27.19 Secondary Structures of Peptides and Proteins

1. 27.19Secondary Structuresof Peptides and Proteins

2. Levels of Protein Structure • Primary structure = the amino acid sequence plus disulfide links • Secondary structure = conformational relationship between nearest neighbor amino acids • a helix pleated b sheet

3. Levels of Protein Structure The a-helix and pleated b sheet are both characterized by: • planar geometry of peptide bond • anti conformation of main chain • hydrogen bonds between N—H and O=C

4. Pleated b Sheet • Shown is a b sheet of protein chains composed of alternating glycine and alanine residues. • Adjacent chains are antiparallel. • Hydrogen bonds between chains. • van der Waals forces produce pleated effect.

5. Pleated b Sheet • b Sheet is most commonly seen with amino acids having small side chains (glycine, alanine, serine). • 80% of fibroin (main protein in silk) is repeating sequence of —Gly—Ser—Gly—Ala—Gly—Ala—. • b Sheet is flexible, but resists stretching.

6. a Helix • Shown is an a helix of a protein in which all of the amino acids are L-alanine. • Helix is right-handed with 3.6 amino acids per turn. • Hydrogen bonds are within a single chain. • Protein of muscle (myosin) and wool (a-keratin) contain large regions of a-helix. Chain can be stretched.

7. 27.20Tertiary Structureof Peptides and Proteins

8. Tertiary Structure • Refers to overall shape (how the chain is folded) • Fibrous proteins (hair, tendons, wool) have elongated shapes • Globular proteins are approximately spherical • most enzymes are globular proteins • an example is carboxypeptidase

9. Carboxypeptidase • Carboxypeptidase is an enzyme that catalyzes the hydrolysis of proteins at their C-terminus. • It is a metalloenzyme containing Zn2+ at its active site. • An amino acid with a positively charged side chain (Arg-145) is near the active site.

10. Carboxypeptidase Disulfide bond Zn2+ Arg-145 N-terminus C-terminus tube model ribbon model

11. R What happens at the active site? •• O O H2N •• + C – + peptide H3N NHCHC Arg-145 C O H2N

12. R What happens at the active site? • The peptide or protein is bound at the active site by electrostatic attraction between its negatively charged carboxylate ion and arginine-145. •• O O H2N •• + C – + peptide H3N NHCHC Arg-145 C O H2N

13. Zn2+ R What happens at the active site? • Zn2+ acts as a Lewis acid toward the carbonyl oxygen, increasing the positive character of the carbonyl carbon. •• O O H2N •• + C – + peptide H3N NHCHC Arg-145 C O H2N

14. Zn2+ R H O •• •• H What happens at the active site? • Water attacks the carbonyl carbon. Nucleophilic acyl substitution occurs. •• O O H2N •• + C – + peptide H3N NHCHC Arg-145 C O H2N

15. Zn2+ •• O H2N •• + – •• C peptide H3N O •• O •• + – H3NCHC R O What happens at the active site? H2N + Arg-145 C

16. 27.21Coenzymes

17. Coenzymes • The range of chemical reactions that amino acid side chains can participate in is relatively limited. • acid-base (transfer and accept protons)nucleophilic acyl substitution • Many other biological processes, such as oxidation-reduction, require coenzymes, cofactors, or prostheticgroups in order to occur.

18. Coenzymes • NADH, coenzyme A and coenzyme B12 are examples of coenzymes. • Heme is another example.

19. H2C CH CH3 H3C CH CH2 N N Fe N N H3C CH3 HO2CCH2CH2 CH2CH2CO2H Heme • Molecule surrounding iron is a type of porphyrin.

20. C-terminus N-terminus Myoglobin Heme • Heme is the coenzyme that binds oxygen in myoglobin (oxygen storage in muscles) and hemoglobin (oxygen transport).

21. 27.22Protein Quaternary Structure:Hemoglobin

22. Protein Quaternary Structure • Some proteins are assemblies of two or more chains. The way in which these chains are organized is called the quaternary structure. • Hemoglobin, for example, consists of 4 subunits. • There are 2 a chains (identical) and 2 b chains (also identical). • Each subunit contains one heme and each protein is about the size of myoglobin.