Enzyme-linked receptors are a major type of receptor on the cell surface. Researchers initially recognized them by their response to extracellular signaling proteins that promote the differentiation, proliferation, growth, and survival of cells in animal tissues.
The extracellular signaling proteins that activate cells are collectively known as growth factors. They mediate cell growth even at very low concentrations, just 10-9 to 10-11 M.
The cell’s responses to growth factors are typically slow, measured in hours, and involve a series of signaling steps that lead to gene expression.
Researchers have discovered some enzyme-linked receptors that mediate rapid, direct effects on the cellular cytoskeleton, which controls the way the cell moves and modifies its shape. The extracellular proteins that activate these rapid responses are not diffusible across the cell membrane but are instead attached to the surfaces over which the cell moves.
Abnormal signaling through enzyme-linked receptors can interfere with cell differentiation, proliferation, migration, and survival, and contribute to the development of cancer.
Enzyme-linked receptors are similar to G-protein-linked receptors in that their binding domain lies on the outer surface of the plasma membrane. However, instead of having a cytosolic ligand-binding domain associated with a trimeric G-protein, their cytosolic domain either associates directly with an enzyme or has intrinsic enzyme activity.
A G-protein-linked receptor typically has seven transmembrane segments, but each subunit of most enzyme-linked receptors typically has just one.
The Classes of Enzyme-linked Receptors
1 – Receptor tyrosine kinases phosphorylate specific tyrosines on a limited number of intracellular signaling proteins.
2 – Tyrosine-kinase-associated receptors are paired with intracellular proteins that have tyrosine kinase activity.
3 – Receptor-like tyrosine phosphatases strip phosphate groups from tyrosines of a small group of intracellular signaling proteins. They are termed “receptor-like” because their putative presumptive ligands have not yet been identified. Their receptor function has not been demonstrated directly.
4 – Receptor serine/threonine kinases phosphorylate specific threonines or serines on the latent gene regulatory proteins with which they are coupled.
5 – Receptor guanylyl cyclases act as direct catalysts of cyclic GMP in the cytosol.
6 – Histidine-kinase-associated receptors activate a “two-component” signaling pathway. In this two-step process, the kinase phosphorylates itself on histidine. It then immediately transfers the phosphate to a second intracellular signaling protein.
This process is vividly demonstrable to students who observe the chemotaxis of bacteria in culture. Activation of a phosphate group on the protein structure of the bacterium’s flagellum or flagella “steer” the bacterium toward attractants and away from repellents.
How to Introduce Enzyme Kinetics to Students
You can introduce your students to a basic understanding of enzyme kinetics with Modern Biology’s IND-14: Enzyme Kinetics. In the first experiment in this mini-program, students prepare a wheat germ extract.
Then they determine the Vo (initial velocity) of the reaction catalyzed by acid phosphatase present in the extract compared to pure acid phosphatase.
In the second experiment, your students examine the effects of substrate concentration on reaction velocity. They analyze data with Lineweaver-Burk and Michaelis-Menton plots to calculate Km and Vmax.
Each of these experiments requires two to three hours. All materials for the exercise are provided with the Modern Biology kit.
IND-14 and every other experiment manufactured by Modern Biology matches a clear and concise lesson, with notes to help teachers prepare for it. Teachers can also develop test questions from the lesson guide.
Why choose Modern Biology Experiments
Every experiment kit made by Modern Biology supports scientific thinking. Students test their hypotheses. They never just watch demonstrations. They are never limited to facts and vocabulary.
Every Modern Biology helps students develop the manual dexterity and note-taking skills that they will carry to future study and their careers. Modern Biology is developed by working scientists for future working scientists.
Every Modern Biology kit includes all the reagents and test materials teachers need for their laboratory exercise. There’s no ordering separate reagents, fussing about missing shipments, or checking out lab materials from the supply room.
Modern Biology supplies the safe, non-toxic, reliable reagents and measurement materials you need for every laboratory exercise. And because every Modern Biology experiment is available at a fixed cost, it’s easier to budget your supply cost for each class for each term.
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