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Synthesis of Lipids
Lipids are essential constituents of cell membranes and play a key role in the generation of bioactive signalling molecules. Their synthesis and metabolism form the basis of cell function and are linked to major diseases such as Type II diabetes, cardiovascular disease and cancer.
Lipid synthesis occurs in the membranes of intracellular organelles including mitochondria, endosomes and the Golgi apparatus. These membranes form a network of tubules and flattened sacs that surround the entire cytosol. This organisation allows for lipid exchange between the different membranes.
The lipids phosphatidylcholine (phosphatidylcholine), cholesteryl esters and fatty acids are the building blocks of cellular membranes. Phospholipid synthesis requires the transfer of a lipid across the bilayer from one leaflet to the other. This is achieved by the action of specialised transport proteins called scramblases, flippases and floppases.
Fatty acid biosynthesis starts with the condensation of acetyl-CoA and malonyl-CoA by the multifunctional enzyme FASN. The addition of two fatty acids by FASN produces the highly saturated lipid palmitic acid and subsequent elongation and desaturation reactions produce a wide range of unsaturated fatty acids.
Synthesis of Hormones
The endocrine glands (hypothalamus, pituitary gland, adrenal glands, gonads, thyroid gland, and pancreas) produce hormones that act on various cell types throughout the body. The cellular responses elicited by these hormones influence metabolism, growth, and development. Hormones also control blood pressure and heart rate, body temperature, the sleep-wake cycle, sexual functions, and emotional states.
The types of hormones differ in their molecular structures, but all hormones have a similar chemical structure and the ability to enter cells and interact with receptor proteins that are already attached to specific regions of the DNA. This interaction results in a series of biochemical changes that modify the activity or function of the affected genes.
The hypothalamus and pituitary gland secrete peptide hormones such as antidiuretic hormone (ADH), oxytocin, and vasopressin. These are stored in the posterior pituitary gland until they are needed. Other peptide hormones are cleaved from inactive precursors in the smooth endoplasmic reticulum, including insulin and glucagon.
Synthesis of Proteins
Protein synthesis is one of the most important processes in the cell. It is the process of converting genetic information encoded in DNA into polypeptide chains of amino acids, which are then folded and post-translationally modified to determine their final role. Proteins are the major structural components of cells, as well as enzymes and hormones.
To begin protein synthesis, the gene strand is copied from DNA into an RNA molecule called messenger RNA (mRNA). This molecule then leaves the nucleus and travels to the ribosome in the cytoplasm. Once there, the second step in protein synthesis begins: translation.
The mRNA molecule contains a sequence of three letters (called codons) that specify which amino acid should be added to the polypeptide chain. The ribosome then adds these amino acids to form a protein. The start of the polypeptide chain is determined by a special initiator tRNA that recognizes the correct sequence of coded bases in the mRNA and inserts methionine into the chain. This is a key difference between prokaryotes and eukaryotes, as the tRNA that initiates protein synthesis in prokaryotes contains an N-formyl methionine, whereas eukaryotic tRNA does not.
Synthesis of Nucleic Acids
Nucleic acids contain the hereditary information carried by chromosomes. They consist of a sugar-phosphate backbone and nitrogenous bases, which are connected to each other by hydrogen bonds. They were first isolated from white blood cells by Johann Friedrich Miescher in 1870 and were found to be composed of DNA and RNA.
DNA is a polymeric macromolecule that has a distinctive double-helix structure. Each chain contains a series of nitrogenous bases that are arranged in pairs and separated by a sugar-phosphate backbone, as shown in the diagram below. The sugars are called deoxyribose and ribose, and the nitrogenous bases are distinguished by their chemical structures, which have been given names based on the letters of the alphabet: Adenosine, Cytidine, Guanosine and Uridine.
A pair of nucleotides is formed by combining one nitrogenous base with four deoxynucleoside triphosphates. This reaction is catalyzed by enzymes called polymerases, and the resulting chain of mononucleotides is known as a DNA molecule.