Peptide Bond Formation PDF

AMINO ACID PEPTIDE FORMATION __________ Prof JULITO A. AGUDONG, EdD, LPT CASE/MaPS  Amino acids can form peptide bonds Amino acid residue Proteins are molecules that consist peptide units...

AMINO ACID PEPTIDE FORMATION __________ Prof JULITO A. AGUDONG, EdD, LPT CASE/MaPS  Amino acids can form peptide bonds Amino acid residue Proteins are molecules that consist peptide units of one or more polypeptide chains dipeptides tripeptides oligopeptides polypeptides Peptides are linear polymers that range from ~8 to 4000 amino acid residues How many different naturally occurring amino acids are there in most species encoded by the genome? Linear arrays of amino acids can make a huge number of molecules Consider a peptide with two amino acids AA1 AA2 21 x 21 = 441 different molecules AA1 AA2 AA3 21 x 21 x 21 = 9261 different molecules For 100 amino acid protein the # of possibilities are: 20 = 1.27 x10 100 130 80 The total number of atoms in the universe is estimated at 4x10  Peptide bonds Proteins are sometimes called polypeptides since they contain many peptide bonds R1 O H R2 O + H3N C C OH + H N C C O- H H R1 O R2 O + + H2O H3N C C N C C O- H H H  Amide character in the peptide bond Since the peptide bond is also an amide bond, it also undergoes resonance. R1 O R2 O + H3N C C N C C O- H H H Therefore, peptides are rigid due to resonance around the amide bond, having ≈ 40% double-bond character. This restricts the rotation due to delocalization of electrons and overlap of the O-C-N  orbitals.  Peptide Bond Formation is a dehydration synthesis reaction (also known as a condensation reaction).  Peptide Bond Formation Mnemonic device for writing structural formula of peptides. H N-terminal C-terminal An example of a tetrapeptide with three (3) peptide bonds (yellow bond)  Peptides The Peptide (Amide) Bond – The amide nitrogen is sp2 hybridized and the lone pair is conjugated with the carbonyl group – There is considerable C–N double-bond character – Rotation about the C–N bond is difficult O O C C N N H H  Peptides The Peptide (Amide) Bond – The amide groups are planar. – The R-groups are on opposite sides of the plane. gly-ser-phe-ala  Peptides The Peptide (Amide) Bond – The amide groups are planar. – The R-groups are on opposite sides of the plane. gly-ser-phe-ala  Peptides Peptides are amino acid polymers containing 2–50 individual units Peptides with >50 units are called proteins By convention, peptide structures are written with the N-terminal amino acid on the left and the C-terminal amino acid on the right. O H3N CH2 C NH CH CO2 CH3 A dipeptide glycylalanine = gly-ala  Peptides glycylalanine = gly-ala O H3N CH2 C NH CH CO2 CH3 glycine amino-terminal amino acid  Peptides glycylalanine = gly-ala O H3N CH2 C NH CH CO2 CH3 alanine carboxy-terminal amino acid  Peptides glycylalanine = gly-ala O H3N CH2 C NH CH CO2 CH3 peptide bond  Peptides A tetrapeptide: O O O H3N CH2 C NH CH C NH CH C NH CH2 CO2 N-terminus CH2OH CH2C6H5 C-terminus glycylserylphenylalanylglycine gly-ser-phe-gly  Peptides ▫ peptide: the name given to a short polymer of amino acids joined by peptide bonds; they are classified by the number of amino acids in the chain ▫ dipeptide: a molecule containing two amino acids joined by a peptide bond ▫ tripeptide: a molecule containing three amino acids joined by peptide bonds ▫ polypeptide: a macromolecule containing many amino acids joined by peptide bonds ▫ protein: a biological macromolecule of molecular weight 5000 g/mol or greater, consisting of one or more polypeptide chains  Peptides The only other type of covalent bond between amino acids in proteins and peptides is the disulfide linkage between two cysteine units: NH HN O C C O CH CH2 S S CH2 CH HN NH  Peptides Note: – Thiols are readily oxidized to disulfides. – Disulfides are readily reduced to thiols. [O] R SH + HS R R S S R [H]  Peptides Disulfide links serve to connect polypeptide chains:     S S            Peptides … or can form a macrocycle: Copyright © 2010 Pearson Education, Inc. Cys.Tyr.Ile.Glu.Arg.Cys.Pro.Leu.Gly.NH2 S S bovine oxytocin  Peptides Conotoxin G I (12 Amino acid residues) 2 disulfide bonds (AA2 & AA7) and (AA3 & AA12) S S Glu-Cys-Cys-Asn-Pro-Ala-Cys-Gly-Arg-His-Tyr-Cys-NH2 S S – A peptide neurotoxin isolated from the venom of the fish-hunting cone snail Conus geographicus. – It is a paralytic toxin that blocks nicotinic cholinoreceptors.  Peptides Conotoxin G I S S Glu-Cys-Cys-Asn-Pro-Ala-Cys-Gly-Arg-His-Tyr-Cys-NH2 S S  Using the mnemonic device, write the structure of a tetrapeptide if R1=ala R2=ser R3= gly & R4= asp; what is the name of the tetrapeptide? By substituting the R-group of the given amino acids in the mnemonic, below is the structure of the tetrapeptide. The name of the tetrapeptide is alanylserylglycylaspartic acid ala gly CH3 H H CH2 CH2- C-terminal N-terminal OH COOH ser asp  ASSIGNMENT 1. Using the mnemonic device, draw the structure of a heptapeptide with the following amino acids: R1=thr R2=glu R3=asn R4=phe R5=lys R6= cys R7=ile 2. Name the heptapeptide 3. Use a red pen for drawing the peptide bond. Thank You!