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Components of Cells
The Macromolecules
Proteins: weak forces

Weak forces

Holding a protein in its correct three dimensional shape are four different types of forces, or bonds, some of which are much weaker than the covalent bonds found in the backbone of the molecule (the peptide bonds), but nevertheless essential for maintaining the correct structure.

Disulfide bridges

These are covalent bonds that form between the R-groups of two cysteine amino acids located at different positions in the primary sequence.

Each of the cysteine amino acids has a sulfur atom as part of its R-group. In a reversible reaction a covalent bond can be created between the sulfur atom in one amino acid, with a similar sulfur atom in a different cysteine amino acid in a very different position in the sequence of amino acids.

This disulfide bridge "spot welds" the long polypeptide chain together at this point. Although this is a very strong force, it can be broken by reducing agents such as mercaptoethanol. Proteins that are treated with such agents lose their shape and also their function.

Ionic bonds

Many of the R-groups found as part of the structure of amino acids have combinations of atoms that, under appropriate conditions (such as pH) dissociate into ions. Some of these ionic forms carry one positive charge, others carry one negative charge.

Opposite charges attract one another so a positively charged R-group at one location in the polypeptide chain will be pulled towards a negatively charged R-group at a different location along the polypeptide chain.

These forces of attraction (sometimes called ionic bonds, or electrostatic bonds are very weak, and they can be changed or eliminated by changes in pH, but they often play significant roles in holding the protein in it's final shape.

Hydrogen bonds

Different R-groups at different locations on a polypeptide chain can share a hydrogen ion (H+, which is actually just a proton).

Both the R-groups must be close to each other, but they can have either oxygen or nitrogen as part of their structure.

Although hydrogen bonds are very, very weak forces of attraction, they are very important forces in holding parts of a large macromolecule, such as a protein or DNA molecule, in the right shape. Opposite strands in a DNA molecule, for example, are specifically paired one to another using combinations of hydrogen bonds (between the -G-C- and -A-T- base pairs).

Hydrophobic interactions

Some amino acid R-groups are at their most stable when not in contact with water molecules. These hydrophobic structures repel water, and are made even more stable when they come together to form even larger areas where water is excluded.

Non-polar amino acid R-groups in highly folded globular proteins are at their most stable when positioned inside the large protein structure where they are the furthest away from the surrounding water. The interior of a protein, therefore, can become very hydrophobic, repel water, and become even more compact.


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© 2003, Professor John Blamire