Download Understanding Protein Structure: Conformations, Dihedral Angles, and Secondary Structures and more Study notes Biology in PDF only on Docsity! Structure
Non superimposable 3D arrangements that are interconvertible without
breaking covalent bonds > CONFORMATIONS
Secondary Tertiary Quaternary
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20 amino acids differing in SIDE CHAINS‐ these side chains must confer 3D structure (otherwise all would look the same!) All amino acids except glycine are chiral h i i i i f‐ t ey can ex st n m rror mage orms All backbones are the same L or D TheL‐form reads CORN in clockwise direction The translational machinery for protein synthesis has evolved only to use L‐forms So – can φ and ψ have any value? NO!!! Most combinations of ψ and φ for an amino acid are not allowed because of steric collisions between the side chains and main chain. Th i ll if h i l ll 180o h leoret ca y t e tors on ang es a are we ave a pure y trans system ( N, Cα, C’) if the torsion angles are all 0 we have the purely cis‐ arrangement. Which is preferred? For the peptide bond (ω) [Ci’ – Ni+1] The trans is preferred 1000:1 S t i lik dj t i ido‐ pro e ns e a acen am no ac side chains to point away from each other. The exception is when the i+1 residue is a Pro Trans only favored 4:1 so you can get cis‐trans isomerization The values of φ and ψ that are possible through the geometric constraints were first determined by Ramachandran and are usually plotted against φ and ψ angles. Ramachandran Plots Way to visualize dihedral angles φ against ψ of amino acid residues in protein structure. It shows the possible conformations of φ and ψ angles for a polypeptide. 180o ψ 0o φ ‐180o 0o 180 o Regular conformations of polypeptides: MOTIFS OF PROTEIN STRUCTURE The main driving force for folding water soluble globular protein molecules is to pack hydrophobic side chains into the interior of the molecule thus creating a HYDROPHOBIC core and a HYDROPHILIC surface. The problem with creating such a hydrophobic core from a protein chain is– to bring the side chains into the core the main chain must also come into the interior. The main chain is highly polar and therefore hydrophilic, with one hydrogen bond donor NH and one acceptor C = O for each peptide unit. In a hydrophobic environment these main chain polar groups must be neutralized by the formation of hydrogen bonds. This problem is solved very elegantly by the formation of regular secondary structure within the interior. Two Types of 2o structure: α helices or β sheets Both are characterized by hydrogen bonding between main chain NH and C = O groups and they are formed when a number of consecutive residues have the same φ and ψ angles. α‐Helix The right handed α‐helix has 3 6 id t d. res ues per urn an a translation per residue of 1.5 A which means 5 41A per . turn. The atoms of the backbone pack closely making favorable Van der Waal’s interactions. In natural proteins a slightly different geometry is seen. The CO groups tend to point out away from the helix axis and the H‐bonds are consequently not as straight and so φ ~ ‐62o and ψ ~‐41o instead of the classical ‐57 to 60o and ‐47 to 50o This geometry actually appears more stable than the classical α‐helix b h b d h f hcase ecause it permits eac CO oxygen to H‐ on to t e NH o t e i + 4 residue and also the solvent and/or other donors. Variations on the classical α‐helix: when the chain is more loosely or more tightly coiled, with hydrogen bonds to residues i + 5 (π‐helix) or i + 3 (310 – helix),respectively, NOT i + 4 The 310‐helix is so called as it has 3 residues per turn and contains 10 atoms between the hydrogen bond donor and acceptor. Both the π and 310 helices are rare and usually occur at the ends of l h li i l h li Th ’ i llregu ar e ces or as a s ng e turn e ces. ey re not energet ca y favorable for the most part—the backbone atoms are packed too tight in the 310 or too loose in the π helix that there is a hole in the middle. α‐helices vary in length from 4/5 residues to over 40 residues. Average ~10 ( ~3 turns) The rise per residue of an α‐helix is 1.5 A along the helical axis, which corresponds to about 15 A for an average helix. An α‐helix can in theory be right or left handed depending on the screw di ti f th h irec on o e c a n. Left handed ones don’t exist!!! Because of steric interactions between side chains and CO groups ALWAYS right handed! (* 3‐5 residue seen rarely) The α‐helix has a dipole moment: All the hydrogen bonds in an α‐helix point in the same direction because the peptide units are aligned in the same orientation along the helical axis. Since a peptide unit has a dipole moment arising from the different polarity of NH and CO groups, these dipole moments are also aligned along the helical axis. The overall effect is a significant net dipole for the α‐helix giving a partial net positive charge at the amino end and a negative charge at C‐terminal end. Although they appear structurally to be very ordered, isolated α‐helices are usually marginally stable in aqueous soltn. A helix forms quickly (10‐5 to 10‐7 sec) but can unravel almost as quickly. Interestingly formation is generally independent of length, though unraveling isn’t. The most common location for a helix in a protein is along the outside with f l d h h d h bone ace out into so tn an one into t e y rop o ic core. ‐helix wants to have a hydrophobic and hydrophilic side With the 3.6 residues per turn, there is a tendency for side chains to change from hydrophobic to hydrophilic with a periodicity of 3‐4 residues. Not a general hard and fast rule as some helices are buried– but not a bad rule of thumb. With this in mind many α‐helices are amphipathic in that they have predominantly non‐polar side chains along one side of the helical cylinder and polar residues along the remainder of its surface. Such helices often aggregate with each other or with other non‐polar surfaces. Helical wheel‐ helps to visualize The view is down the helix with the hydrophobic core being in the middle. The helix repeats itself after 5 turns or 18 residues so the 19‐21 residues are offset. Polar charges ½ hydrophilic All polar tend to cluster as do hydrophobics ½ hydrophobic α‐helices that cross membranes are all hydrophobic. So if we see a sequence of long stretches of hydrophobics‐ we can often guess that these maybe membrane spanning helices.