Chapter 12 - Proteins

Transcription

1 Roles of Biomolecules Carbohydrates Lipids Proteins 1) Catalytic 2) Transport 3) Regulatory 4) Structural 5) Contractile 6) Protective 7) Storage Nucleic Acids Amino Acids Chapter 12 - Proteins Amino acids Mammals require all 20 -amino acids for protein synthesis amino acids organized according to. (See Table 12.1 next page) 1) 2) 3) 4) Notes: proline glycine 1

2 2

3 Box 12.1 Proteins in Diet minimal reserves for protein production proteins synthesized only when all 12 amino acids present Complete Protein Incomplete Proteins Grains (rice, corn, oats, wheat) Legumes (beans, peas) Nuts 12.2 The Zwitterion Structure of -amino acids Strong secondary forces solubility melting point L-alanine vs. L-lactic acid 12.3 Peptides Peptide Polypeptide Protein 3

4 physiological function determined largely by. # isomers = OMIT SECTIONS: The Peptide Bond and Ionization of Peptides 12.4 Chemical Reactions of Peptides Disulfide bridge formation: 12.5 The 3-D Structure of Proteins Peptide a polyamide formed from amino acids linked by peptide bonds Polypeptide a few to hundreds/thousands of amino acids Protein a polypeptide is considered a protein if 1) 2) Ex. Hemoglobin simple protein conjugated protein amino acid sequence Levels of Structure: Primary (1 ) Secondary (2 ) Tertiary (3 ) Quaternary (4 ) 4

5 Determinants of Protein Conformation 1) 2) the number of allowed conformation are limited by single-bond rotation The following Give rise to the stability of one conformation over others: 1) Shielding of nonpolar -amino acids from water 2) Hydrogen Bonding between peptide groups 3) Attractive interactions between side groups of amino acids a) hydrophobic attractions b) hydrogen-bonding c) salt bridge (ionic) attractions What type of attraction would exist between side chains of the following amino acids? 1) 2) 3) a) Pro-His b) Ser-Tyr c) Pro-Phe d) Lys-Glu e) Ser-Val 4) Attractive interactions of side groups of polar -amino acids with water 5) Disulfide Bridges native conformation Concept Check: (pg. 389) between peptide groups is mainly responsible for the secondary structure of polypeptides. Disulfide bridges and secondary interactions between side groups are mostly responsible for the and structures of proteins. are more resistant to environmental changes such as those in temperature and ph than are the various secondary attractive forces. 5

6 Basic Patterns of protein conformation When favored 1) a-helix 2) b-pleated sheet 3) b-turn (b-bend) 4) Loop conformations 12.6 Fibrous Proteins Key features: solubility elongated shapes having one dimension much longer than the others serve as and proteins usually have repetition of a throughout all or most of chain described as containing no structure usually have structure -keratins Hierarchical structure based on helical polypeptide chains Ex. Hair Individual polypeptides Mostly -helices 6

7 Collagen major stress-bearing component of connective tissues such as bone, cartilage, cornea, ligament, teeth, tendons, and the fibrous matrices of skin and blood vessels. Hierarchical structure based on helical polypeptide chains Compared to -keratins more and contains little L-hand helix bonding and between peptide chains contribute to strength. -keratins and Silk Fibroins 12.7 Globular Proteins Key Features: don t aggregate into macroscopic structures Recall: highly branched molecules have do metabolic work,,, and contain large numbers of amino acids with side chains. possess structure Myoglobin and Hemoglobin Hemoglobin in cells; picks up oxygen in lungs and transports to tissues Myoglobin in cells; picks up oxygen and stores it as reserve Myoglobin polypeptide w/ -amino acids ¾ of residues are folded into -helices connected by nonpolar residues on polar residues on 7

8 Except two histidines prosthetic group, heme, held in cavity inside by hydrophobic attractive forces oxygen binds at site Hemoglobin polypeptide chains ( and ) -helices separated by -turns hydrophobic residues where myoglobin has hydrophilic which serve to each polypeptide carries a with is ion. de-oxygenated hemoglobin has a 2,3-bisphosphoglycerate ion (BPG) in central cavity. hemoglobin w/oxygen hemoglobin no oxygen Carbon monoxide poisoning Lysozyme helps dispose of bacteria after they have been killed by other means all four types secondary structures 12.8 Mutations: Sickle-cell Hemoglobin KEY: protein function ultimately depends on. genetic mutation the substitution of an a-amino acid residue similar often has minimal effect on the three-dimensional structure and may have no effect on function. Ex. for Val for Glu for Ser The substitution of very different amino acids may result in a large change in 3-D structure. Ex. for Gly for Glu for Ala Normal hemoglobin - sickle-cell hemoglobin Val-His-Leu-Thr-Pro-Glu-Glu-Lys- Val-His-Leu-Thr-Pro-Val-Glu-Lys- regions critical to binding oxygen changed sickling is the aggregation of the hemoglobins (hydrophobic attractions between the hydrophobic pocket and residue 6 (Val) Sickle-cell anemia found in likely because of the high incidence of. 8

9 If Val-His-Leu-Thr-Pro-Ala-Glu-Lys- no effect on hemoglobin function Ex. Sickle-cell hemoglobin deoxygenated red blood cells containing sickle-cell hemoglobin take on an elongated sickle shape instead of the normal biconcave disk shape. the sickled cells aggregate together into long rodlike structures that do not move easily though the blood capillaries. the capillaries become inflamed, causing considerable pain. Sickle-cell anemia Sickle-cell trait 12.9 Denaturation Denaturation - alters structures. denaturation involves breaking bonds unlike digestion which breaks bonds and alters structure. resistant to denaturation Globular proteins have secondary forces compared to fibrous proteins and are thus denatured. (whipping eggs) Methods of Denaturation: 1) Increased temperature 5) Organic Chemicals 2) Ultraviolet and ionizing radiations 6) Salts of heavy metals 3) Mechanical Energy 7) Oxidizing and reducing Agents 4) Changes in ph Prion diseases Alzheimer s Disease 9