Understanding Proteins and Protein Synthesis: A Look into Cellular Structure and Function, Study notes of Communication

Download Understanding Proteins and Protein Synthesis: A Look into Cellular Structure and Function and more Study notes Communication in PDF only on Docsity! Pr CE Feature Proteins and Protein Synthesis: An Overview Bob Caruthers, CST, PhD AST Professional Development Manager n understanding of the basic bioscience involved in protein synthesis adds insight into the genesis of litcle-understood diseases, such as Creutzfeldt-Jakob disease, thar develop as a result of protein synthesis and/or genetic mutations. Because protein synthesis occurs at the cellular level and is critical to cell function, one must first understand basic cellular structure before studying protein synthesis. A review of cellular structure is therefore presented in the following paragraphs. CELLULAR STRUCTURE The two major cellular components—the nucleus and the cytoplasm (protoplasm of the cell outside the tiucleus)!—are separated from other portions of the cell by membranes: The nuclear membrane separates the nucleus from the cytoplasm, and the cell mem- brane separates the cytoplasm from the external environment, The organelles, which are highly specialized structures found in the cytoplasm, are also lined by membranes, which are composed of lipids and proteins. The lipid portion of the organelle membrane inhibits movement of water and water-soluble substances across the membrane while the protein portion penetrates the lipid framework and allows for the movement of certain substances through the membrane. Comparing the organelle membrane with a child's toy aids in understanding how the membrane functions. The toy—a wooden box with a variety of shapes cut out of it—requires a child to match puzzle- like objects with the cut-out shapes on the box in order to insert the shapes into the box. Much like the organelle membrane, the box is not penetrable unless the child matches the correctly shaped object with the same-shaped hole in the box. In the case of the membrane, the protein “hole” allows passage of substances that match its biochemical requirements.’ A list of the basic types of organelles follows: * Endoplasmic reticulum — A network of tubular and flat vesicular structures that allow for inera- cytoplasmic transport and communication. ¢ Ribosomes — Organelles attached co the endo- plasmic reticulum, which are the usual site of protein synthesis for proteins transported outside the cell, or organelles thar free-float in the cytoplasm and are the usual site of protein synthe- sis for proteins used within the cell. * Golgi apparatus — A multilayered, membrane- like structure that secretes lysosomes, secretory vesicles, and other cytoplasmic components. © Lysosomes ~~ Vesicular organelles formed by the Golgi apparatus that disperse throughout the cytoplasm and provide for intracytoplasmic digestion. ¢ Peroxisomes — Organelles similar to lysosomes, which contain oxidases and protect the cell from potentially poisonous substances. * Secretory vesicles — Storage compartments for secreted substances. * Mitochondria — A self-replicating structure that synthesizes adenosine triphosphate (ATP), which ‘The Serglast Yeskaategiat July 1997 33 Figure 1—The two major components of a typical cell, the nucleus and the cytoplasm, ate separated from other portions of the cell by membranes. The various organelles are highly specialized structures found within che cycoplasm. diffuses throughout the cell and provides energy for cellular functions. @ Tubules and Filaments — Protein strands used to provide structure to cells and perform specific actions depending on cell type. © Nucleus — The cell’s control center, which has several substructures and contains large quantities of DNA? The basic components of a typical cell are depicted in Figure 1. The cell’s five primary substances— water, proteins, electrolytes, lipids, and carbohydrates—are referred to collec- tively as “protoplasm.” Cells are made up AA July 1997 The Surgiael Techastegter of 70% to 85% water, in which chemical reactions occur among various dissolved or suspended particles. Proteins, the second most abundant cellular substance, constitute 10% to 20% of the cell mass. Cellular electrolytes—particles that are charged electrically when dissociated in solution—include potassium, magnesium, phosphate, sulfate, and bicarbonate, as well as small quantities of calcium and sodium chloride. Lipids, substances soluble in fat solvents, occur mainly in the form of phospholipids and choles- terol. Carbohydrates, which play an essential role in cellular nutrition, are of limited importance structurally.? The structure of the cell membrane, largely consisting of lipid and protein molecules, is depicted in Figure 2. PROTEINS AND PROTEIN SYNTHESIS Amino acids have been called the “building blocks” of protein. Cells are differentiated based on their protein structure, which regulates the type and rate of cellular chemical change (see Figure 3).* Hereditary information, passed from cell to cell chrough a set of amino acid sequences, determines the types of protein that a cell may produce. DNA within the nucleus contains a genetic code for sequencing amino acids. Base triplets form “code words” that ultimately determine the sequence of three amino acids. The code words are bound to the second strand of DNA’s double-stranded structure. While protein synthesis occurs in the cell's cytoplasm, DNA is confined co the nucleus. DNA code words must therefore be transcribed and transmitted for protein synthesis to occur. The transerip- tion process requires a second type of nucleic acid--RNA—which contains triplet sequences called codons. The DNA double strand separates temporarily and one strand forms a template for the RNA strand. The RNA strand resembles the DNA template with two exceptions: Sugar deoxyribose is replaced with ribose, and the base thymine is replaced with uracil. The resulting nucleotides activate when two phosphate radicals are added, producing high-energy phosphate bonds derived from ATP molecules. The activated nucleotides are then assembled into a complex RNA molecule through enzymatic action and released into the nucleoplasm, where enzymes “edit” or Temove unwanted sequences from the