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.
Pernicious anemia is a disease where not enough red blood cells are produced due to a deficiency of vitamin B12. Those affected often have a gradual onset. The most common initial symptoms are feeling tired and weak. Other symptoms may include shortness of breath, feeling faint, a smooth red tongue, pale skin, chest pain, nausea and vomiting, loss of appetite, heartburn, numbness in the hands and feet, difficulty walking, memory loss, muscle weakness, poor reflexes, blurred vision, clumsiness, depression, and confusion. Without treatment, some of these problems may become permanent.
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 or 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.
The Schilling test was a medical investigation used for patients with vitamin B12 (cobalamin) deficiency. The purpose of the test was to determine how well a patient is able to absorb B12 from their intestinal tract. The test is now considered obsolete and is rarely performed, and is no longer available at many medical centers. It is named for Robert F. Schilling.
Megaloblastic anemia is a type of macrocytic anemia. An anemia is a red blood cell defect that can lead to an undersupply of oxygen. Megaloblastic anemia results from inhibition of DNA synthesis during red blood cell production. When DNA synthesis is impaired, the cell cycle cannot progress from the G2 growth stage to the mitosis (M) stage. This leads to continuing cell growth without division, which presents as macrocytosis. Megaloblastic anemia has a rather slow onset, especially when compared to that of other anemias. The defect in red cell DNA synthesis is most often due to hypovitaminosis, specifically vitamin B12 deficiency or folate deficiency. Loss of micronutrients may also be a cause.
Methionine synthase (MS, MeSe, MTR) is primarily responsible for the regeneration of methionine from homocysteine in most individuals. In humans it is encoded by the MTR gene (5-methyltetrahydrofolate-homocysteine methyltransferase). Methionine synthase forms part of the S-adenosylmethionine (SAMe) biosynthesis and regeneration cycle, and is the enzyme responsible for linking the cycle to one-carbon metabolism via the folate cycle. There are two primary forms of this enzyme, the Vitamin B12 (cobalamin)-dependent (MetH) and independent (MetE) forms, although minimal core methionine synthases that do not fit cleanly into either category have also been described in some anaerobic bacteria. The two dominant forms of the enzymes appear to be evolutionary independent and rely on considerably different chemical mechanisms. Mammals and other higher eukaryotes express only the cobalamin-dependent form. In contrast, the distribution of the two forms in Archaeplastida (plants and algae) is more complex. Plants exclusively possess the cobalamin-independent form, while algae have either one of the two, depending on species. Many different microorganisms express both the cobalamin-dependent and cobalamin-independent forms.
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.
Methylcobalamin (mecobalamin, MeCbl, or MeB12) is a cobalamin, a form of vitamin B12. It differs from cyanocobalamin in that the cyano group at the cobalt is replaced with a methyl group. Methylcobalamin features an octahedral cobalt(III) centre and can be obtained as bright red crystals. From the perspective of coordination chemistry, methylcobalamin is notable as a rare example of a compound that contains metal–alkyl bonds. Nickel–methyl intermediates have been proposed for the final step of methanogenesis.
Folate deficiency, also known as vitamin B9 deficiency, is a low level of folate and derivatives in the body. This may result in megaloblastic anemia in which red blood cells become abnormally large, and folate deficiency anemia is the term given for this medical condition. Signs of folate deficiency are often subtle. Symptoms may include fatigue, heart palpitations, shortness of breath, feeling faint, open sores on the tongue, loss of appetite, changes in the color of the skin or hair, irritability, and behavioral changes. Temporary reversible infertility may occur. Folate deficiency anemia during pregnancy may give rise to the birth of low weight birth premature infants and infants with neural tube defects.
Methylmalonic acid (MMA) is a dicarboxylic acid that is a C-methylated derivative of malonic acid.
Adenosylcobalamin (AdoCbl), also known as coenzyme B12, cobamamide, and dibencozide, is, along with methylcobalamin (MeCbl), one of the biologically active forms of vitamin B12.
Vitamin B12 deficiency, also known as cobalamin deficiency, is the medical condition in which the blood and tissue have a lower than normal level of vitamin B12. Symptoms can vary from none to severe. Mild deficiency may have few or absent symptoms. In moderate deficiency, feeling tired, headaches, soreness of the tongue, mouth ulcers, breathlessness, feeling faint, rapid heartbeat, low blood pressure, pallor, hair loss, decreased ability to think and severe joint pain and the beginning of neurological symptoms, including abnormal sensations such as pins and needles, numbness and tinnitus may occur. Severe deficiency may include symptoms of reduced heart function as well as more severe neurological symptoms, including changes in reflexes, poor muscle function, memory problems, blurred vision, irritability, ataxia, decreased smell and taste, decreased level of consciousness, depression, anxiety, guilt and psychosis. If left untreated, some of these changes can become permanent. Temporary infertility, reversible with treatment, may occur. A late finding type of anemia known as megaloblastic anemia is often but not always present. In exclusively breastfed infants of vegan mothers, undetected and untreated deficiency can lead to poor growth, poor development, and difficulties with movement.
Vitamin B12, also known as cobalamin, is a water-soluble vitamin involved in metabolism. It is one of eight B vitamins. It is required by animals, which use it as a cofactor in DNA synthesis, and in both fatty acid and amino acid metabolism. It is important in the normal functioning of the nervous system via its role in the synthesis of myelin, and in the circulatory system in the maturation of red blood cells in the bone marrow. Plants do not need cobalamin and carry out the reactions with enzymes that are not dependent on it.
Cyanocobalamin is a form of vitamin B
12 used to treat and prevent vitamin B
12 deficiency except in the presence of cyanide toxicity. The deficiency may occur in pernicious anemia, following surgical removal of the stomach, with fish tapeworm, or due to bowel cancer. It is used by mouth, by injection into a muscle, or as a nasal spray.
Methionine synthase reductase, also known as MSR, is an enzyme that in humans is encoded by the MTRR gene.
Haptocorrin (HC) also known as transcobalamin-1 (TC-1) or cobalophilin is a transcobalamin protein that in humans is encoded by the TCN1 gene. One essential function of haptocorrin is protection of the acid-sensitive vitamin B12 while it moves through the stomach. A second function is serum HC binding of the great majority of circulating vitamin B12, rendering it unavailable for take-up by cells. This is conjectured to be a circulating storage function.
Imerslund–Gräsbeck syndrome is a rare autosomal recessive, familial form of vitamin B12 deficiency caused by malfunction of the "Cubam" receptor located in the terminal ileum. This receptor is composed of two proteins, amnionless (AMN), and cubilin. A defect in either of these protein components can cause this syndrome. This is a rare disease, with a prevalence about 1 in 200,000, and is usually seen in patients of European ancestry.
In molecular biology, the vitamin B12-binding domain is a protein domain which binds to cobalamin. It can bind two different forms of the cobalamin cofactor, with cobalt bonded either to a methyl group (methylcobalamin) or to 5'-deoxyadenosine (adenosylcobalamin). Cobalamin-binding domains are mainly found in two families of enzymes present in animals and prokaryotes, which perform distinct kinds of reactions at the cobalt-carbon bond. Enzymes that require methylcobalamin carry out methyl transfer reactions. Enzymes that require adenosylcobalamin catalyse reactions in which the first step is the cleavage of adenosylcobalamin to form cob(II)alamin and the 5'-deoxyadenosyl radical, and thus act as radical generators. In both types of enzymes the B12-binding domain uses a histidine to bind the cobalt atom of cobalamin cofactors. This histidine is embedded in a DXHXXG sequence, the most conserved primary sequence motif of the domain. Proteins containing the cobalamin-binding domain include:
Cobalamin biosynthesis is the process by which bacteria and archea make cobalamin, vitamin B12. Many steps are involved in converting aminolevulinic acid via uroporphyrinogen III and adenosylcobyric acid to the final forms in which it is used by enzymes in both the producing organisms and other species, including humans who acquire it through their diet.
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