Mechanisms of Hormone Action

3.1. How Hormones Change Their Target Cells Hormones are chemical messengers that invoke profound changes within target cells. There are two fundamental mechanisms by which such changes occur: • Activation of enzymes and other dynamic molecules: Most enzymes shuttle between conformational states that are catalytically active versus inactive, on versus off. Many hormones affect their target cells by inducing such transitions, usually causing an activation of one of more enzymes. Because enzymes are catalytic and often serve to activate additional enzymes, a seemingly small change induced by hormone-receptor binding can lead to widespread consequences within the cell. • Modulation of gene expression: Stimulating transcription of a group of genes clearly can alter a cell's phenotype by leading to a burst of synthesis of new proteins. Similarly, if transcription of a group of previously active genes is shut off, the corresponding proteins will soon disappear from the cell. More specifically, when a receptor becomes bound to a hormone, it undergoes a conformational change which allows it to interact productively with other components of the cells, leading ultimately to an alteration in the physiologic state of the cell. Considerable information about a how a hormone acts can be gained by knowing the type of receptor it uses. Despite the molecular diversity of hormones, all hormone receptors can be categorized into one of two types, based on their location within the cell. Thus, if introduced to a new steroid hormone, one can quickly deduce that it has an intracellular receptor and acts upon its target cells by affecting transcription.