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BIOL 130
Heidi Engelhardt

You are an amino acid in the lumen of the small intestine of a newborn mammal. You are looking at intestinal epithelial cells that bring important maternal proteins (immunoglobulins) across their apical surfaces by receptor-mediated endocytosis. Your ambition is to be part of a receptor that does that job. You will be asked to describe your adventures during one of three phases of your journey: A. beginning from the challenge of entering the cell, until you have been loaded onto an appropriate tRNA. Pg (352-355) How amino acids enter - In the small intestine, epithelial cells exist. However the small intestine is covered with villi which are responsible for absorption of nutrients. Amino acids in particular are absorbed by active transport. In a way, this is a challenge for the cell because energy must be used. Active Transport - Draw diagram of sodium potassium pump - There are two main methods of active transport. One method involves the hydrolysis of ATP into ADP + P. This reaction releases energy which is used to allow ions to move against the concentration gradient. The other method uses pumps/active transporters. There are many pumps but the first one that was discovery was the sodium-potassium pump. This pump works by using ATP to transfer three molecules of Na+ outside the cell and two molecules of K+ into the cell. As a result, a concentration gradient is created. You have high [Na+] on outside and low [Na+] on inside. This concentration gradient is key because we know Na+ wants to go move inside the cell again. But other molecules, such as amino acid, can be co-transported with Na+ into the cell. More specifically this is called symport (Na+ being cotransported with amino acids). Translation and loading into tRNA - a tRNA molecule consists of a single strand of RNA. tRNA has a 3d structure because of the hydrogen bonds that can form between the RNA strand. There are two main parts of tRNA. An anticodon, a triplet of nucleotides that can form base pairs with a complementary codon. However, the other part we’re interested in here is the amino acid attachment site. Loading of amino acid into tRNA - Amino acids can covalently link to tRNA at these sites. This is done through the help of enzymes called aminoacyl tRNA synthetase. There are 20 different types of these enzymes, 1 to help bind each type of amino acid. The way it is by allowing both the amino acid and ATP binding on the active site. ATP loses two phosphate groups and becomes AMP. This gives the amino acid high potential energy. This allows the appropitae tRNA to come in and bind to the amino acid, AMP is released. The product is then released from aminoacyl tRNA and is ready for translation. The product is called aminoacyl tRNA. B. beginning from initiation of translation, until your incorporation into a folded protein. Initiation The purpose of translation is to create a polypeptide chain. Translation can be split into three main steps. Initiation, elongation and termination. Translation occurs in the ribosome (draw a ribosome as seen on pg 356) Ribosomes help with the coupling of tRNA anticodons with codons on the mRNA strand. This process begins when a specific region of mRNA binds to a secquence of RNA on the ribosome. The specific region of mRNA is called the Shine-Dalgarno sequence/ribosomal binding site. Proteins called initiation factors help with the binding of these two sequences. Once binded, an aminoacyl tRNA with a methionine binds to the AUG start codon at the P site. Elongation This marks the beginning of the second stage which is elongation. A and E sites are empty, and an mRNA codon is exposed at this site. A corresponding aminoacyl tRNA binds to the A site. Once the situation occurs where the A and P sites are filled, a peptide bond between the respective amino acids can form. Protein synthesis is catalyzed by a ribozyme. Once a peptide bond forms, the polypept
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