We’ve talked about amino acids before, but this is where they get interesting. Amino acids combine to make up proteins: proteins are polymers and amino acids are its monomers. There are twenty amino acids, which would be useful to memorise if you’re continuing with biology. At pH 7, ten are non-polar and uncharged, five are polar and uncharged, three are polar and charged positive, and two are polar and charged negative. This is a handy table:
(Source: Pearson Ed via University of Illinois at Chicago)
These amino acids have structures that generally look like this:
The “R” group is a specific side chain that varies among the amino acids, making each one unique—and hence determines their charge.
Amino acids joined together through a dehydration reaction, where a water molecule is formed and removed to form a covalent bond called a peptide bond. A structure resulting from a bunch of these bonds repeating over and over is called a polypeptide. Like DNA molecules, polypeptides have a direction: they’ve got an amino acid at one end (the N-terminus) and a carboxyl group at the other (the C-terminus).
Proteins are polypeptides, accounting for 50% of dry mass in almost all cells. They’re incredibly diverse, and are instrumental in almost everything that organisms do, so it’s only natural that there must be a whole lot of different types—enzymatic proteins which accelerate chemical reactions, receptor proteins which respond to chemical stimuli, defensive proteins that protect against disease… And that just scratches the surface.
Protein function is dictated by structure, and proteins structures are hierarchical in nature—there are four different levels:
- Primary: This is the amino acid sequence. There are twenty types of amino acids, and proteins are chains of 127 of them, so there are thousands of different combinations. The particular sequence of amino acids is determined by genetic information, so essentially, DNA dictates how proteins are built.
- Secondary: This is the way the amino acids are folded into regular units. These are the result of hydrogen bonds between the backbones of the amino acid and carboxyl groups. These bonds are usually formed to keep non-polar parts away from water. There are two main types: alpha helix and beta pleated sheet.
- Tertiary: The overall 3D way the polypeptide folds up. This is the result from the interactions between the side chains (R groups) of the amino acids, and the whole tertiary structure is held together by a bunch of different forces, such as hydrogen bonding, weak dispersive forces, and disulphide bonds.
- Quaternary: Some proteins stop at the tertiary structure, but others go further, joining up a bunch of 3D units to form a larger functional molecule, held together by weak interactions like the forces listed above.
The kind of structure a protein has fully determines what function it serves. The structure depends on a variety of different conditions—if the chemical and physical environment, pH, salt concentration or temperature change, the protein structure might unravel and denature, and thus become unable to perform its function.
Body images sourced from Wikimedia Commons
Further resources: Video to get your head around the levels of structure and another about function