Lipid[ edit ] Lipid membrane bilayer Many intricate macromolecules are synthesized in a pattern of simple, repeated structures. Fatty acids are hydrocarbon derivatives; they contain a carboxyl group "head" and a hydrocarbon chain "tail".
A third major membrane component, cholesteroldoes not contain these fatty acid units. However, the first step in phospholipid synthesis involves the formation of phosphatidate or diacylglycerol 3-phosphate at the endoplasmic reticulum and outer mitochondrial membrane.
The pathway starts with glycerol 3-phosphate, which gets converted to lysophosphatidate via the addition of a fatty acid chain provided by acyl coenzyme A. These ceramides are synthesized from the acylation of sphingosine. As the image denotes, during sphingosine synthesis, palmitoyl CoA and serine undergo a condensation reaction which results in the formation of dehydrosphingosine.
Not only does it serve as a component of lipid membranes, it is also a precursor to several steroid hormones, including cortisoltestosteroneand estrogen. More generally, this synthesis Synthesising ba v itamins in three stages, with the first stage taking place in the cytoplasm and the second and third Synthesising ba v itamins occurring in the endoplasmic reticulum.
The synthesis of isopentenyl pyrophosphatethe "building block" of cholesterol 2. The formation of squalene via the condensation of six molecules of isopentenyl phosphate 3.
The conversion of squalene into cholesterol via several enzymatic reactions Nucleotides[ edit ] The biosynthesis of nucleotides involves enzyme- catalyzed reactions that convert substrates into more complex products.
Nucleotides are composed of a five-membered ring formed from ribose sugar in RNA, and deoxyribose sugar in DNA; these sugars are linked to a purine or pyrimidine base with a glycosidic bond and a phosphate group at the 5' location of the sugar. The DNA nucleotides adenosine and guanosine consist of a purine base attached to a ribose sugar with a glycosidic bond.
In the case of RNA nucleotides deoxyadenosine and deoxyguanosinethe purine bases are attached to a deoxyribose sugar with a glycosidic bond. The purine bases on DNA and RNA nucleotides are synthesized in a twelve-step reaction mechanism present in most single-celled organisms.
Higher eukaryotes employ a similar reaction mechanism in ten reaction steps.
Purine bases are synthesized by converting phosphoribosyl pyrophosphate PRPP to inosine monophosphate IMPwhich is the first key intermediate in purine base biosynthesis. The first step in purine biosynthesis is a condensation reactionperformed by glutamine-PRPP amidotransferase. This enzyme transfers the amino group from glutamine to PRPP, forming 5-phosphoribosylamine.
The following step requires the activation of glycine by the addition of a phosphate group from ATP.
FGAM cyclase catalyzes ring closure, which involves removal of a water molecule, forming the 5-membered imidazole ring 5-aminoimidazole ribonucleotide AIR. The final step involves the enzyme IMP synthasewhich performs the purine ring closure and forms the inosine monophosphate intermediate.
Uridine monophosphate biosynthesis involves an enzyme that is located in the mitochondrial inner membrane and multifunctional enzymes that are located in the cytosol. Aspartate carbamoyltransferase condenses carbamoyl phosphate with aspartate to form uridosuccinate.
Dihydroorotase performs ring closurea reaction that loses water, to form dihydroorotate. Dihydroorotate dehydrogenaselocated within the mitochondrial inner membrane,  oxidizes dihydroorotate to orotate.
Phosphate addition to UMP is catalyzed by a kinase enzyme.
The enzyme CTP synthase catalyzes the next reaction step: However, uracil is only found in RNA. This conversion involves the enzyme ribonucleoside triphosphate reductase. This reaction that removes the 2'-OH of the ribose sugar to generate deoxyribose is not affected by the bases attached to the sugar.
This non-specificity allows ribonucleoside triphosphate reductase to convert all nucleotide triphosphates to deoxyribonucleotide by a similar mechanism. Cells do not normally contain thymine bases that are linked to ribose sugars in RNA, thus indicating that cells only synthesize deoxyribose-linked thymine.
DNA[ edit ] As DNA polymerase moves in a 3' to 5' direction along the template strand, it synthesizes a new strand in the 5' to 3' direction Although there are differences between eukaryotic and prokaryotic DNA synthesis, the following section denotes key characteristics of DNA replication shared by both organisms.
DNA is composed of nucleotides that are joined by phosphodiester bonds. There are more than amino acids found in nature of which only twenty, known as the standard amino acidsare the building blocks for protein. Mammals can only synthesize ten of the twenty standard amino acids.
The other amino acids, valinemethionineleucineisoleucinephenylalaninelysinethreonine and tryptophan for adults and histidineand arginine for babies are obtained through diet. The different amino acids are identified by the functional group.
With the exception of prolineall of the amino acids found in life have the L-isoform conformation. In cells, there are two major pathways of incorporating nitrogen groups. One pathway involves the enzyme glutamine oxoglutarate aminotransferase GOGAT which removes the amide amino group of glutamine and transfers it onto 2-oxoglutarateproducing two glutamate molecules.
In this catalysis reaction, glutamine serves as the nitrogen source. An image illustrating this reaction is found to the right.Book flights to London, New York and many other international holiday destinations with British Airways.
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