The Very Important Difference Between A Target And A Signal

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The Very Important Difference Between A Target And A Signal

What exactly are hormones and how are they different from “non-hormones”? Hormones are chemical messengers secreted into blood or extracellular fluid by one cell that affect the functioning of other cells.

Most hormones circulate in blood, coming into contact with essentially all cells. However, a given hormone usually affects only a limited number of cells, which are called target cells . A target cell responds to a hormone because it bears receptors for the hormone.

In other words, a particular cell is a target cell for a hormone if it contains functional receptors for that hormone, and cells which do not have such a receptor cannot be influenced directly by that hormone. Reception of a radio broadcast provides a good analogy. Everyone within range of a transmitter for National Public Radio is exposed to that signal (even if they don’t contribute!). However, in order to be a NPR target and thus influenced directly by their broadcasts, you have to have a receiver tuned to that frequency.

Hormone receptors are found either exposed on the surface of the cell or within the cell, depending on the type of hormone. In very basic terms, binding of hormone to receptor triggers a cascade of reactions within the cell that affects function. Additional details about receptor structure and function are provided in the section on hormone mechanism of action.

A traditional part of the definition of hormones described them as being secreted into blood and affecting cells at distant sites. However, many of the hormones known to act in that manner have been shown to also affect neighboring cells or even have effects on the same cells that secreted the hormone. Nonetheless, it is useful to be able to describe how the signal is distributed for a particular hormonal pathway, and three actions are defined:

  • Endocrine action : the hormone is distributed in blood and binds to distant target cells.
  • Paracrine action : the hormone acts locally by diffusing from its source to target cells in the neighborhood.
  • Autocrine action : the hormone acts on the same cell that produced it.

Two important terms are used to refer to molecules that bind to the hormone-binding sites of receptors:

  • Agonists are molecules that bind the receptor and induce all the post-receptor events that lead to a biologic effect. In other words, they act like the “normal” hormone, although perhaps more or less potently. Natural hormones are themselves agonists and, in many cases, more than one distinct hormone binds to the same receptor. For a given receptor, different agonists can have dramatically different potencies.
  • Antagonists are molecules that bind the receptor and block binding of the agonist, but fail to trigger intracellular signalling events. Antagonists are like certain types of bureaucrats – they don’t themselves perform useful work, but block the activities of those that do have the capacity to contribute. Hormone antagonists are widely used as drugs.

Finally, a comment on the names given hormones and what some have called the tyranny of terminology . Hormones are inevitably named shortly after their discovery, when understanding is necessarily rudimentary. They are often named for the first physiologic effect observed or for their major site of synthesis. As knowledge and understanding of the hormone grow, the original name often appears inappropriate or too restrictive, but it has become entrenched in the literature and is rarely changed. In other situations, a single hormone will be referred to by more than one name. The problem is that the names given to hormones often end up being either confusing or misleading. The solution is to view names as identifiers rather than strict guidelines to source or function.

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Difference Between Calmodulin and Troponin C

January 28, 2020 Posted by Madhu

The key difference between calmodulin and troponin C is that calmodulin can bind with only calcium ions whereas troponin C is able to bind with both calcium and magnesium ions.

Calmodulin and troponin C are proteins in eukaryotes. Both act as calcium-binding messenger proteins. More importantly, calmodulin can bind with only calcium while troponin C can bind with both calcium and magnesium.

CONTENTS

What is Calmodulin?

Calmodulin refers to the calcium modulated protein. It can be found in all eukaryotic cells. It can also act as a multifunctional intermediate calcium-binding protein. This protein acts as an intercellular target for secondary messenger calcium ions. Moreover, for the activation of the calmodulin protein, the binding of the secondary messenger calcium ions is neccessary. Once it is activated, it can act as a part of the calcium signal transduction pathway.

Figure 01: Calmodulin

When considering the structure of this protein, it is a small protein having about 148 amino acids. It has approximately two globular regions. Each of these regions contains two EF-hand motifs which can bind with the calcium ions. There is a flexible linker region between these globular regions. Therefore, the calmodulin molecule has four sites for calcium ion binding.

Moreover, the calmodulin protein can bind with a variety of target molecules. Therefore, it is very important to have flexibility in this protein. The generic shape of the non-polar grooves in the binding sites allows it to bind with a variety of targets.

What is Troponin C?

Troponin C is a protein that exists as a part of the troponin complex. There are four EF motifs in the troponin C molecule for the bonding of calcium ions. Moreover, this protein exists as a component in thin filaments in combination with actin and tropomyosin.

A troponin C molecule contains two lobes: N lobe and C lobe. The C lobe is important as a structural component and helps in binding to the N domain of the troponin I. Also, the C lobe is able to bind with either calcium ions or magnesium ions. However, the N lobe binds only with calcium ions. It is the regulatory lobe of this protein and after biding with a calcium ion, it can bind with the C domain of the troponin I.

Figure 02: Structure and Binding of Troponin C

There are two subtypes of troponin C as slow troponin and fast troponin. Moreover, there are a number of mutations for this protein as well. These mutations can cause structural alterations of troponin C and in the binding of calcium and magnesium ions as well. These mutations can cause abnormalities in muscle contractions.

What is the Difference Between Calmodulin and Troponin C?

Calmodulin and troponin C are proteins in eukaryotes. Both these proteins have four EF-hand motifs that can bind with calcium (and/or magnesium) ions. However, the key difference between calmodulin and troponin C is that calmodulin can bind with only calcium ions whereas troponin C is able to bind with both calcium and magnesium ions.

Below infographic summarizes the difference between calmodulin and troponin C .

Summary – Calmodulin vs Troponin C

Calmodulin and troponin C are proteins in eukaryotes. The key difference between calmodulin and troponin C is that calmodulin can bind with only calcium ions whereas troponin C is able to bind with both calcium and magnesium ions.

Reference:

1. Issa, Ziad F., et al. “Ventricular Arrhythmias in Inherited Channelopathies.” Clinical Arrhythmology and Electrophysiology: A Companion to Braunwalds Heart Disease, 2020, pp. 645–684., doi:10.1016/b978-1-4557-1274-8.00031-2.

Image Courtesy:

1. “Calmodulin” By User Magnus Manske on en.wikipedia – Originally from en.wikipedia: 13:27, 22 June 2004 Magnus Manske 564×519 (34,897 bytes) (<>) (Public Domain) via Commons Wikimedia
2. “Troponino” By Arcadian – Troponino.jpg via Commons Wikimedia

About the Author: Madhu

Madhu is a graduate in Biological Sciences with BSc (Honours) Degree and currently persuing a Masters Degree in Industrial and Environmental Chemistry. With a mind rooted firmly to basic principals of chemistry and passion for ever evolving field of industrial chemistry, she is keenly interested to be a true companion for those who seek knowledge in the subject of chemistry.

Difference Between Excitatory and Inhibitory Neurotransmitters

February 16, 2020 Posted by Samanthi

Key Difference – Excitatory vs Inhibitory Neurotransmitters

Neurotransmitters are chemicals in the brain which transmits signals across a synapse. They are classified into two groups based on their action; these are called excitatory and inhibitory neurotransmitters. The key difference between excitatory and inhibitory neurotransmitters is their function; excitatory neurotransmitters stimulate the brain whereas inhibitory neurotransmitters balance the excessive simulations without stimulating the brain.

What are Neurotransmitters?

Neurons are specialized cells designated to transmit signals through the nervous system. They are the basic functional units of the nervous system. When one neuron transmits a chemical signal to another neuron, a muscle or gland, they use different chemical substances which carry the signal (message). These chemical substances are known as neurotransmitters. Neurotransmitters carry the chemical signal from one neuron to the adjacent neuron or to target cells and, facilitate the communication between cells as shown in figure 01. Different types of neurotransmitters are found in the body; for example, Acetylcholine, Dopamine, Glycine, Glutamate, Endorphins, GABA, Serotonin, Histamine etc. Neurotransmission occurs via the chemical synapses. Chemical synapse is a biological structure which allows two communicating cells to transmit chemical signals to each other using neurotransmitters. Neurotransmitters can be divided into two main categories known as excitatory neurotransmitters and inhibitory neurotransmitters based on the influence they have on the postsynaptic neuron after binding with its receptors.

Figure_1:
Neuron synapse during neurotransmitter re-uptake.

What is Neuron Action Potential?

Neurons transmit signals using action potential. Neuron action potential can be defined as a quick rise and fall of the electrical membrane potential (voltage difference across the plasma membrane) of the neuron as shown in figure 02. This happens when the stimulus causes the depolarization of the cell membrane. Action potential is generated when the electrical membrane potential becomes more positive and exceeds the threshold potential. At that moment, the neurons are in the excitable stage. When the electrical membrane potential becomes negative and is not able to generate an action potential, neurons are in the inhibitory state.

Figure_2: Action Potential

What are Excitatory Neurotransmitters?

If the binding of a neurotransmitter causes the depolarization of the membrane and creates a net positive charge exceeding the threshold potential of the membrane and generates an action potential to fire the neuron, these types of neurotransmitters are called excitatory neurotransmitters. They cause the neuron to become excitable and stimulate the brain. This happens when the neurotransmitters bind with ion channels permeable to cations. For, example Glutamate is an excitatory neurotransmitter which binds to a postsynaptic receptor and causes sodium ion channels to open up and allow sodium ions to go inside the cell. Entry of sodium ions increases the concentration of the cations, causing the depolarization of the membrane and creating an action potential. At the same time, potassium ion channels open up and permit the potassium ions to exit the cell with the objective of maintaining the charge within the membrane. Potassium ion efflux and closing of sodium ion channels at the peak of the action potential, hyperpolarize the cell and normalize the membrane potential. However, the action potential generated within the cell will transmit the signal to the presynaptic end and then to the neighboring neuron.

Examples of Excitatory Neurotransmitters

– Glutamate, Acetylcholine (excitatory and inhibitory), Epinephrine, Norepinephrine Nitric oxide, etc.

What are Inhibitory Neurotransmitters?

If the binding of a neurotransmitter to the postsynaptic receptor does not generate an action potential to fire the neuron, the type of neurotransmitter is known as inhibitory neurotransmitters. This follows the production of negative membrane potential below the threshold potential of the membrane. For example, GABA is an inhibitory neurotransmitter which binds with GABA receptors located on the postsynaptic membrane and opens the ion channels permeable to chloride ions. The influx of chloride ions will create more negative membrane potential than the threshold potential. The summation of the signal transmission will happen due to the inhibition caused by hyperpolarization. Inhibitory neurotransmitters are very important in balancing the brain stimulation and keeping the brain functions smoothly.

Examples of Inhibitory Neurotransmitters

– GABA, Glycine, Serotonin, Dopamine, etc.

What is the difference between Excitatory and Inhibitory Neurotransmitters?

Excitatory vs Inhibitory Neurotransmitters

Excitatory neurotransmitters stimulate the brain. Inhibitory Neurotransmitters calm the brain and balance the brain stimulation. Generation of action potential This creates positive membrane potential generates an action potential. This creates negative membrane potential farther threshold potential to generate an action potential Examples Glutamate, Acetylcholine, Epinephrine, Norepinephrine, Nitric oxide GABA, Glycine, Serotonin, Dopamine

Summary – Excitatory vs Inhibitory Neurotransmitters

Excitatory neurotransmitters will depolarize the membrane potential and generate a net positive voltage that exceeds the threshold potential, creating an action potential. Inhibitory neurotransmitters keep the membrane potential in a negative value farther from threshold value which cannot generate an action potential. This is the main difference between excitatory and inhibitory neurotransmitters.

Reference:
1. Purves, Dale. “Excitatory and Inhibitory Postsynaptic Potentials.” Neuroscience. 2nd edition. U.S. National Library of Medicine, 01 Jan. 1970. Web. 13 Feb. 2020.
2. Adnan, Amna. “Neurotransmitters and its types.” Neurotransmitters and its types. N.p., n.d. Web. 13 Feb. 2020.

Image Courtesy:
1. “Action potential”By Original by en:User:Chris 73, updated by en:User:Diberri, converted to SVG by tiZom – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Reuptake both”By Sabar – self-made, created with Corel Painter and Adobe Photoshop (Public Domain)

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