Pathways

The ubiquitin-proteasome pathway is the pathway in which molecules, specifically proteins, are broken down into smaller molecules in the cytosol or in the nucleus.This pathway subsequently has effects in many other pathways and processes. This pathway uses 2 distinct steps. The first step is that the protein being broken down is tagged by multiple ubiquitin units attaching to the protein. The second step is that the protein that has been tagged degrades as it is catalyzed by the 26S proteasome. This pathway is important for DNA repair, regulating the amount of proteins, and the creation of antigen-peptide.

The ubiquitin-proteasome pathway is the pathway in which molecules, specifically proteins, are broken down into smaller molecules in the cytosol or in the nucleus.This pathway subsequently has effects in many other pathways and processes. This pathway uses 2 distinct steps. The first step is that the protein being broken down is tagged by multiple ubiquitin units attaching to the protein. The second step is that the protein that has been tagged degrades as it is catalyzed by the 26S proteasome. This pathway is important for DNA repair, regulating the amount of proteins, and the creation of antigen-peptide.

The ubiquitin-proteasome pathway is the pathway in which molecules, specifically proteins, are broken down into smaller molecules in the cytosol or in the nucleus.This pathway subsequently has effects in many other pathways and processes. This pathway uses 2 distinct steps. The first step is that the protein being broken down is tagged by multiple ubiquitin units attaching to the protein. The second step is that the protein that has been tagged degrades as it is catalyzed by the 26S proteasome. This pathway is important for DNA repair, regulating the amount of proteins, and the creation of antigen-peptide.

The ubiquitin-proteasome pathway is the pathway in which molecules, specifically proteins, are broken down into smaller molecules in the cytosol or in the nucleus.This pathway subsequently has effects in many other pathways and processes. This pathway uses 2 distinct steps. The first step is that the protein being broken down is tagged by multiple ubiquitin units attaching to the protein. The second step is that the protein that has been tagged degrades as it is catalyzed by the 26S proteasome. This pathway is important for DNA repair, regulating the amount of proteins, and the creation of antigen-peptide.

The ubiquitin-proteasome pathway is the pathway in which molecules, specifically proteins, are broken down into smaller molecules in the cytosol or in the nucleus.This pathway subsequently has effects in many other pathways and processes. This pathway uses 2 distinct steps. The first step is that the protein being broken down is tagged by multiple ubiquitin units attaching to the protein. The second step is that the protein that has been tagged degrades as it is catalyzed by the 26S proteasome. This pathway is important for DNA repair, regulating the amount of proteins, and the creation of antigen-peptide.

Ubiquinone is also known as coenzyme Q10. It is a 1,4-benzoquinone, where Q refers to the quinone chemical group, and 10 refers to the isoprenyl chemical subunits. Ubiquinone is a carrier of hydrogen atoms (protons plus electrons) and functions as an ubiquitous coenzyme in redox reactions, where it is first reduced to the enzyme-bound intermediate radical semiquinone and in a second reduction to ubiquinol (Dihydroquinone; CoQH2). Ubiquinone is not tightly bound or covalently linked to any known protein complex but is very mobile. In eukaryotes ubiquinones were found in the inner mito-chondrial membrane and in other membranes such as the endoplasmic reticulum, Golgi vesicles, lysosomes and peroxisomes. The benzoquinone portion of Coenzyme Q10 is synthesized from tyrosine, whereas the isoprene sidechain is synthesized from acetyl-CoA through the mevalonate pathway. The mevalonate pathway is also used for the first steps of cholesterol biosynthesis. The enzyme para-hydroxybenzoate polyprenyltransferase catalyzes the condensation of p-hydroxybenzoate with polyprenyl diphosphate to generate ubiquinone.

Ubiquinone is also known as coenzyme Q10. It is a 1,4-benzoquinone, where Q refers to the quinone chemical group, and 10 refers to the isoprenyl chemical subunits. Ubiquinone is a carrier of hydrogen atoms (protons plus electrons) and functions as an ubiquitous coenzyme in redox reactions, where it is first reduced to the enzyme-bound intermediate radical semiquinone and in a second reduction to ubiquinol (Dihydroquinone; CoQH2). Ubiquinone is not tightly bound or covalently linked to any known protein complex but is very mobile. In eukaryotes ubiquinones were found in the inner mito-chondrial membrane and in other membranes such as the endoplasmic reticulum, Golgi vesicles, lysosomes and peroxisomes. The benzoquinone portion of Coenzyme Q10 is synthesized from tyrosine, whereas the isoprene sidechain is synthesized from acetyl-CoA through the mevalonate pathway. The mevalonate pathway is also used for the first steps of cholesterol biosynthesis. The enzyme para-hydroxybenzoate polyprenyltransferase catalyzes the condensation of p-hydroxybenzoate with polyprenyl diphosphate to generate ubiquinone.

Ubiquinone’s distinctive structure is defined by a polyisoprenoid side chain connected to a benzoquinone ring. It serves multiple roles in plants, functioning as an electron transporter in inner mitochondrial membranes, as well as acting as an antioxidant to protect against free radicals. The biosynthesis of ubiquinone is connected to the biosynthesis of tyrosine, tryptophan, and phenylalanine through the shared compounds L-tyrosine and chorismate. Its biosynthesis also takes place in many organelles, with key steps occurring in the mitochondria, chloroplasts, and peroxisomes of plant cells. The compound L-tyrosine begins in the cytoplasm and is converted to homogentisic acid before it can enter the chloroplast through the transporter homogentisate prenyltransferase. Once in the chloroplast, homogentisic acid can follow one of three different sets of reactions, ultimately forming five different compounds, plastoquinol-9, α-tocopherol, β-tocopherol, α-tocotrienol and β-tocotrienol. Pyrophosphate compounds from reactions early in all three sets are provided as products from terpenoid backbone biosynthesis. Meanwhile, chorismate, which also begins in the cytoplasm, can follow two distinct pathways. The first involves its transfer into the mitochondrion, where it undergoes a series of reactions until it forms ubiquinone. This ubiquinone can be used for oxidative phosphorylation within the mitochondrion. The second pathway chorismate follows brings it into the chloroplast, where multiple PHYLLO enzymes catalyze a series of reactions to form 2-succinyl benzoate. With the addition of coenzyme A, 2-succinyl benzoyl-CoA can be moved out of the chloroplast and into the peroxisome. Through a pair of reactions, this compound is ultimately hydrolyzed to form 1,4-dihydroxy-2-naphthoate, which is transported back into the chloroplast to form phylloquinol. Phylloquinone can also react with a hydrogen ion to form phylloquinol in the cell membrane.

Ubiquinone (coenzyme Q) biosynthesis in bacteria is a vital metabolic pathway responsible for producing ubiquinone, a lipid-soluble molecule that plays a central role in the electron transport chain (ETC) and cellular respiration. The process begins with the precursor 4-hydroxybenzoate (4-HB), which is derived from tyrosine or chorismate. Through a series of enzymatic reactions, a polyprenyl side chain is attached to 4-HB by 4-hydroxybenzoate polyprenyltransferase to form 4-hydroxy-3-polyprenylbenzoate, which is subsequently modified by hydroxylation, methylation, and decarboxylation steps. These modifications involve key enzymes such as monooxygenases and methyltransferases, leading to the production of ubiquinone. The length of the polyprenyl side chain varies among species and is determined by specific polyprenyl synthases. Ubiquinone acts as a mobile electron carrier within the bacterial membrane, shuttling electrons between complexes in the ETC and contributing to the generation of a proton gradient used for ATP synthesis. This pathway is essential for energy metabolism in aerobic bacteria, and disruptions can severely impair cellular respiration. Furthermore, ubiquinone also functions as an antioxidant, protecting bacterial cells from oxidative damage, underscoring its dual importance in bioenergetics and stress defense.