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week-07-aa-prot-G&D-hormones

week-07-aa-prot-G&D-hormones - BIOL 101 Fall 2007 Week...

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Week 7 - Page - 1- Fig. 2 . Peptide bond formation, linking two amino acids Fig. 1. Diagram showing a disulfide bridge between two cysteine molecules. BIOL 101 - Fall 2007- Week 7 -AA - TL - G&D - Hormones - TC Please note - all lecture notes are© Timothy C. Hall, Biology Dept., Texas A&M University AMINO ACIDS: PROTEIN SYNTHESIS AND TRANSLATION Proteins are linear polymers composed of up to 20 different amino acids that have the general formula: 2 R -(CH.NH )-COOH in which R can be any of 20 different radicles (groups), the simplest being H, in glycine (Gly; G). Because the hydroxyl residue (in the 2 carboxyl, COOH, group) has a -ve charge and the NH group a +ve charge, amino acids are differentially charged at each end and are examples of zwitterions . The different characters of the R groups confer dramatically different properties to the amino acids: Acidic amino acids aspartic (Asp; D) and glutamic (Glu; E) acids have additional carboxyl groups. Basic amino acids 2 lysine (Lys; K); arginine (Arg; R) and histidine (His; H) have additional NH groups. Nonpolar amino acids regions of proteins containing these amino acids are hydrophobic and are often embedded in membranes. Polar amino acids regions of proteins containing these amino acids are hydrophilic and are readily miscible with aqueous solutions. Interactions occur between amino acids within and between polypeptide chains. A common and important interaction is the formation of bonds between two cysteine residues (Fig. 1). This ‘disulfife bridge’ is maintained in reducing conditions but broken in oxidizing situations. Peptide : a structure of two ( dipeptide ) or 2 more amino acids, joined by a peptide bond (O=C-NH), formed by removal of H O (a condensation reaction) using an OH from the carboxyl group of one amino acid and H from the 2 NH group of another. Note that, in Fig. 2, the left-hand amino acid retains a free amino group; this defines the N -terminal end. the polypeptide chain grows by addition of amino acids to the
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Week 7 - Page - 2- right-hand, carboxy- ( C -) terminal, end. In eukaryotic proteins the first ( N -terminal) amino acid is almost always methionine ( Met , M). A polypeptide consists of many amino acids. A Protein is the final structure attained by the polypeptide. Folding may be intrinsic to the protein sequence, or assisted by chaperones . Structure Primary : the amino acid sequence. Secondary : results from hydrogen bonding to give $ - (pleated) sheets ; " -helix . Tertiary : disulfide (between S residues of cysteines) and other bonds give unique 3-D structure. Quaternary : the structure obtained if more than one polypeptide ( subunit ) is involved in the protein. Denature is when the shape of the protein is altered, e.g. by heating. This may be irreversible ( coagulation ) or reversible by renaturation . Functions important examples include: enzymes , cell walls , membranes , storage proteins , glycoproteins .
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