Chemical Components of a Major Site

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Synthesis of Lipids

Most membrane lipids, including glycerophospholipids, sphingolipids and, in limited types of cells, sterols, are synthesized in the endoplasmic reticulum (ER). The ER also synthesizes many of the proteins that comprise a cell’s membranes. Proteins destined for the plasma membrane, Golgi apparatus, lysosomes and plastids are transported from the cytosol into these organelle membranes by transport vesicles.

These vesicles contain the lipids needed for their destination. Lipids cannot be dissolved in water and must therefore use active mechanisms to facilitate their transport.

In eukaryotic cells, phospholipids are synthesized in three steps from choline, two fatty acids and glycerol 3-phosphate. The first step of synthesis involves acyl transferases that add the two fatty acids to glycerol 3-phosphate. This reaction is regulated to control the length of the fatty acid chains in the final product. Glycerol 3-phosphate is released to the cytosol and can then be used in further reactions for lipid synthesis.

Synthesis of Hormones

Hormones have a wide range of biological functions and physiological outcomes. Their chemical structure and physical properties determine whether they can enter cells and interact with their receptors. The interaction triggers a series of events that modify the cell’s activity or function.

Steroid hormones, such as E2, are produced in the ovaries and other tissues in the female body for prenatal development, bone growth, sex drive, and many other effects. They are synthesized from the amino acid tyrosine by successive iodination of the phenol ring positions on tyrosine. One iodinated site forms monoiodotyrosine; two sites form diiodotyrosine.

Polypeptide and protein hormones are synthesized from inactive precursors (preprohormones) on the rough endoplasmic reticulum of different endocrine glands. Once complete, they are packaged into secretory granules and stored in the cytoplasm until appropriate stimuli result in their release into the extracellular fluid. These hormones are water soluble and can diffuse across the lipid bilayer of cell membranes and interact with their receptors on the cellular surface.

Synthesis of Amino Acids

All amino acids except glycine are chiral molecules, which exist in two optically active asymmetric forms that are the mirror images of each other (enantiomers). Most proteins contain only l-amino acid residues. The d-amino acids are found mainly in bacteria and some antibiotics.

Glycine is synthesized in animal cells through the reversible reaction catalyzed by glutamate dehydrogenase with nicotinamide adenine dinucleotide phosphate (NADPH) as a reducing agent. The resulting a-keto acid is converted into an amino acid through transamination reactions or to glucose via gluconeogenesis. The nitrogen skeletons of amino acids can also be conserved as carbohydrate or as fatty acid.

Three of the eight essential amino acids – arginine, methionine and phenylalanine — cannot be directly synthesized in the body and must be supplied through the diet. The methionine that is synthesised provides the sulfur needed for cysteine synthesis, and phenylalanine supplies methyl groups for metabolism.

Synthesis of Proteins

During protein synthesis, nucleic acid genetic information is converted to polypeptide chains of amino acids. This process is essential in forming structural components of the cell, producing hormones and enzymes and carrying out other important functions.

The first step, transcription, produces an exact copy of a gene out of the DNA in the nucleus. The RNA copy, known as messenger RNA (mRNA) is then transported to a ribosome in the cytoplasm. Here, the mRNA is’read’ by a sequence of three codes called codons. Then transfer RNA (tRNA) carries the correct amino acids into the ribosome in the correct order.

Amino acids are then assembled into a chain, which is folded and modified by various cellular mechanisms into its final form. The resulting protein may then bind to other proteins, lipids and carbohydrates to form lipoproteins or glycoproteins. Proteins can also be processed by the endoplasmic reticulum, and then exported to the Golgi apparatus for further modification.