Polar uncharged amino acids

7 amino acids are polar uncharged: gly, ser, cys, asn, gln, tyr, thr

These amino acids are relatively hydrophilic (water loving) because they possess polar functional groups (in side chains) i.e. oxygens and nitrogens, which can participate in hydrogen bonding with water so capable of interacting with water.  

Gly is included here although it is not polar (no oxygens or nitrogens), but H of gly can be used in H-bonding with other polar groups. Gly is the smallest AA so functions in polypeptide chain as a linker, a spacer, provides minimum steric hindrance to rotations bending etc.

Ser, thr and tyr have OH groups, which provide polarity to otherwise hydrophobic side chains. Since they are uncharged they tend to be found inside proteins to enable intramolecular hydrogen bonding between regions of chain to stabilise structure & folding.

In addition OH group is employed to provide functional activity as well as structural H-bonding, e.g. in a number of enzymes (such as serine proteases) ser is located at active site of the enzyme, and the OH of ser participates in the hydrolytic reaction catalysed by the protease - functional role.  

Ser and thr have yet another role in proteins. To enable binding of non-amino acid molecules to protein. In this case ser & thr are located near surface, and OH is used in oxygen bridges to bind sugars to make glycoproteins. So the hydroxy amino acids have very important roles in structure, function and control of proteins.

Tyr has a special OH - due to strong electron withdrawing power of ring structure the OH is ionisable so has a pK value. Value is 10.07 so at pH 7.4 not ionised very much. However tyr used at active site of some enzymes where localised disturbance of pH can be generated and allow the ionisable OH to participate in catalysis.

Cys is polar because of the presence of the S in the sulphydryl group SH, which is weakly acidic pK 8.4. In many cases two cys residues are covalently linked to each other through oxidation of the SH groups to produce a disulphide bridge. This is important in protein structural stability.

Cys is another amino acid with a multi-functional role. In some enzymes cys occurs at the active site and participates in enzyme catalysis through the SH group. A number of proteins contain iron (Fe), but no haem group as in Hb, and in these proteins the Fe is stabilised by being bonded to seven S atoms from cys residues. So called Fe-S proteins e.g. ferredoxins, some enzymes. Thus cys has an important structural role in binding metal ions for proteins. 

Asn and gln are derived from asp and glu where the acidic carboxylic acid group is modified to become an amide group. The amide group is not acidic, but it is very polar because of the N. So these residues play important structural role inside proteins by providing hydrogen bonding. Gln also participates in a special covalent linkage between separate protein chains i.e. an inter-linkage as opposed to an intra-linkage (disulphide bridge). This link usually occurs when proteins have to be bound very tightly together e.g. in keratin molecules forming hair, and when fibrin molecules are cross-linked to form a fibrin blood clot. The link is called an isopeptide linkage.

Asn also used to bind sugars to proteins to form glycoproteins. Asn binds different types of sugars from ser & thr. Asn also involved in cancer therapy. Very difficult to kill cancer cells as almost identical to normal cells. Been found that some cancer cells cannot make own asparagine, and depend on uptake of asn from blood. Use of enzyme asparaginase to starve cancer cells by destroying asn in blood so cancer cells cannot make new protein and die. Normal cells can synthesize asn for themselves, but still some problems to overcome. This has shown that there may are important metabolic differences between normal cells and cancer cells that may be exploited for cancer therapy.


For more detail on this topic see the text book pages 60-64

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