The deficiency of the enzyme phenylalanine hydroxylase PAH affects the biosynthesis of amino acids in human metabolism by disrupting the normal conversion of phenylalanine to tyrosine. This deficiency leads to a condition known as phenylketonuria PKU .Phenylalanine hydroxylase is responsible for converting the essential amino acid phenylalanine into another amino acid, tyrosine, in the liver. This conversion is a crucial step in the catabolism of phenylalanine and the biosynthesis of other important molecules such as neurotransmitters and hormones.When there is a deficiency of PAH, phenylalanine accumulates in the blood and body tissues, leading to abnormally high levels of phenylalanine hyperphenylalaninemia . This excess phenylalanine can be toxic to the brain and other organs, as it interferes with the normal transport of other amino acids across the blood-brain barrier, ultimately affecting the synthesis of proteins, neurotransmitters, and other essential molecules.The potential health consequences of PAH deficiency and PKU include:1. Intellectual disability: High levels of phenylalanine in the brain can lead to intellectual disability, as it impairs the normal development and function of the brain.2. Neurological problems: PKU can cause various neurological issues such as seizures, tremors, and poor coordination.3. Behavioral and psychiatric disorders: Children with PKU may exhibit behavioral problems, hyperactivity, and psychiatric disorders such as depression and anxiety.4. Skin and hair abnormalities: Due to the reduced production of melanin, individuals with PKU may have lighter skin, hair, and eye color compared to their non-affected family members.5. Growth and developmental delays: Children with PKU may experience growth and developmental delays, as well as failure to thrive.6. Musty body odor: Excess phenylalanine can lead to the production of phenylacetic acid, which has a musty odor and can be detected in the breath, urine, and sweat of individuals with PKU.Early diagnosis and treatment of PKU through a strict low-phenylalanine diet can help prevent or minimize these health consequences. This diet involves the restriction of high-phenylalanine foods such as meat, fish, eggs, dairy products, and certain grains, as well as the use of specialized medical foods and supplements to provide adequate nutrition. Regular monitoring of blood phenylalanine levels is essential to ensure proper management of the condition.