Proteins

Overview

This module looks at the products of translation (and the ultimate products of gene expression), the proteins. Aspects of protein structure will be considered and related to protein function in cells and tissues.

Objectives

1. Know the basic structure of amino acids, and how a peptide bond forms. It is not necessary to know the structures of all of the individual amino acids.
2. Know the different levels of protein structure, and how they relate to function.
3. Realize that proteins have a diverse array of functions, and know what some of those basic functions are.

What Are Proteins?

We have seen that cells invest a tremendous amount of resources and energy in the production of proteins. (Imagine for a moment everything that goes into transcribing a single gene, and translating the message, and multiply that by the thousands of genes being transcribed in a cell at any given time, and you begin to get an idea of the kind of investment being made!) This suggests that the synthesis of proteins is important to the cell. This is underscored by the fact that after water, proteins are by far the most prevalent components of cells. So just what do they do? As we'll see (or at least begin to see), they do just about everything.

Protein Structure

The proper term for a single protein molecule is polypeptide. The term protein is used to describe the general class of molecule, and to refer to a functional unit. In other words, if a single polypeptide functions on its own, it can be called a protein. If, on the other hand, it must be complexed with other polypeptides in order to function, then the whole complex can be called a protein, but the individual polypeptides are not.

Polypeptides are linear, unbranched chains of amino acids. There 20 different amino acids used to make polypeptides, and they all share a similar architecture:

All amino acids have an amino group at one end and a carboxyl group at the other end. Where each amino acid differs is in the 'R' group. The chemical nature of the R group gives the amino acid its properties. Some are hydrophobic (avoid contact with water), some are positively charged, some are negatively charged, etc.

Amino acids are joined together by peptide bonds. Peptide bonds form between the carboxyl group of one amino acid and the amino group of the next amino acid, as shown in the following figure:

A polypeptide chain, therefore, has one end with an amino group, and one end with a carboxyl group. So polypeptides have a directionality (amino end/carboxyl end) just as nucleic acids do (5' end/3' end). During translation, each successive amino acid is added to the carboxyl end of the chain. (For more on the synthesis of polypeptides, see the module on translation.)