Chains with backbones
Proteins are chains of amino acids, and if we ignore the uniquene brought by the amino acids we find that one protein chain is a lot line another. They all have a backbone consisting of 3 atoms in a row: nitrogen, carbon, carbon. Repeat as many times as needed. That’s the backbone of the protein. It’s what’s connected to the backbone that gives each protein its uniqueness in the world of proteins.
Here’s series of nicely artistic textbook figures illustrating the key aspects of protein manufacture. They are copied from Dealing with Genes by Berg and Singer, and illustrated by Georg Klatt (their book is sadly out of print and is a real steal if you find it at a used book store). Note how according to the central dogma of molecular biology, genetic information is first transcribed from DNA chains into messenger RNA chains, and is then translated by ribosomes from messenger RNA chains into protein chains using transfer RNA to bring each newly added amino acid to the growing protein chain.
Amino acids: Elements of a charm bracelet.
Each protein chain is a linear polymer having two distinct ends (N and C). The units (the 20 aa’s) are joined by peptide bonds. The “sequence” of a protein chain is given as the list of amino acids in its chain, from N to C. The illustration below (also from Berg and Singers textbook) lists the twenty amino acids, complete with their generic amino group and carboxyl group (shaded in gray) and their unique side-chain (shaded in yellow). The full name and the three-letter abbreviation are given for each amino acid. Each amino acid is also commonly abbreviated by a single letter. The only letters of the English language that are not associated with an amino acid are B, J, O, U, X and Z.
The amino acids can be described according to three main chemical characteristics:
- Is the side-chain oil-like (hydrophobic)?
- Is it wettable by water (hydrophilic)?
- Is it positively or negatively charged?
In addition, some of the amino acids play special roles. For example, a cysteine can form a cross-link with another cysteine. Glycine is special because it has such a tiny side-chain. Proline is special because its side-chain is locked into a small ring that restricts flexibility.
Practice exercise 1: Jot down the one-letter abbreviation for each amino acid. Here’s a flashcard quiz to help you memorize the amino acid abbreviations.
Practice exercise 2: The following is a numbered list of the twenty side-chains. Can you assign each number with the name of the amino acid?