Bioinformatica I The amino acids

Things to do today Proteins (high speed sneak preview) – Primary structure – Secondary structure – Tertiary structure The amino acids – One amino acid – Our first protein – A closer look at the amino acids – Secondary structure preferences

Our goal for today: a different view on proteins

Protein? Protein!

Proteins Primary structure – A.K.A. “the sequence” Secondary structure – Short stretches form distinct ‘substructures’ Helices Strands Turns & Loops Tertiary structure – The arrangement of secondary structure elements with respect to each other

Primary structure

Proteins Primary structure – A.K.A. “the sequence” Secondary structure – Short stretches form distinct ‘substructures’ Helices Strands Turns & Loops Tertiary structure – The arrangement of secondary structure elements with respect to each other

Secondary structure - helix

Secondary structure - strands

Secondary structure - turn

Proteins Primary structure – A.K.A. “the sequence” Secondary structure – Short stretches form distinct ‘substructures’ Helices Strands Turns & Loops Tertiary structure – The arrangement of secondary structure elements with respect to each other

From sequence to structure?

“When you understand the amino acids, you understand everything”

The amino acids A short introduction

One amino acid - Cα is at the heart of the amino acid - Cα, C N and O are called backbone atoms - R can be any of the 20 side chains

Our first protein

We now have an oligomer Proteins are made up from 20 different amino acids String of amino acids is called “primary structure”

The 20 amino acids AAlaAlanine C CysCysteine D Asp Aspartic acid (Aspartate) E GluGlutamic acid (Glutamate) F PhePhenylalanine G GlyGlycine H HisHistidine I IleIsoleucine K LysLysine L LeuLeucine M MetMethionine N AsnAsparagine P ProProline Q GlnGlutamine R ArgArginine S SerSerine T ThrThreonine V ValValine W TrpTryptophan Y TyrTyrosine

The 20 amino acids The side chains, R, determine the differences in the structural and chemical properties of the 20 ‘natural’ amino acids. The 20 amino acids can, for example, be classified as follows: Hydrophobic AliphaticAla, Leu, Ile, Val AromaticPhe, Tyr, Trp, (His) Hydrophilic PolarAsn, Gln AlcoholicSer, Thr, (Tyr) ChargedArg, Lys, Asp, Glu, (His) Inbetween: Sulfur-containingMet, Cys SpecialGly (no R), Pro (cyclic) Several amino acids belong in more than one category.

There are many ways to characterize the properties of amino acids. The ones most useful and most commonly used are: Hydrophobicity Size Charge Secondary structure preference Alcoholicity Aromaticity And on top of that there are some special characteristics like bridge forming by cysteines, rigidity of prolines, titrating at physiological pH of histidine, flexibility of glycines, etc.

Hydrophobic

Aromatic

Hydrophilic - neutral

Hydrophilic - charged

Sulfur - containing

Really special

Cysteines are extra special

amino acids don’t fall neatly into classes--they are different combinations of small/large, charged/uncharged, polar/nonpolar properties the properties of a residue type can also vary with conditions/environment Key points about the character of amino acid side chains

Obviously, there are relations between the physico-chemical characteristics of the amino acids and their secondary structure preference.

Secondary structure preferences

Secondary structure - helix

Helices pack because of the hydrogen bonds and because of the hydrophobic packing of side chains along the length of the helix. Certain residues do this hydrophobic packing better than others, and those residues are thus good for a helix. Remember: AMELK

Secondary structure - strands

Also strands pack because hydrophobic packing of side chains along the length of the strand. Certain residues do this hydrophobic packing better than others, and those residues are thus good for a strands.  -branched residues (Ile, Thr, Val) are very good for strands, and so are the large hydrophobic residues. Remember: VITWYF

Secondary structure - turn

Secondary structure - turns To create a turn the backbone needs to be bent pretty sharply, and some residues are really good at that. Glycine is special because it is so flexible, so it can easily make the sharp turns and bends needed in a  -turn. Proline is special because it is so rigid; you could say that it is pre-bent for the turn. Aspartic acid, asparagine, and serine have in common that they have short side chains that can form hydrogen bonds with the own backbone. These hydrogen bonds compensate the energy loss caused by bending the chain into a Remember: PSDNG

A common theme Most secondary structure elements are located at the surface of the protein For helices and strands that means that there is a part facing the ‘outside’ and a part that’s facing the ‘inside’

What goes where?

Hydrophobicity Hydrophobicity is the most important characteristic of amino acids. It is the hydrophobic effect that drives proteins towards folding. Actually, it is all done by water. Water does not like hydrophobic surfaces. When a protein folds, exposed hydrophobic side chains get buried, and release water of its sad duty to sit against the hydrophobic surfaces of these side chains. Water is very happy in bulk water because there it has on average 3.6 H-bonds and about six degrees of freedom. So, whenever we discuss protein structure, folding, and stability, it is all the entropy of water, and that is called the hydrophobic effect.

When hydrophobic objects come together in water, the number of unhappy waters go down, and that is good for stability. Free waters are happy waters.