Final answer:
Proteins can be turned 'on' or 'off' by second messengers like cAMP and protein kinases, apart from the phosphorylation involving phosphate groups. cAMP activates protein kinases that modulate protein activity, while transcription factors control gene expression by binding to DNA.
Step-by-step explanation:
Other than phosphate, which is commonly involved in the regulation of protein activity by the process of phosphorylation, molecules known as second messengers also play a critical role in the activation or inhibition of proteins. Specifically, cyclic adenosine monophosphate (cAMP) is a vital second messenger that is produced when adenylyl cyclase converts ATP into cAMP in response to external signals. Once formed, cAMP activates a group of proteins called protein kinases, which contribute to the phosphorylation process and can thereby change the activity of target proteins within the cell. This phosphorylation leads to a change in the protein's structure, which may either activate or inactivate the protein's function.
These protein kinases and other related enzymes, such as phosphatases which remove phosphate groups, are responsible for a variety of cellular processes including gene expression. Proteins known as transcription factors are another set of proteins that can regulate gene activity by binding to specific regions of DNA and promoting or inhibiting the transcription of genes. Thus, through different mechanisms like phosphorylation by kinases and the binding of transcription factors, proteins within a cell can be turned 'on' or 'off' in response to various signals, allowing the cell to perform necessary functions and respond to changes in the environment.