Lecture Notes on Peptides and Proteins | CHEM C1260, Study notes of Chemistry

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CHM333 LECTURE 8: 9/10/07 FALL 2007 Professor Christine Hrycyna 38 PEPTIDES and PROTEINS - Learned basic chemistry of amino acids – structure and charges - Chemical nature/charges of amino acids is CRUCIAL to the structure and function of proteins - Amino acids can assemble into chains (peptides, polypeptides, proteins) o Can be very short to very long  Dipeptide = two amino acids linked  Tripeptide = three amino acids linked - Amino acids sometimes called RESIDUES - Identity and function of a protein or peptide is determined by o Amino acid composition o Order of amino acids in the chain o Enormous variety of possible sequences  e.g., if you have a protein with 100 aa, there are 1.27 X 10130 possible sequences! - Amino acids are linked by COVALENT BONDS = PEPTIDE BONDS - Peptide bond is an amide linkage formed by a condensation reaction (loss of water) - Brings together the alpha-carboxyl of one amino acid with the alpha-amino of another - Portion of the AA left in the peptide is termed the amino acid RESIDUE o Amino acids sometimes called RESIDUES - R groups remain UNCHANGED – remain active - N-terminal amino and C-terminal carboxyl are also available for further reaction - Reaction is NOT thermodynamically favorable (not spontaneous) o Need energy and other components and instructions to correctly assemble  This is the process of protein translation FORMATION OF THE PEPTIDE BOND CHM333 LECTURE 8: 9/10/07 FALL 2007 Professor Christine Hrycyna 39 Animation of peptide bond formation: http://www.cat.cc.md.us/courses/bio141/lecguide/unit4/peptide.html http://www.specialedprep.net/MSAT%20SCIENCE/Cellular%20Biology/Proteins1.htm#HOW - Peptides are always written in the N  C direction - Each peptide has ONLY ONE free amino group and ONE free carboxyl group; others are neutralized by formation of the peptide bond IMPORTANT FEATURES OF THE PEPTIDE BOND: 1. Peptide bonds have double bond character resulting from resonance stabilization (C-N bond has 40% double bond character) C-N and C-O have partial double bond character Animation: http://www.mcphu.edu/netbiochem/movies/peptresn.mov - Results in the peptide bond being PLANAR o C, N, H, O are all in the same plane (Cα’s are also in plane) o p orbitals can overlap to form partial double bonds between the nitrogen and carbon and the carbon and oxygen - Stronger than a normal bond because of the double bond character CHM333 LECTURE 8: 9/10/07 FALL 2007 Professor Christine Hrycyna 42 SUMMARY: 1. Cα , O, C, N and H atoms are planar 2. No rotation around the peptide bond 3. Cα (with R groups) groups are in trans configuration (i.e. H on the amide Nitrogen is opposite to the Oxygen of the carbonyl) 4. Peptide backbone is rigid DRAWING PEPTIDES GIVEN THEIR AMINO ACID SEQUENCE: - Peptide = ALDS - Left  Right = N  C-terminus (amino to carboxyl termini) - Want to draw it at pH 5 1) Draw the PEPTIDE BACKBONE fully protonated for as many amino acids as you have. For convenience, convention and ease, put the R groups down and the carbonyls up. In our case, we have FOUR amino acids: CHM333 LECTURE 8: 9/10/07 FALL 2007 Professor Christine Hrycyna 43 2. Add in the side chains onto the Cα carbons – the ones that have the CH group – in order left to right. Draw side chains fully protonated as well. 3. Determine if groups are ionized at the pH that you are interested in. Remember if pH > pKa, the H+ is OFF (deprotonated) Let’s say pH 5: - Terminal carboxyl group is DEPROTONATED (5 > 2.21) - Terminal amino group is NOT DEPROTONATED (5 < 9.69) - Carboxyl group on Aspartate in DEPROTONATED (5 > 3.9) Go back to fully protonated figure above and remove 2 protons: One from the terminal carboxyl, and one from the side chain of Asp. Note: In di, tri, or polypeptides, there is only ONE free amino group (N-terminus) and ONE free carboxyl group UNLESS present in R groups. You should be able to write out the full structures of peptides at different pH values. CHM333 LECTURE 8: 9/10/07 FALL 2007 Professor Christine Hrycyna 44 Calculate an approximate pI for the peptide Val-Cys-Arg-Phe-His-Asp-Gln. α-carboxyl COOH (pKa 2.2) Asp R-COOH (pKa 3.9) His imidazole + (pKa 6.0) α-amino NH3 + (pKa 9.6) Cys-SH (pKa 8.3) Arg guan. + (pKa 12.5) Sketch out starting with fully protonated and remove protons in order until you get the neutral species. Take the average of the pKa’s surrounding the neutral species. Net charge at low pH = +3, so have to add 3 equivalents of base to get to net charge zero. 3 equiv. base would completely titrate the α-carboxyl, R-carboxyl, and His imidazole, so pI would be halfway between pK of imidazole and pK of α-amino group, i.e. (6.0 + 8.3)/2 so pI = 7.15. You need to be able to do these types of problems! Interesting Peptide in Biological Systems: 1. Glutathione a. Tripeptide of glutamate, cysteine, glycine b. Regulates oxidation/reduction reactions in cells c. Destroys destructive free radicals by scavenging oxidizing agents 2. Oxytocin and Vasopressin a. Pituitary gland peptide hormones b. Nonapeptides cyclized by disulfide bond c. Oxytocin stimulates uterine contractions during childbirth – induces labor d. Vasopressin stimulates water resorption by kidneys and increases blood pressure (anti-diuretic hormone)