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Section B Protein Structure Molecular Biology Course.

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Presentation on theme: "Section B Protein Structure Molecular Biology Course."— Presentation transcript:

1 Section B Protein Structure Molecular Biology Course

2 B1 Amino acids: structure, side chains (charged, polar uncharged, nonpolar aliphatic, aromatic) B2 Protein structure and function: Structure: size and shapes, primary  secondary  tertiary  quaternary (prosthetic groups) ; Biological functions Structure & function: Domains, motif, and family B3 Protein analysis Purification  Determine sequence, mass, and structure (X-ray crystallography and NMR) Molecular Biology Course Cross with Biochemistry

3 B1 Amino acids- basic structure H3NH3N COO - R H CC 4.A few proteins contain nonstandard amino acids that are formed by post-translational modification of proline and lysine. proline Common structure of 19 AAs  -carbon is chiral (asymmetric) except in glycine (R is H) 2.Amino acids are dipolar ions (zwitterions [ 兼性离子 ] ) in aqueous solution and are amphoteric ( 兼性的同时有酸碱性或正负 电荷的 ) 3.The side chains (R) differ in size, shape, charge and chemical reactivity Protein structure

4 1. “Acidic” amino acids (2): containing additional carboxyl groups which are usually ionized Form salt bridges, are hydrophilic ( 亲水 ) aspartic acid (Asp, D ,天冬氨酸 ) glutamic acid ( Glu, E ,谷氨酸 ) Protein structure B1 Amino acids- charged (5)

5 2. “Basic” amino acids (3): containing positively charged groups Lysine (Lys, K, 赖氨酸 ) The imidazole group ( 咪唑基 ) has a pK a near neutrality. This group can be reversibly protonated under physiological conditions, which contribute to the catalytic mechanism of many enzymes. Arginine (arg, R, 精氨酸 )   a guanidino group ( 胍基) Histidine (His, H ,组氨酸 ) Protein structure

6 Contain groups that form hydrogen bonds with water, hydrophilic Contain hydroxyl groups. Asparagine (Asn, N ,天冬酰氨 ) B1 Amino acids- polar uncharged (5) Serine (Ser, S ,丝氨酸 ) Threonine (Thr, T ,苏氨酸 ) Glutamine (Gln, Q ,谷氨酰氨 ) Protein structure

7 Cysteine (Cys, C ,半胱氨酸 ) has a thiol ( 巯醇 ) group , which is often oxidizes to cystine x-S-S-x Protein structure

8 Glycine (Gly, G ,甘氨酸 ) Proline (Pro, P ,脯氨酸 ) : imino acid ( 亚氨基酸 ) Methionine (Met, M ,甲硫氨酸 ): contains a sulfur atom Protein structure B1 Amino acids- nonpolar aliphatic (7) (hydrophobic 疏水 )

9 Alkyl ( 烷基 ) side chains Alanine (Ala, A ,丙氨酸 ) Valine (Val, V ,缬氨酸 ) Leucine (Leu, L ,亮氨酸 ) Isoleucine (Ile, I ,异亮氨酸 ) Protein structure

10 Phenylalanine (Phe, F ,苯丙氨酸 ) Tyrosine (Tyr, Y ,酪氨酸 ) Tryptophan (Trp, W ,色氨酸 ) Protein structure B1 Amino acids- aromatic (3) Accounts for most of UV absorbance of proteins at 280 nm hydrophobic (疏水的 )

11 B2Protein structure and function Molecular Biology Course Structure: size and shapes, primary  secondary  tertiary  quaternary, prosthetic groups ( 辅 基, nonprotein molecules of conjugated proteins ( 结合蛋白 ) Domains, motif, and family Protein function

12 B2Protein structure - Sizes 1.A few thousands Daltons (x 10 3 ) to more than 5 million Daltons (x 10 6 ) 2.Some proteins contain bound nonprotein materials (prosthetic groups or other macromolecules), which accounts for the increased sizes and functionalities of the protein complexs. Protein structure

13 Globular proteins: enzymes Complementary fit of a substrate molecule to the catalytic site on an enzyme molecule. Protein structure B2Protein structure - Shapes

14 Fibrous proteins : important structural proteins (silk fibroin, keratin in hair and wools ) keratin in hair Keratin ( 角蛋白 ) Protofibril ( 初原纤维 ) microfibril ( 微管 ) Protein structure

15 Polypeptides contain N- and C- termini and are directional, usually ranging from aa Formation of a peptide bond (shaded in gray) in a dipeptide. Protein structure B2Protein structure - Primary

16 Structure of the pentapeptide Ser-Gly-Tyr-Ala-Leu. N terminus C terminus Protein structure

17  -helix right-handed 3.6 aa per turn hydrogen bond N-H···O=C Collagen triple helix: three polypeptide intertwined A stereo, space-filling representation Protein structure B2Protein structure - Secondary

18  -sheet: hydrogen bonding of the pepetide bond N-H and C=O groups to the complementary groups of another section of the polypeptide chain A stereo, space-filling representation of the six- stranded antiparallel  sheet. Parallel  sheet: sections run in the same direction Antiparallel  sheet: sections run in the opposite direction Protein structure

19 B2Protein structure - Tertiary The different sections of  -helix,  -sheet, other minor secondary structure and connecting loops of a polypeptide fold in three dimensions

20 Noncovalent interaction between side chains that hold the tertiary structure together: van der Waals forces, hydrogen bonds, electrostatic salt bridges, hydrophobic interactions Covalent interaction: disulfide bonds Denaturation of protein by disruption of its 2 o and 3 o structure by heat and extremes of pH will lead to a random coil conformation Protein structure

21 Many proteins are composed of two or more polypeptide chains (subunits). These subunits may be identical or different. The same forces which stabilize tertiary structure hold these subunits together. This level of organization called quaternary structure. The quaternary structure of hemoglobin 。  1-yellow;  1-light blue;  2-green;  -dark blue; heme-red back Protein structure B2Protein structure - Quaternary

22 Advantages of the quaternary structure: 1.It allows very large protein molecules to be made, such as tubulin 。 2.It can provide greater functionality to a protein by combining different activities into a single entity. 3.The interactions between the subunits can often be modified by binding of small molecules and lead to the allosteric effects seen in enzyme regulation Protein structure

23 Covalently or noncovalently attached to many conjugated proteins, and give the proteins chemical functionality. Many are co-factors in enzyme reactions. Examples : heme groups in hemogobin (Figure)Figure Protein structure B2Protein structure - Prosthetic groups

24 Protein structure B2Protein structure - Domains, motifs and families Domains: structurally independent units (tertiary) of a protein, being connected by sections with limited higher order structure within the same polypeptide. (Figure)(Figure) They can also have specific function such as substrate binding

25 Protein structure Structural motifs: Groupings of secondary structural elements that frequently occur in globular proteins Often have functional significance and represent the essential parts of binding or catalytic sites conserved among a protein family  motif

26 Protein families: structurally and functionally related proteins from different sources The primary structures of c-type cytochromes from different organisms Protein structure Motif

27 The tertiary structures of the above c-type cytochromes Cys, Met and His side chains covalently linked to the heme to the protein Protein structure Domain Back

28 Enzymes: substrate binding, side chain in catalysis Signaling: cell membrne Transport and storage: hemoglobin transports oxygen Structure and movement: collagen, keratin, tubulin in cytoskeleton, actin and myosin for muscle contraction Nutrition: casein ( 酪蛋白 ) and ovalbumin( 卵清蛋白 ) Immunity: antibodies Regulation: transcription factors Protein structure B2Protein functions

29 B3Protein analysis 1. Purification: to obtain enough pure sample for study 2. Sequencing: determine the primary structure of a pure protein sample 3. Mass determination: determine the molecular weight (MW) of an interested protein. 4. X-ray crystallography and NMR: determine the tertiary structure of a given sample. Protein structure

30 Protein purification To purify the interested protein from other proteins and nonprotein molecules existing in the cells An essential experimental step prior to study of any individual protein Protein structure

31 The principal properties of proteins used for purification 1.Size: gel filtration chromatography 2. Charge: ion-exchange chromatography, isoelectric focusing, electrophoresis 3. Hydrophobicity: hydrophobic interaction chromatography 4. Affinity: affinity chromatography 5. Recombinant techniques: involving DNA manipulation and making protein purification so easy Protein structure

32 1.Gel filtration chromatography Protein structure

33 Bead: the matrix determine the size of the pore Protein structure

34 Ultracentrifugation: can be used to separate proteins according to their size and shape that determine their sedimentation rate. Very large protein complex. Protein structure

35 2. Charge: ion-exchange chromatography, isoelectric focusing, electrophoresis Isoelectric point (pI): the pH at which the net surface charge of a protein is zero pH=pI pH < pI pH > pI Protein structure

36 Charge 1: Ion-exchange chromatography Column + anions Sample mixture Protein binding Column + proteins Column + anions Ion displacing Purified protein Protein structure

37 + Charge 2: Electrophoresis Protein migrate at different position depending on their net charge Protein structure

38 Charge 3: Isoelectric focusing A protein will stop moving at position corresponding to its isoelectric point (pI) in a pH gradient gel. Protein structure

39 3. Hydrophobicity: hydrophobic interaction chromatography Similar to ion-exchange chromatography except that column material contains aromatic or aliphatic alkyl groups Protein structure

40 4. Affinity chromatography Enzyme-substrate binding Receptor-ligand binding Antibody-antigen binding Protein structure

41 5. Recombinant techniques: Clone the interested protein encoding gene in an expression vector with a purification tag added at the 5’- or 3’ end of the gene Protein overexpression in a cell Protein purification with affinity chromatography. Protein structure

42 Mass Determination  Gel filtration chromatography and SDS-PAGE Comparing of the unknown protein with a proper standard Popular SDS-PAGE: cheap and easy with a 5-10% error SDS: sodium dodecyl sulfate, makes the proteins negatively charged and the overall charge of a protein is dependent on its mass. Protein structure

43 Mass spectrometry: Molecules are vaporized and ionized, and the degree of deflection (mass-dependent) of the ions in an electromagnetic field is measured extremely accurate, but expensive MALDI can measure the mass of proteins smaller than 100 KDa Helpful to detect post-translational modification Protein sequencing: relying on the protein data base

44 Protein sequencing : Determine the primary structure of a protein Specific enzyme/chemical cleavage: Trypsin cleaves after lusine(K) or arginine (R) V8 proteease cleaves after glutamic acid (E) Cyanogen bromide cleaves after methionine (M) Edaman degradation: Performed in an automated protein sequencer Determine the sequence of a polypeptide from N- terminal amino acid one by one. Expensive and laborious Protein structure

45 Most protein sequences are deduced from the DNA/cDNA sequence Direct sequencing: determine the N-terminal sequences or some limited internal sequence  construction of an oligonulceotide or antibody probe  fishing the gene or cDNA Protein structure

46 X-ray crystallography and NMR Determing the tertiary structure (3-D) of a protein X-ray crystallography: Measuring the pattern of diffraction of a beam of X-rays as it pass through a crystal. The first hand data obtained is electron density map, the crystal structure is then deduced. A very powerful tool in understanding protein tertiary structure Many proteins have been crystallized and analyzed Protein structure

47 Measuring the relaxation of protons after they have been excited by radio frequencies in a strong magnetic field Measure protein structure in liquid but not in crystal Protein measured can not be larger than 30 KDa NMR

48 Protein Folding: chaperones are involved in vivo (1)The rapidly reversible formation of local secondary structure (2)Formation of domains through the cooperative aggregation of folding nulcei (3)Assembly of domains into a “molten” globule (4)Conformational adjustment of the monomer (5)Final conformational adjustment of the dimeric protein to form the native structure. back Protein structure

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