Why globular proteins are water soluble
Most globular proteins (compact, ball-shaped) dissolve in water because their water-avoiding side chains (the variable groups of amino acids) are buried inside and their water-loving ones face the surface.
Remember
A protein stays in solution when hydrophobic (water-avoiding) side chains are buried in its core and hydrophilic (water-loving) side chains cover its outside, where they interact with water.
Memory aid
oil inside, water outside
Picture a soluble protein as an oil droplet in a water-loving coat: the water-avoiding groups sit in the middle and the water-loving groups face the solution.
When a polypeptide chain folds in water, its side chains sort themselves by how they get on with water. In most globular proteins, the hydrophobic side chains are buried in the middle of the protein, away from the water. The hydrophilic ones, which are polar (they have an uneven spread of electric charge) or charged, are left on the outside.
That water-friendly surface forms hydrogen bonds with the surrounding water molecules, so the protein dissolves. This lets a globular protein stay in solution in the cytosol (the watery fluid inside the cell), as many enzymes do, and in blood plasma (the watery, liquid part of blood), as albumin does.
Buried hydrophobic groups become exposed when a protein loses its normal fold, and the protein then tends to clump (aggregate) and come out of solution. The same logic explains an engineered change in which several surface lysines (positively charged side chains) are swapped for leucine, a non-polar side chain (a part of an amino acid that avoids water). Hydrophobic patches are created on the surface, and the variant is prone to aggregation.
Fibrous proteins such as collagen are long and thread-like, and features such as cross-links (bonds between chains) are typical of them. Cross-links explain strength and insolubility, not solubility.
Key facts
- Folding
- in most globular proteins, hydrophilic side chains are left on the surface, while hydrophobic side chains are buried in the core.
- Why it dissolves
- water molecules interact with the polar surface, so the protein stays in solution.
- Where it works
- most globular proteins stay dissolved in watery fluids such as the cytosol and blood plasma.
- Exposed hydrophobic groups
- they would make proteins stick to each other and come out of solution.
- Globular versus fibrous
- globular proteins are compact and ball-shaped, whereas fibrous proteins such as collagen are long and thread-like.
Exam traps
- Exposed hydrophobic side chains do not make a protein soluble, because a soluble globular protein buries them and puts polar groups outside, and a hydrophobic surface would make it poorly soluble.
- Quaternary structure (two or more polypeptide chains assembled together) does not on its own explain why a protein dissolves, because it tells you how many chains are present, not what chemistry the surface has.
- Beta-pleated sheet (a flat, zig-zag fold of the chain) does not by itself set solubility, because many soluble proteins and many insoluble proteins contain it; solubility depends on which side chains face the water.
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