<P> Hormone receptor proteins bind to a hormone as a result of an accumulation of weak interactions . Because of the relatively large size of enzymes and receptors, the large amount of surface area provides the basis for these weak interactions to occur . This binding is actually highly specific because of the complementarity of these interactions between polar, non-polar, charged, neutral, hydrophilic, or hydrophobic residues . Upon binding, the receptor often undergoes a conformational change and may bind further signaling ligands in order to activate a signaling pathway . Because of these highly specific and high affinity interactions between hormones and their receptors, very low concentrations of hormone are needed to produce significant cellular response . Receptors can have various different structures depending on the function of the hormone and the structure of its ligand . Therefore, hormone binding to its receptor is a complex process which can be mediated by cooperative binding, reversible and irreversible interactions, and multiple binding sites . </P> <P> The presence of hormone or multiple hormones enables a response in the receptor which begins a cascade of signaling . The hormone receptor interacts with different molecules in order to induce a variety of changes, such as the increase or decrease of nutrient sources, growth, and other metabolic functions . These signaling pathways are complex mechanisms mediated by feedback loops where different signals activate and inhibit other signals . If a signaling pathway ends with the increase in production of a nutrient, that nutrient is then a signal back to the receptor that acts as a competitive inhibitor to prevent further production . Signaling pathways regulate cells through activating or inactivating gene expression, transport of metabolites, and controlling enzymatic activity in order to manage growth and functions of metabolism . </P> <P> Intracellular and nuclear receptors are a direct way for the cell to respond to internal changes and signals . Intracellular receptors are activated by hydrophobic ligands which pass through the cellular membrane . All nuclear receptors are very similar in structure, and are described with intrinsic transcriptional activity . Intrinsic transcriptional involves the three following domains: transcription - activating, DNA - binding, and ligand - binding . These domains and ligands are hydrophobic and are able to travel through the membrane . The movement of macromolecules and ligand molecules into the cell enables a complex transport system of intracellular signal transfers through different cellular environments until response is enabled . Nuclear receptors are a special class of intracellular receptor which specifically aid the needs of the cell to express certain genes . Nuclear receptors often bind directly to DNA by targeting specific DNA sequences in order to express or repress transcription of nearby genes . </P> <P> The extracellular environment is able to induce changes within the cell . Hormones, or other extracellular signals are able to induce changes within the cell by binding to membrane - bound receptors . This interaction allows the hormone receptor to produce second messengers within the cell to aid response . Second messengers may also be sent to interact with intracellular receptors in order to enter the complex signal transport system that eventually changes cellular function . </P>

Lipophilic hormones bind to what type of receptors