Direct Modification of Protein by Signaling Molecules

A special case of signal transduction is represented by a class of small, reactive signaling molecules, such as NO (see chapter 6.10). NO is synthesized in a cell in response to an external signal and is dehvered to the extracellular fluid. Either by diffusion or in a protein-boimd form, the NO reaches neighboring cells and modification of target enzymes ensues, resulting in a change in the activity of these enzymes. NO is characterized as a mediator that lacks a receptor in the classical sense. 

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