A reader pointed me to the ThermoFisher Scientific website. ThermoFisher Scientific is a major supply of scientific equipment and supplies. They created their life sciences wesite to help inform their customers and sell more products. The page I'm interested in is: Overview of Post-Translational Modifications (PTMs). It begins with,
Within the last few decades, scientists have discovered that the human proteome is vastly more complex than the human genome. While it is estimated that the human genome comprises between 20,000 and 25,000 genes (1), the total number of proteins in the human proteome is estimated at over 1 million (2). These estimations demonstrate that single genes encode multiple proteins. Genomic recombination, transcription initiation at alternative promoters, differential transcription termination, and alternative splicing of the transcript are mechanisms that generate different mRNA transcripts from a single gene (3).
The increase in complexity from the level of the genome to the proteome is further facilitated by protein post-translational modifications (PTMs). PTMs are chemical modifications that play a key role in functional proteomics, because they regulate activity, localization and interaction with other cellular molecules such as proteins, nucleic acids, lipids, and cofactors.