Peptide Hormones Definition
Peptide hormones are a class of proteins which are bound by receptor proteins and enable or disable a biological pathway. Hormones, in general, are biological molecules used in multicellular organisms to direct and coordinate development, growth, and reproduction. The word peptide refers to peptide bonds between amino acids. A peptide hormone, therefore, is a chain of amino acids which serves the function of a biological communication molecule.
Peptide hormones have a short half-life, meaning they break apart quickly. This allows organisms to use peptide hormones to direct processes quickly and efficiently, without the signal lingering for a long time. This makes peptide hormones ideal candidates for intracellular hormones, operating within cells. However, many peptide hormones are also found in extracellular applications. Peptide hormones can be found in insects, all vertebrates, and many other creatures. Other hormones, such as steroid hormones, must be broken down and excreted through the urine or feces.
Synthesis of Peptide Hormones
Like all proteins, peptide hormones are described in the DNA, translated into the form of a protein, and modified or altered appropriately. A large majority of protein synthesis happens within the endoplasmic reticulum. Large protein complexes known as ribosomes read the messenger RNA and convert the message into a sequence of amino acids. Peptide hormones can be any length, from only a few amino acids to several hundred.
Typically, cells secret peptide hormones via one of two pathways. The first, called regulated secretion works by producing lots of the hormone and storing it in a secretory granule or vesicle. When a signal is given to release the hormone, the granule bursts and hormone is released either into the cell, out of the cell, or into the environment.
Other peptide hormones are released via constitutive secretion. In this form of hormone release something signals the DNA to start producing the peptide hormone. A regulator protein may be removed, or a growth factor may somehow signal enzymes in the nucleus to produce the peptide hormones. As they are produces, they are simultaneously released without being first stored. When the signal is over, the DNA is again protected, and the organism stops producing the peptide hormones.
Peptide Hormones Examples
Insulin
Insulin is one of the most commonly known peptide hormones. Insulin is one of many peptide hormones found in animals which helps regulate the amount of glucose within cells and the blood. Insulin acts on all cells of the body, binding to receptor proteins on the surface of cells and enabling the uptake of glucose. Most importantly, in healthy individuals, insulin is self-regulating because its release is cause by a high level of glucose in the blood. This is shown in the image below.
This image represents a cell within the pancreas, the gland responsible for secreting insulin. Within the pancreas specialized islet beta cells have an important receptor on their surface responsible for taking in glucose, the GLUT2 receptor. This protein transports glucose into the cell, where the glucose undergoes the process of glycolysis. Once broken into smaller pieces, it enters the mitochondria where it goes through the Krebs cycle and eventually oxidative phosphorylation to produce ATP.
In the presence of an increase concentration of ATP, the ATP-sensitive potassium channel closes. This means that the potassium ions can no longer escape the cell. Ions on either side of the membrane build up a certain electrical potential, which is completely disrupted by the closing of the potassium channel. This creates a depolarization of the membrane, much like a nerve reaction.
The depolarization travels around the membrane until it reaches voltage-gated calcium channels. The depolarization causes these channels to open, allowing calcium ions to flood the cell. These calcium ions activate secretory vesicles, which carry the insulin. These small sacs fuse with the plasma membrane and dump their pre-made peptide hormones into the bloodstream. There, they can circulate and tell cells to uptake glucose. When the concentration goes back down the ATP within the islet beta-cell will decrease, and the system will reset.
Insulin is one of the peptide hormones which is released by regulated secretion. Long before the signal was received, insulin was transcribed from the DNA and processed by ribosomes. Insulin is one of the longer peptide hormones, at 51 amino acids. The peptide hormone is then passed through the endoplasmic reticulum and the Golgi apparatus before becoming package in secretory vesicles. This arrangement ensures that a lot of insulin can be released in a short period when needed.
Other Human Peptide Hormones
Besides insulin, the human bodily relies on a wide range of other peptide hormones. Among those are prolactin, a hormone responsible for acting on the mammary glands, and growth hormone, which is responsible for controlling many aspects of growth and development. Like insulin, these hormones must be timely and controlled by the DNA. This ensures that the organism develops in the proper fashion.
Peptide Hormones in Other Organisms
Essentially all known organisms use some form of peptide hormones. While hormones in plants have typically been restricted to 5 categories, scientists have recently confirmed the use of peptide hormones within plants. In the animal world, all use some form of peptide hormone. This is likely due to the ease with which peptide hormone pathways would be created evolutionarily. Other hormones, which require entirely novel pathways to create, likely also require more mutations and stable evolution to occur. Peptide hormones could be created through novel interactions of the DNA, a trait caused by mutations anyway.
Quiz
1. Which of the following is a peptide hormone?
A. A molecule made of amino acids used to lower the activation energy of a reaction
B. A molecule made of lipids, used to convey a signal to some part of the cell or organism
C. A molecule made of amino acids used to convey a signal to some part of the cell or organism
2. Why is it important that insulin is a peptide hormone?
A. It can be eaten in the diet
B. It can be readily synthesized from DNA
C. It lasts a long time
3. For people with diabetes, the insulin they receive can be produced by bacteria. How can this be?
A. Magic!
B. The bacteria have been given the gene for insulin
C. The human genome is placed within bacteria
References
- Colorado State University. (2018, July 6). Hormone Chemistry, Synthesis and Elimination. Retrieved from VIVO Pathophysiology: http://www.vivo.colostate.edu/hbooks/pathphys/endocrine/basics/chem.html
- Lodish, H., Berk, A., Kaiser, C. A., Krieger, M., Scott, M. P., Bretscher, A., . . . Matsudaira, P. (2008). Molecular Cell Biology (6th ed.). New York: W.H. Freeman and Company.
- Nelson, D. L., & Cox, M. M. (2008). Principles of Biochemistry. New York: W.H. Freeman and Company.