Did You Know?
The term 'hormone' was coined by English physiologists William M. Bayliss and Ernest H. Starling. They are also credited with discovering the first hormone called secretin, in 1902.
Hormones are biochemicals that have a specific regulatory effect on the activity of the target cells or organs. On the basis of their structure, they are classified into four types: steroids, peptides, amino acid derivatives, and fatty acid compounds. The hormones are able to induce a specific effect on the target cell or organ by binding to a specific receptor at the target cell/tissue/organ. It must be noted that they only interact with these receptors, thereby producing a specific effect.
It is the part of the brain called hypothalamus that acts as a link between the endocrine system and the nervous system. The hypothalamus produces certain releasing hormones that stimulate the secretion of certain hormones by the pituitary gland, which in turn cause specific effects in the target tissues or organs of the body. It must be noted that the location of receptors in the target cell varies, depending on the type of hormone. Steroid and peptide hormones also differ in terms of the location of the receptors.
Steroid Hormone vs. Peptide Hormone
While receptors of certain hormones are present in the nucleus of the target cell, some might be located in the cell cytoplasm or on the surface of the cell membrane. For the hormone to cause the intended effect, it has to attach to a specific receptor at the target cell. The receptor is basically a protein synthesized by the target cell. Once the hormone binds to the receptor, a chain of events takes place, which eventually leads to the generation of proteins. These proteins perform several vital functions, such as producing enzymes that accelerate vital chemical reactions or other vital cellular processes. Let's understand the formation of hormone-receptor complex in case of steroid and peptide hormones.
While steroid hormones are lipids or fat-soluble molecules that are made from cholesterol, peptide hormones are chains of amino acids that are water-soluble in nature. Cortisol, estrogen, androgens, progesterone, and calcitriol (active metabolite of vitamin D3) are examples of steroid hormones. One of the major differences between steroid and peptide hormone is the location of the receptor. In case of the steroid hormone, the receptors are found in the cytoplasm or the nucleus. Since peptide hormones are water-soluble, they cannot enter the target cell. The receptors for the peptide hormone are located at the surface of the cell.
As the peptide hormone binds to a receptor on the surface of the cell membrane, it activates a signaling pathway that induces intracellular activity. Receptors for peptide hormones are proteins and glycoproteins embedded in the cell membrane that travel across the membrane at least once. This allows the receptor to come in contact with the extracellular, as well as intracellular environment. There are different types of cell surface hormone receptors that facilitate the synthesis of proteins that are required for bringing about the intended effect of the peptide hormone. The hormone that triggers the signaling pathway is referred to as the first messenger. For instance, in some cases, the hormone-receptor complex activates a G-protein (first messenger) that is also located in the membrane.
The subunits of G-protein bring about changes in other effectors such as enzymes (adenylate cyclase or phospholipase C) or ion channels. This leads to the production of a second messenger, such as cyclic AMP. Cyclic AMP binds to protein kinase A, which activates this protein and leads to phosphorylation (the addition of a phosphate group to a protein or organic molecule) of other proteins. Thus, the process involves a cascading signal transduction, which can lead to a series of events causing multiple changes in the target cells. This might stimulate mitosis, secretion, or an increase in the uptake of a molecule.
Receptors can also carry out other functions. For instance, the receptor for insulin also acts as an enzyme. It can also add a phosphate group to tyrosine residues, which serve as sites where the downstream signaling proteins can fit. Thus, the second messenger in the cytoplasm can affect the concentration of proteins that already exist within the cell. The second messenger might activate enzymes and cellular proteins. These might also alter gene expression. Insulin, glucagon, somatostatin, adrenocorticotropic hormone, antidiuretic hormone, and prolactin are some of the examples of peptide hormones. These hormones regulate vital bodily processes such as metabolism, lactation, growth, and reproduction.
Steroid hormones can diffuse across the lipid bilayer of the plasma membrane of the target cell to bind to the receptor on account of being fat-soluble. In case of a steroid hormone, specific regions of the receptor carry out specific tasks such as identifying the hormone, binding and forming the hormone-receptor complex and attaching to a specific region on chromosomal DNA, and regulating the transcriptional complex. The formation of this hormone-receptor complex leads to the activation or release of associated proteins. After moving into the nucleus, the complex binds to regions of DNA that are located near specific steroid-regulated genes.
It must be noted that the information in the DNA is not directly decoded into proteins. One strand of DNA is transcribed or copied into mRNA (messenger RNA) molecules from specific genes, which are then used for the synthesis of proteins. Thus, the hormone-receptor complex affects the synthesis of mRNA molecules, thereby determining the amount of corresponding protein that is synthesized from this RNA. This is followed by the synthesis of new proteins from those mRNA molecules. The hormone-receptor complexes act as transcription regulators.
While steroid hormones are directly involved in the transcription of mRNA, as they bind to the DNA of their target cells and facilitate the transcription of specific genes, peptide hormones bind to the receptor on the surface of the membrane of the target cell. They do not exert a direct influence on the process of transcription of mRNA, as they cannot diffuse through the cell membrane.
On a concluding note, the major difference between steroid and peptide hormones is that the former is lipid-soluble and can diffuse easily into the cell membrane of the target cell to connect with receptors, whereas the latter is water-soluble and connects with receptors at the membrane because it can't diffuse through the membrane. Since peptide hormones bind to receptors at the surface, their effect is faster than steroid hormones. Both these hormones perform vital bodily functions, and any problems in the working of these hormones and their receptors can have a serious repercussion on one's health.