependent equilibrium can be represented as below:
H3N+—— COOH —–K1—–> H3N —–COO– —–K2—> H2N—- COO– zwitterions
The present assignment assumes that Amino acids separated from foodstuff are available as amino acid hydrochlorides. An experiment is done for determining the abrupt change in pH when titrated with Sodium hydroxide.
Weighed quantities of the unknown Amino acid hydrochlorides (Say 0.2 gm) in progressive experiments and placed them in a 150 ml beaker and 50 ml of distilled water is added to it. It is placed on a stand and heated until the solid completely dissolves. The electrode of a pH meter is placed in the solution and the reading is noted.
NaOH solution (0.1M) is prepared, taken in a burette and set up for titration. A magnetic stirrer is also placed in the beaker for stirring during titration. The NaOH is gradually added simultaneously noting the progressive readings of additions of NaOH and noting the corresponding pH readings. The pH reading progresses slowly at the initial stages of addiction and towards the end, the addition has to be made drop by drop so that the endpoint is not crossed. Note the point when the pH changes suddenly.
The volume of NaOH added at stages with corresponding pH readings are plotted on a graph with the volume of NaOH on the X-axis and pH on the Y-axis. The equivalence point can be seen from this graph where the pH steeply rises. The molecular equivalent of NaOH corresponds to the molecular equivalent of the amino acid hydrochloride. The molecular weight of the amino acid hydrochloride (and that of the amino acid) can thus be calculated. From a chart giving the value of the molecular weight and the pH value of various amino acids, the identity of the unknown amino acid can be known.
The amino acid to be tested for the absence of special diets to be recommended for persons having PKU is Phenylalanine. The molecular mass of Phenylalanine is 165.9 and the corresponding equilibrium pH is 9.31(“Equilibrium curves of Amino acids”).
References
“Equilibrium curves of Amino Acids”. Web.
“Genetics Home Reference”. Web.
“Medicine” . Web.