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ABCG and ABCA Regulation of Sterol Metabolism
ATP Binding Cassette transporters comprise a large family of membrane-spanning proteins that are responsible for transporting a variety of substrates in prokaryotes and eukaryotes. The most intriguing and, arguably, the most important membrane proteins for this purpose are the ABC (ATP-Binding Cassette) transporters. These proteins, found in all species, use [...]
Abscisic Acid Metabolism in Arabidopsis
ABA (Abscisic Acid) is a plant hormone that plays important role during many phases of the plant life cycle, including seed development and dormancy, embryo maturation, cell division and elongation, and in plant responses to various environmental stresses such as drought, salinity, cold, pathogen attack and UV radiation. However, despite [...]
all-trans-Retinoic Acid Signaling in Brain
Retinoic Acid, the active form of Vitamin-A (all-trans-Retinol), is a lipophilic molecule, and is known to affect gene transcription. Retinoic Acid is made available in the body through dietary intake and subsequent metabolism in the liver. Vitamin-A is secreted from its storage pools and circulates in blood. In the liver [...]
Biosynthesis and Degradation of Catecholamines
Catecholamine is the common term for the important hormones Norepinephrine (Noradrenaline), Epinephrine (Adrenaline) and Dopamine. Chromaffin cells of the adrenal medulla are the key site of catecholamine synthesis and collections of these cells are also found in heart, liver, kidney, gonads, adrenergic neurons of the postganglionic sympathetic system, and CNS [...]
Biosynthesis and Degradation of Thyroid Hormones
Iodide, which is ingested in food and water, is actively concentrated by the thyroid gland, converted to organic iodine by TPO (Thyroid Peroxidase also known as Iodide Peroxidase), and incorporated into tyrosine in intrafollicular thyroglobulin within the colloid at the basal cell surface of the thyroid follicular cell. The tyrosines [...]
Biosynthesis of Arginine in Bacteria
In bacteria, biosynthesis of arginine proceeds from glutamate in eight enzymatic steps initiated by the acetylation of glutamate by N-Acetyl Glutamate Synthetase. The N-Acetylated intermediates lead to ornithine. The synthesis of ornithine, like that of proline, involves the activation and reduction of the 5-carboxyl group of glutamate. In prokaryotes the [...]
Biosynthesis of Chorismate in Bacteria and Plants
In micro-organisms and plants the biosynthesis of aromatic compounds proceeds via the common seven-step aromatic or shikimate pathway to the branch point intermediate chorismate. This intermediate is subsequently converted to the three aromatic amino acids via specific terminal pathways. Many other aromatic compounds are derived either partially or entirely from [...]
Biosynthesis of Cysteine from Serine in Bacteria and Plants
Inorganic sulfur in the environment (primarily sulfate, but also sulfur, and sulfite) must undergo fixation to be utilized by organisms. The fixation of sulfate is largely confined to plants and bacteria and biosynthesis of cysteine represents the final step of sulfate assimilation in these organisms. Fixation begins with the formation [...]
Biosynthesis of Cysteine in Mammals
Cysteine is not an essential amino acid, but it arises in mammals from methionine, which is essential, and serine, which is not. In the absence of dietary methionine, animals cannot make cysteine. Mammals require homocysteine (from the degradation of Methionine) as source of sulfur and produce cysteine. Homocysteine reacts with [...]
Biosynthesis of Glycine and Serine
Intermediates in energy production pathways such as glycolysis and the Krebs cycle are commonly the starting point for the biosynthesis of amino acids. The biosynthesis of serine and glycine constitute a major metabolic pathway that plays a central role in the formation of other amino acids, nucleic acids and phospholipids. [...]
Biosynthesis of Isoleucine
Isoleucine is essential amino acid, only synthesized in plants and bacteria, and required in the diet by animals. In proteins, the hydrophobic Isoleucine side-chain tends to reside with other hydrophobic residues in the interior of globular proteins or in transmembrane domains. Isoleucine biosynthesis begins with the common metabolic intermediate Pyruvate, [...]
Biosynthesis of Leucine
Leucine is a branched chain essential amino acid whose carbon skeleton originates from Pyruvate. Leucine is a strictly ketogenic amino acid. Alpha-ketoIsovalerate is a precursor for Leucine. The Leucine biosynthesis starts from the oxo acid precursor of Valine and resembles the initial 3 steps of the citrate cycle (Ref.1). The [...]
Biosynthesis of Lysine
The amino acid L-lysine is synthesized by plants and microorganisms by two different pathways, one proceeding via Diaminopimelate and the other via Alpha-Aminoadipate. Humans, however, cannot synthesise the compound and so it is an essential part of their diet. The Diaminopimelate pathway operates in bacteria, lower fungi and green plants. [...]
Biosynthesis of Melanin
Melanins are naturally occurring pigments found in animals and plants. They are complex heterogeneous polymers whose chemical structure has not been satisfactorily determined. In Plants Melanins occur as Allomelanin, whereas in animals there are two major classes of natural Melanins, the black-brown Eumelanin found in human black hair and in [...]
Biosynthesis of Neurotransmitters
For neuronal signal transduction, the presynaptic cell synthesizes neurotransmitters that then traverse the synaptic cleft. Neurotransmitters are signaling molecules used by neurons to communicate across chemical synapses. The receptors for neurotransmitters include GPCRs (G-Protein Coupled Receptors) and ligand-gated ion channels. The biogenic amines are neurotransmitters derived from amino acids, and [...]
Biosynthesis of Phenylalanine and Tyrosine in Bacteria and Plants
Chorismate is a branch point for different pathways leading to Phenylalanine and Tyrosine. A mutase reaction yields Prephenate, which undergoes either a Dehydratase/Decarboxylase or a reduction/Decarboxylase reaction (in some bacteria) resulting in aromatization, the oxo analogues of phenylalanine and tyrosine obtained in this way are then converted into the respective [...]
Biosynthesis of Plasmalogens
Plasmalogens are glycerophospholipids of neural membranes containing vinyl ether bonds. Their synthetic pathway is located in peroxisomes and endoplasmic reticulum. About 20% of mammalian Glycerophospholipids are Plasmalogens. The exact percentage varies both from species to species and from tissue to tissue within a given organisms. While Plamalogens comprise only 0.8% [...]
Biosynthesis of Progesterone
In mammals, the binding of FSH (Follicle-Stimulating Hormone), LH (Luteinizing Hormone) or ACTH (Adreno Cortico-Trophic Hormone) to their receptors at the adrenal cortex, ovary or testes membranes starts a G-Protein mediated reaction sequence. This leads to a transport of cholesterol from cytoplasmic inclusion droplets into the mitochondria, assisted by SCP [...]
Biosynthesis of Proline
Proline is unique in having a cyclic structure with its side chain connected to the amino group to create a secondary amine. As a consequence of its cyclic structure, Proline constrains the structure of proteins where it occurs, disrupting Alpha-helices. Isomerization between the cis and trans forms of Proline in [...]
Biosynthesis of Spermidine and Spermine
The polyamines Spermidine and Spermine are naturally occurring ubiquitous polycations involved in the regulation of cell growth, differentiation, apoptosis and progression through the cell cycle. (Ref.1). Cellular polyamine levels are regulated by multiple pathways such as synthesis from amino acid precursors, cellular uptake mechanisms that salvage polyamines from diet and [...]
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