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Proteins Chapter 8_07

Proteins Chapter 8_07 - Chapter 5 Proteins From structure...

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Chapter 5 Proteins: From structure to information flow within a cell 5.1 To fry an egg When you fry an egg there is a remarkable transformation from a clear gelatinous goo to a soft white opaque solid. As heat is transferred from the pan, the molecules of protein in the egg white rearrange and change their internal structures. This causes them to link together to form a solid and to reflect light giving them a white appearance. Egg white is made up primarily of one protein, albumin, whose role in the egg is to provide food for the growing embryo. During formation and growth of the embryo, proteins also serve to relay and receive signals that are essential for the cells to coordinate their activities. Within each of the cells of the growing embryo, proteins provide structure and serve to speed up chemical reactions. This diversity of roles may explain why, in many ways, proteins are more complex than DNA or RNA. This starts at the level of building blocks, where DNA and RNA have only four, while proteins have 20. But it goes way beyond the increase in combinatorial possibilities of the building blocks. Proteins fold in many different configurations that determine how they act and interact. Their interactions with each other can transfer information, build structural scaffolds, generate macromolecular machines or mark one partner for degradation. Probably because proteins play so many different roles, they have been most recalcitrant to the introduction of high throughput methods for their analysis. Major progress has been made, however, and more is likely in the near future. 5.2 How are proteins synthesized? Recall that the central dogma of molecular biology is that information is transferred from DNA to RNA to protein. In bacteria this is a tightly coordinated set of events. As RNA is transcribed from the DNA, the process of protein synthesis begins almost immediately (Fig. 5.1). Synthesis of protein is performed by specialized conglomerates of RNA and protein called ribosomes (Fig. 5.1). These little factories are built on a scaffold of the highly abundant ribosomal RNA ( rRNA ) molecules, which associate with a specialized set of ribosomal proteins. Ribosomes are made of two subunits, the larger of which is the first to attach to the nascent RNA molecule, after which the smaller one attaches itself (Fig. 5.2). In addition to the ribosome and the RNA molecule, protein synthesis requires two other players. A small species of RNA called transfer RNA ( tRNA ) ferries the protein building blocks, the amino acids to the ribosome (Fig. 5.3). The information as to which amino acid to add to the growing protein chain is found in the codons in the RNA molecule. At the end of each tRNA there is a set of three bases that match exactly one codon. It is the remarkable specificity of hybridization that allows only the tRNA carrying the right bases (called the anticodon ) to match the codon in the RNA and thus bring the correct amino acid to the growing protein (Fig. 5.3). In the ribosome there are three positions for tRNA molecules carrying amino acids. At one end is the amino-acyl site where
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an incoming tRNA plus amino acid sits down on the correct codon (Fig. 5.5).
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Proteins Chapter 8_07 - Chapter 5 Proteins From structure...

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