ABSTRACT
Each cell produces thousands of proteins, each with a specific function. Some proteins are expressed at very low levels, a few copies per cell, while others such as housekeeping gene products are extremely abundant. They may be expressed during short periods during the life of an individual, for example during embryonic development, while others may be continually expressed but with very short half lives. The collection of proteins in a cell is known as the proteome, and, unlike the genome which is constant irrespective of cell type, it differs from cell to cell and is constantly changing through its biochemical interactions with the genome and the environment (Figure 12.1). It changes from moment to moment in response to tens of thousands of intra-and extra-cellular environmental signals, such as other proteins, pH, hypoxia and drug administration, and changes continuously during multigenic processes such as ageing, stress or disease. Proteomics consists not only of the identification and
quantification of proteins but also involves the comprehensive study of their structure, localization, modification, interactions, activities and function of all proteins in body fluids, tissues or cell types under given conditions.1
