Vitamin B6 is one of the B vitamins, and thus an essential nutrient. The term refers to a group of six chemically similar compounds, i.e., "vitamers", which can be interconverted in biological systems. Its active form, pyridoxal 5′-phosphate, serves as a coenzyme in more than 140 enzyme reactions in amino acid, glucose, and lipid metabolism.
Methionine is an essential amino acid in humans.
Homocysteine or Hcy: is a non-proteinogenic α-amino acid. It is a homologue of the amino acid cysteine, differing by an additional methylene bridge (-CH2-). It is biosynthesized from methionine by the removal of its terminal Cε methyl group. In the body, homocysteine can be recycled into methionine or converted into cysteine with the aid of vitamin B6, B9, and B12.
Methylmalonic acidemias, also called methylmalonic acidurias, are a group of inherited metabolic disorders, that prevent the body from properly breaking down proteins and fats. This leads to a buildup of a toxic level of methylmalonic acid in body liquids and tissues. Due to the disturbed branched-chain amino acids (BCAA) metabolism, they are among the classical organic acidemias.
Homocystinuria (HCU) is an inherited disorder of the metabolism of the amino acid methionine due to a deficiency of cystathionine beta synthase or methionine synthase. It is an inherited autosomal recessive trait, which means a child needs to inherit a copy of the defective gene from both parents to be affected. Symptoms of homocystinuria can also be caused by a deficiency of vitamins B6, B12, or folate.
A heterozygote advantage describes the case in which the heterozygous genotype has a higher relative fitness than either the homozygous dominant or homozygous recessive genotype. Loci exhibiting heterozygote advantage are a small minority of loci. The specific case of heterozygote advantage due to a single locus is known as overdominance. Overdominance is a rare condition in genetics where the phenotype of the heterozygote lies outside of the phenotypical range of both homozygote parents, and heterozygous individuals have a higher fitness than homozygous individuals.
Hypophosphatasia (; also called deficiency of alkaline phosphatase, phosphoethanolaminuria, or Rathbun's syndrome; sometimes abbreviated HPP) is a rare, and sometimes fatal, inherited metabolic bone disease. Clinical symptoms are heterogeneous, ranging from the rapidly fatal, perinatal variant, with profound skeletal hypomineralization, respiratory compromise or vitamin B6 dependent seizures to a milder, progressive osteomalacia later in life. Tissue non-specific alkaline phosphatase (TNSALP) deficiency in osteoblasts and chondrocytes impairs bone mineralization, leading to rickets or osteomalacia. The pathognomonic finding is subnormal serum activity of the TNSALP enzyme, which is caused by one of 388 genetic mutations identified to date, in the gene encoding TNSALP. Genetic inheritance is autosomal recessive for the perinatal and infantile forms but either autosomal recessive or autosomal dominant in the milder forms.
Hypoprothrombinemia is a rare blood disorder in which a deficiency in immunoreactive prothrombin, produced in the liver, results in an impaired blood clotting reaction, leading to an increased physiological risk for spontaneous bleeding. This condition can be observed in the gastrointestinal system, cranial vault, and superficial integumentary system, affecting both the male and female population. Prothrombin is a critical protein that is involved in the process of hemostasis, as well as illustrating procoagulant activities. This condition is characterized as an autosomal recessive inheritance congenital coagulation disorder affecting 1 per 2,000,000 of the population, worldwide, but is also attributed as acquired.
Tetrahydrobiopterin deficiency (THBD, BH4D) is a rare metabolic disorder that increases the blood levels of phenylalanine. Phenylalanine is an amino acid obtained normally through the diet, but can be harmful if excess levels build up, causing intellectual disability and other serious health problems. In healthy individuals, it is metabolised (hydroxylated) into tyrosine, another amino acid, by phenylalanine hydroxylase. However, this enzyme requires tetrahydrobiopterin as a cofactor and thus its deficiency slows phenylalanine metabolism.
Methylmalonyl-CoA mutase is a mitochondrial homodimer apoenzyme that focuses on the catalysis of methylmalonyl CoA to succinyl CoA. The enzyme is bound to adenosylcobalamin, a hormonal derivative of vitamin B12 in order to function. Methylmalonyl-CoA mutase deficiency is caused by genetic defect in the MUT gene responsible for encoding the enzyme. Deficiency in this enzyme accounts for 60% of the cases of methylmalonic acidemia.
Hyperhomocysteinemia is a medical condition characterized by an abnormally high level of total homocysteine in the blood, conventionally described as above 15 μmol/L.
Dihydropyrimidine dehydrogenase deficiency is an autosomal recessive metabolic disorder in which there is absent or significantly decreased activity of dihydropyrimidine dehydrogenase, an enzyme involved in the metabolism of uracil and thymine.
In medical genetics, compound heterozygosity is the condition of having two or more heterogeneous recessive alleles at a particular locus that can cause genetic disease in a heterozygous state; that is, an organism is a compound heterozygote when it has two recessive alleles for the same gene, but with those two alleles being different from each other. Compound heterozygosity reflects the diversity of the mutation base for many autosomal recessive genetic disorders; mutations in most disease-causing genes have arisen many times. This means that many cases of disease arise in individuals who have two unrelated alleles, who technically are heterozygotes, but both the alleles are defective.
Cystathionine-β-synthase, also known as CBS, is an enzyme (EC 4.2.1.22) that in humans is encoded by the CBS gene. It catalyzes the first step of the transsulfuration pathway, from homocysteine to cystathionine:
Cystathionine is an intermediate in the synthesis of cysteine from homocysteine. It is produced by the transsulfuration pathway and is converted into cysteine by cystathionine gamma-lyase (CTH).
The enzyme cystathionine γ-lyase (EC 4.4.1.1, CTH or CSE; also cystathionase; systematic name L-cystathionine cysteine-lyase (deaminating; 2-oxobutanoate-forming)) breaks down cystathionine into cysteine, 2-oxobutanoate (α-ketobutyrate), and ammonia:
The transsulfuration pathway is a metabolic pathway involving the interconversion of cysteine and homocysteine through the intermediate cystathionine. Two transsulfurylation pathways are known: the forward and the reverse.
Methionine synthase reductase, also known as MSR, is an enzyme that in humans is encoded by the MTRR gene.
Pyridoxine-dependent epilepsy (PDE) is a rare genetic disorder characterized by intractable seizures in the prenatal and neonatal period. The disorder was first recognized in the 1950s, with the first description provided by Hunt et al. in 1954. More recently, pathogenic variants within the ALDH7A1 gene have been identified to cause PDE.
Methylenetetrahydrofolate reductase deficiency is the most common genetic cause of elevated serum levels of homocysteine (hyperhomocysteinemia). It is caused by genetic defects in MTHFR, which is an important enzyme in the methyl cycle.
