soluble in water, not organic solvents, amphoteric, Zwitterions- electrically neutral, dipolar ion - hydrocarbon R groups - hydrophobic and unreactive - Localized inside molecule NON-POLAR (Hydrophobic) POLAR uncharged (Hydrophillic) - polar R groups (–OH & –NH) lots of O - no net charge. - hydrophilic in nature. POLAR, charged (Hydrophilic) : - -vely / +vely R group, hydrophilic. - Acidic Aa = net -, (COO - in R) - Basic Aa = net +, (NH2 + in R) PP formation (conden rxn) - N & C Terminus - R gp project fr backbone - Lesser buffering capacity - Biuret Test for peptide bonds -Purple = Pos, Blue = Neg (i) a basic amino group (–NH2) (ii) an acidic carboxyl group (–COOH) (iii) a hydrogen atom and (iv) a variable group known as the R gp which gives 'uniqueness' to the Aa Except for glycine, all Aa have at least one asymmetric C- atom and thus exhibit optical isomerism R groups have important physical (size & shape) and chemical properties that determine the specific properties of a given amino acid. Glycine (R= H) - Hydropohilic / phobic - Smallest Aa, thus allow close contact./. or within pp > flexible, allow pp to form hinges / more Proline (Hydrophobic Aa) - does not fit into 2 o structure - produces kinks or hinges eg: collagen Cysteine - Reactive SH gp - forms S-S group -Intra / inter pp - stabilizes 3D protein

These levels of structure are dependent on each other. If one level changes, the other levels may change as a consequence. Primary - unique # and linear sequence of Aa - determined by genetic DNA sequences (KIV: Gene Expression). - contains info to specify how pp chains will coil, fold and interact - Size, charge, polarity or hydrophobicity of R groups will affect the bonds present at higher levels and the ultimate 3-D conformation n function of protein  Secondary  - regular coiling and folding of pp regions  - resultant repeated patterns is 2 o structure  - by regularly spaced H-bonds formed at the pp backbone between NH group of one Aa & C=O group of other Aa  - H-bonds DO NOT involve R groups. - α-helix and β-pleated sheet.  -helix  -sheet Peptide BondsH-Bonds along pp backbone α-keratin - fibrous protein with an α-helical shape. -3 helices - protofibril (S-S) cross-linkages. - Tough – d/p on o of S-S bonds - Stretchable – H-bonds break & reform easily - Insoluble in water –hydrophobic R groups on outside of the fibre. Silk fibroin -entirely  sheet adj // chains, in ops directions (i) Inelastic – covalently bonded chains (peptide backbone) is nearly fully extended. (ii) Very strong – all C=O and –NH groups are involved in hydrogen bonding. - extended spiral spring. - stabilized by hydrogen bonds, between all (C=O) & (– NH) groups. -nth + 4 residue, (1 turn 3.6 Aa) -- H bonds parallel to the main axis - R groups project perpendicular to main axis. - R groups influence α-helix;s interaction - extended zigzag, sheet-like conformation. - H bonds between C=O and –NH groups of one region pp & the C=O and –NH groups of adjacent regions of the same chain / different chains. - intrachain sheet -interchain sheets -R group steric hindrance, thus aa in β-pleated sheet usually have small R groups. -ANTI-PARALLEL – -PARALLEL

-Interactions between R groups only - 4 main types of interactions -Maintains tertiary structure -Similar bonds allow for interaction with other pp mols -Give rise to quat structure 3 0 & 4 0 structure Fibrous proteinsGlobular proteins Repetitive regular sequence of amino acids Fixed specific sequence of amino acids; non-repetitive Amino acid sequence may vary slightly between two examples of the same fibrous protein Amino acid sequence never varies between two examples of the same globular protein Usually elongated polypeptide chains wrapped around to form multimolecular parallel filaments to strands Much more compact structures owing to highly contorted pattern of folding, bending and twisting along polypeptide chain to form a spherical shape. Has a specific three-dimensional structure. Length of polypeptide chain may vary in two examples of the same fibrous protein Length of polypeptide always identical in two examples of the same globular protein Stable structure due to the numerous intra- and inter-molecular hydrogen and disulphide bonds Relatively unstable structure. This is due to the numerous intra- and inter- molecular non-covalent bonds, such as hydrogen, ionic bonds Generally insoluble in waterGenerally more soluble than fibrous proteins Perform structural functions. Examples include keratin, fibroin and collagen. Perform metabolic functions. Examples include enzymes, hormones, antibodies and haemoglobin. Usually tertiary and quaternary level of protein organisation

Aquaporin

3.6 amino acid residues / turn