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Transcript
If I make a 5-bead necklace and I have 20 different colours of bead. How many different necklaces can I make? 3.2 million!!! Learning goals: • (b) describe, with the aid of diagrams, the structure of an amino acid; • (c) describe, with the aid of diagrams, the formation and breakage of peptide bonds in the synthesis and hydrolysis of dipeptides and polypeptides; • (d) explain, with the aid of diagrams, the term primary structure; • (e) explain, with the aid of diagrams, the term secondary structure with reference to hydrogen bonding; • (f) explain, with the aid of diagrams, the term tertiary structure, with reference to hydrophobic and hydrophilic interactions, disulfide bonds and ionic interactions; • (g) explain, with the aid of diagrams, the term quaternary structure, with reference. The structure of amino acids. Amino acids. • Proteins are made from 20 types of amino acids. • Each amino acid has a different R-group R-group How do the R-groups differ? Where do we get amino acids from? • “Non essential” amino acids are made by the body. • “Essential” amino acids need to be eaten in our diet. How do they form proteins? Condensation reaction -OH from carboxyl group -H from amino group Forms peptide bond Peptides • When two amino acids join together a dipeptide is formed. • Three amino acids form a tripeptide. • Many amino acids form a polypeptide. e.g. • In a polypeptide there is always one end with a free amino (NH2) group, called the N-terminus, and one end with a free carboxyl (COOH) group, called the C-terminus. Structure of proteins • Primary • Secondary • Tertiary • Quarternary The sequence of amino acids in a polypeptide chain is called the primary structure of a protein. Secondary structure • The protein’s secondary structure is formed when a chain of amino acids coils to form an alpha helix or folds into a betapleated sheet. • Both require H-bonding. Alpha helix • Every backbone N-H group donates a hydrogen bond to the backbone C=O group of the amino acid four residues along. Beta pleated sheet • Hydrogen bonds form between chains lying next to one another. Beta sheet challenge Tertiary structure • Further folding or twisting of the whole polypeptide chain. • Bonds form between R groups in the polypeptide chain. • Include disulphide bonds, ionic bonds, hydrogen bonds, hydrophobic interactions. Disulphide bonds • Form between R-groups containing sulphur atoms e.g. cysteine and methionine. Ionic bonds • Form between R groups with charges e.g positive and negative charges. Hydrophobic interactions Non polar R-groups tend to cluster together in water. These weak associations are called hydrophobic interactions. When the polypeptide chain folds, the hydrophobic R-groups tend to be close to each other in the interior of the folded chain, whereas hydrophilic R-groups tend to be on the outside, attracted to water. Label the diagram to show the bonds and interactions in the tertiary structure. Quarternary structure • Almost all working proteins are actually composed of more than one polypeptide chain, and the arrangement of the different chains is called the quaternary structure. There are a huge variety of quaternary structures e.g.: • The primary structure of a protein is the sequence of amino acids, joined by peptide bonds • The secondary protein structure occurs when the sequence of amino acids are linked by hydrogen bonds in an alpha helix or beta pleated sheet. • The tertiary structure describes the folding of a polypeptide chain that result from the molecular interactions among the R groups of the different amino acids( H, disulphide, ionic bonds) • The arrangement of two or more polypeptide chains in a protein make up its quaternary structure Answer the summary questions on amino acids. Amino acids join together into polypeptide chains by .......................... bonds which are formed by ........................... reactions. The linking bonds are formed between the ................ and ................. groups of the amino acids when ........................ is released from the reaction. The polypeptide chains may be folded into secondary structures, such as the ........................... and ........................... . Secondary structures may be further folded into tertiary structures, such as the ........................... . Polypeptides are combined together to give the .......................... structure of the protein. These three dimensional shapes of the protein are held firmly in place by ionic bonds, ...................... bonds and covalent ..................... bonds. Disruption of these bonds, by extremes of pH or exposure to ........................... causes ........................... of the protein. • peptide; condensation; acid/amine;; water; alpha helix,beta pleated sheet;; globular/coil type/shape; quaternary; hydrogen; sulphur; high temperatures; denaturation