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Protein Synthesis

The synthesis of proteins involves the translation of genetic information into a polypeptide chain. It requires a DNA sequence, enzymes and messenger, ribosomal and transfer ribonucleic acids (mRNA, rRNA and tRNA).

The first step in protein synthesis, called initiation, occurs when the RNA polymerase binds to the promoter region of DNA. A special initiator tRNA binds to the 5′ end of mRNA and inserts the first amino acid, methionine. The tRNA also contains a gene-specific start codon and an end codon.

The other steps are elongation, folding and post-translational modification. The elongation step uses tRNA to recognize the appropriate start and stop codes, which are read by ribosomes. Amino acids are added to the peptide chain one at a time by specific molecules of tRNA synthetases. The chain is then folded and modified. The resulting polypeptide is transported to the Golgi apparatus, where it can be released or stored. This process is essential for the growth of all living organisms.

Membrane Synthesis

Lipids are complex organic molecules that serve as building blocks for membrane structures that have multiple functions. Membrane proteins are renewed continuously and most lipids are synthesized in the endoplasmic reticulum. Lipids in general are transported to other organelle membranes through vesicular trafficking and are then incorporated into the respective compartments.

Bulk membrane lipid biogenesis takes place in the endomembrane compartment (ER, Golgi apparatus). Specialized phospholipids are also synthesized in mitochondria and perosixomes. De novo lipid synthesis occurs during the developmental differentiation of secretory cells, and regulated by the XBP-1(S) transcription factor.

In the pGEMM7Dpsd vector, the PlsB, PlsC and CdsA enzymes that catalyze phosphatidylserine (PS) biosynthesis are expressed in giant vesicles in the presence of acyl-CoA and G3P. PS-enriched vesicles are visualized by the membrane-bound fluorescent protein LactC2-eGFP. Upon activation, XBP-1(S) regulates the activity of these enzymes and triggers membrane growth. Similarly, a constitutive expression of the T7 promoter prevents read-through transcription and promotes vesicle accumulation.

Cellular Respiration

Cellular respiration is the process in which biochemical energy is harvested from organic substances (such as glucose, a six-carbon molecule) and stored in energy-carrying molecules called adenosine triphosphate, or ATP. During this process, the organic molecules are broken down by enzyme-catalyzed reactions.

All cells use cellular respiration to get the energy they need to function. Without cellular respiration, cells can only get energy from other molecules through a process called fermentation. Cellular respiration produces significantly more energy than fermentation does, and it can also store the energy in molecules of adenosine triphosphate.

The four stages of cellular respiration are glycolysis, the transition reaction, the Krebs cycle, and oxidative phosphorylation via the electron transport chain. Glycolysis and the transition reaction take place in the cytoplasm, while the Krebs cycle and oxidative phosphorylation occur in the mitochondrion. The electron transport chain is located on the inner mitochondrial membrane. All but the oxidative phosphorylation stage require oxygen, and the oxidative phosphorylation stages produce potential energy in the form of ATP molecules.

Apoptosis

The death of unwanted cells is an important part of the cellular maintenance that allows multicellular organisms to survive. This process is known as programmed cell death or apoptosis.

Unlike necrosis, in which the contents of dying cells spill out and cause inflammation, apoptosis is an orderly process that packages up the cell’s contents into little packets of membrane for “garbage collection” by immune cells. Apoptosis is essential for development, allowing old and damaged cells to be replaced with new ones, or in maintaining the balance of the immune system by eliminating pre-cancerous or virus-infected cells.

Apoptosis research has been revolutionized by a greater understanding of the genetic program and biochemical mechanisms that regulate it. The two main pathways of apoptosis, extrinsic and intrinsic, both require specific triggering signals that initiate an energy-dependent cascade of molecular events culminating in characteristic morphological features including shrinkage, chromatin condensation and formation of cytoplasmic blebs or apoptotic bodies (Figure 1). Molecular analysis has also revealed that granzyme A acts to protect chromatin structure and DNA integrity during apoptosis by inhibiting the activity of a complex of proteins that includes SET, Ape1, p32 and HMG2. Apoptosis is now understood to be a fundamental cellular process that provides insight into many diseases and is poised to influence therapeutic strategy.