Synthesis and metabolism of steroids in molluscs

There are four members of the EP receptor family, EP 1 (EP1), EP 2 (EP2), EP 3 (EP3), and EP 4 (EP4). The EP1 receptor is encoded by the PTGER1 gene located on chromosome and is composed of 3 exons that encode a protein of 402 amino acids. The EP2 receptor is encoded by the PTGER2 gene located on chromosome and is composed of 2 exons that encode a protein of 358 amino acids. The EP3 receptor is encoded by the PTGER3 gene located on chromosome and is composed of 11 exons that generate at least 8 alternatively spliced mRNAs. The EP4 receptor is encoded by the PTGER4 gene located on chromosome and is composed of 7 exons that encode a protein of 488 amino acids. The EP2 and EP4 receptors are coupled to the activation of a G s -type G-protein, the EP1 receptor is coupled to the activation of a G q -type G-protein, and the EP3 receptor has been shown to activate both G q - and G i -type G-proteins.

Now this brings us to the next part - how do we go from glucose to ATP? This is achieved through the process of "oxidation" - and this is carried out through a series of metabolic pathways. Complex chemical transformations in the cell occur in a series of separate reactions to form each pathway, and each reaction is catalyzed by a specific enzyme. Interestingly, metabolic pathways are similar in all organisms, from bacteria to humans. In eukaryotes (plants and animals) many of the metabolic pathways are compartmentalized, with certain reactions occurring in specific organelles. Basically, cells trap free energy released from the breakdown (metabolism) of glucose. This energy gets trapped in the ATP as it converts from ADP to ATP by the addition of phosphate.

As indicated in the Figure above showing the pathway of cholesterol biosynthesis a molecule of geranylpyrophosphate (GPP) and a molecule of isopentenylpyrophosphate (IPP) are condensed into farnesylpyrophosphate (FPP) through the action of the farnesyl diphosphate synthase enzyme which is encoded by the FDPS gene. Through the action of the ER-localized enzyme, dehydrodolichyl diphosphate synthase (encoded by the DHDDS gene), farnesylpyrophosphate is elongated via the sequential head-to-tail addition of multiple isopentenylpyrophosphate groups in a reaction referred to as cis -prenylation. The number of IPP substrates added ultimately determines the overall number of isoprene units in dolichol which in humans ranges from 17 to 21. The DHDDS gene is located on chromosome and is composed of 10 exons that generate five alternatively spliced mRNAs each of which encode a distince protein isoform. The product(s) of the DHDDS reaction is referred to as a polyprenolpyrophosphate. The pyrophosphate is removed by an as yet uncharacterized enzyme activity that may be either a polyprenol pyrophosphate phosphatase or a polyprenol phosphatase resulting in the formation of a polyprenol.

A vitamin is an organic compound needed in small quantities that cannot be made in cells. In human nutrition , most vitamins function as coenzymes after modification; for example, all water-soluble vitamins are phosphorylated or are coupled to nucleotides when they are used in cells. [18] Nicotinamide adenine dinucleotide (NAD + ), a derivative of vitamin B 3 ( niacin ), is an important coenzyme that acts as a hydrogen acceptor. Hundreds of separate types of dehydrogenases remove electrons from their substrates and reduce NAD + into NADH. This reduced form of the coenzyme is then a substrate for any of the reductases in the cell that need to reduce their substrates. [19] Nicotinamide adenine dinucleotide exists in two related forms in the cell, NADH and NADPH. The NAD + /NADH form is more important in catabolic reactions, while NADP + /NADPH is used in anabolic reactions.

Synthesis and metabolism of steroids in molluscs

synthesis and metabolism of steroids in molluscs

A vitamin is an organic compound needed in small quantities that cannot be made in cells. In human nutrition , most vitamins function as coenzymes after modification; for example, all water-soluble vitamins are phosphorylated or are coupled to nucleotides when they are used in cells. [18] Nicotinamide adenine dinucleotide (NAD + ), a derivative of vitamin B 3 ( niacin ), is an important coenzyme that acts as a hydrogen acceptor. Hundreds of separate types of dehydrogenases remove electrons from their substrates and reduce NAD + into NADH. This reduced form of the coenzyme is then a substrate for any of the reductases in the cell that need to reduce their substrates. [19] Nicotinamide adenine dinucleotide exists in two related forms in the cell, NADH and NADPH. The NAD + /NADH form is more important in catabolic reactions, while NADP + /NADPH is used in anabolic reactions.

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