Name: allosteric control of enzyme speed
Uploaded: 2025-10-23T11:48:33.000Z
Duration: 1 min 8 s
Channel: JoVE

ויסות אלוסטרי מתייחס לשליטה באנזים באמצעות אתר מובחן, מלבד האתר הפעיל של האנזים. כאשר מולקולה נקשרת למיקום שכזה, המכונה האתר האלוסטרי, הקשר יכול לגרום לשינוי בסידור המרחבי, או לשינוי צורתי, באנזים. פעולה שכזו עשוייה לגרום לעלייה בזיקה של האתרים הפעילים של האנזים לסובסטרטים שלו, ולהעצמת פעילות האנזים, בתהליך המכונה הפעלה אלוסטרית.בגרף קצב התגובה כתלות בריכוז סובסטרט, ניתן לייצג את ההפעלה האלוסטרית S כעקומה חיובית בצורת, שבה מוצג זמן המתנה עד להפעלת האנזים. ואז, עקב הגידול הפתאומי באתרים הפעילים, ריכוז גבוה של סובסטרט יכול להיקשר ולזרז במהירות את התגובה. לעומת זאת, אם האפקטור נקשר לאנזים וגורם לשינוי בסידור מרחבי, שבתורו מפחית את זיקת קישור הסובסטרט, מכונה התהליך עיכוב אלוסטרי.במקרה הזה, ישנה ירידה בתפקוד האנזים וצניחה בקצב התגובה הכימית מהמצב המופעל.

<ol><li>קורניש-באודן, א. הבנת אינטראקציות אלוסטריות ושיתופיות באנזימים. כתב העת FEBS. 281 (2), 621-6332 (2014).</li><li>Tsai, C.-J., Nussinov, R. מנגנון אלוסטרי מתפתח של הפעלת EGFR בהקשרים פיזיולוגיים ופתולוגיים. כתב עת ביופיזי. 117 (1), 5-13 (2019).</li></ol>

ויסות אלוסטרית

Allosteric Regulation

<div style="direction:rtl">ויסות אלוסטרי של אנזימים מתרחש כאשר קשירה של מולקולת אפקטור לאתר השונה מהאתר הפעיל גורמת לשינוי בפעילות האנזימטית. אתר חלופי זה נקרא אתר אלוסטרי, ואנזים יכול להכיל יותר מאחד מהאתרים הללו. ויסות אלוסטרי יכול להיות חיובי או שלילי, וכתוצאה מכך עלייה או ירידה בפעילות האנזים. רוב האנזימים שמציגים ויסות אלוסטרי הם אנזימים מטבוליים המעורבים בפירוק או סינתזה של מולקולות תאיות ספציפיות.עיכוב אלוסטריבעיכוב אלוסטרי, הקישור של מולקולת אפקטור לאתר האלוסטרי גורמת לשינוי קונפורמציה שמפחית את הזיקה של האנזים לסובסטרט. לעתים קרובות, המעכב האלוסטרי הוא תוצר של האנזים או מסלול האנזים, המאפשר למוצרים אנזימטיים להגביל את הייצור שלהם. סוג זה של עיכוב משוב מונע ייצור יתר של מוצרים. כדוגמה קלאסית, איזולאוצין הוא מעכב אלוסטרי של אנזים החשוב בסינתזה שלו עצמו.הפעלה אלוסטריתלעומת זאת, הפעלה אלוסטרי גורמת לשינוי קונפורמטיבי המגביר את הזיקה של האנזים למצע שלו. הפעלה אלוסטרית מגבירה באופן דרמטי את קצב התגובה, כפי שמיוצג על ידי העקומה ההיפרבולית בגרף קצב תגובה לעומת ריכוז המצע. כדוגמה, קשירת ליגנד חוץ-תאי לקולטן EGF הטרנסממברני גורם לשינוי קונפורציה שגורם לעלייה בפעילות הקינאז התוך-תאית של הקולטן. אם אנזים מורכב ממספר יחידות משנה, קישור של מפעיל אלוסטרי ליחידת משנה אחת יכולה לגרום לעלייה בזיקה ובשינוי צורה עבור כל תת-היחידות הקשורות.קריאה מוצעת</div>

ויסות אלוסטרי של אנזימים מתרחש כאשר קשירה של מולקולת אפקטור לאתר שונה מהאתר הפעיל גורמת לשינוי ב ...

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KEY TERMS AND CONCEPTS

מונחים ומושגים מרכזיים

allosteric control of enzyme speed

Allosteric control changes enzyme speed when a molecule binds to a site other than the active site. That alternate binding spot is called an allosteric site. An enzyme can have more than one allosteric site. This type of control can raise or lower enzyme activity.

Many enzymes with allosteric control are metabolic enzymes. They help with the breakdown or the building of specific molecules in cells. In allosteric inhibition, the effector molecule causes a shape change that lowers the enzyme's ability to bind its substrate. Often, the inhibitor is a product of the enzyme or pathway, so the product helps stop more of itself from being made. This feedback inhibition prevents overproduction.

A classic example is isoleucine, which acts as an allosteric inhibitor of an enzyme needed for its own synthesis. Allosteric activation works in the opposite way. An activator changes the enzyme shape so it binds its substrate more strongly. This can greatly increase reaction rate, and it is shown by a hyperbolic curve on a graph of reaction rate versus substrate concentration.

A well-known example of allosteric activation is the EGF receptor. When an extracellular ligand binds to this transmembrane receptor, it changes shape and increases the receptor's intracellular kinase activity. In enzymes with multiple subunits, binding of one activator to a single subunit can also increase the affinity and shape of the other linked subunits.

Allosteric Control of Enzyme Speed

what is metabolism?

Metabolism, Energy, and Cell Reactions

Metabolism is the set of chemical reactions that happen in a cell. It includes anabolism, which builds molecules, and catabolism, which breaks them down. These reactions help cells manage energy and keep life processes running. Cells use metabolism to turn food into energy, make cell parts, and remove waste products. Food stores potential energy in the bonds between atoms. Catabolic pathways break large food molecules into smaller ones and release that energy. Anabolic pathways use that energy...

first law of thermodynamics

Energy Flow in a Food Web

Energy flow in a food web shows the First Law of Thermodynamics in action. Energy cannot be created or destroyed. It can only change form as it moves through living things and the environment. Sunlight is the starting point. Plants capture radiant energy from the sun during photosynthesis and turn it into chemical energy stored in complex carbohydrates. At the same time, oxygen is released into the atmosphere. Herbivores eat the plants and use those sugars during digestion. The energy is...

second law of thermodynamics

Entropy in Food Web Energy Flow

The Second Law of Thermodynamics explains why entropy increases as energy moves through a system. Entropy means the amount of disorder in a system. Each time energy is transferred or transformed, some energy is lost as unusable heat. That heat can raise the motion of nearby molecules for a short time and make the surroundings less ordered and more random. This idea also appears in a food web. Primary producers, such as plants, take in energy from the sun and make food. During this process,...

kinetic energy

Motion as a Source of Energy

Kinetic energy is the energy of motion. It appears when an object is moving and can do work or cause change. A waterfall is one clear example. Water behind a dam has potential energy, but when it rushes downward, that moving water has kinetic energy. When the water reaches the bottom and stops moving, its energy can become potential energy again. Kinetic energy also shows up in living systems. Plants absorb light particles, called photons, which move very fast and help create chemical energy...

potential energy

Stored Energy in Motion and Bonds

Stored energy in motion and bonds is called potential energy. It depends on the structure of an object or the position of matter in space. Raising a kettlebell changes its location and increases its potential energy. A stretched rubber band also stores potential energy, which can later change into kinetic energy. Chemical bonds between atoms hold another form of stored energy called chemical energy. These bonds involve attractive forces, and that energy can change into other forms during a...

free energy

Gibbs Free Energy in Chemical Reactions

Gibbs free energy is a measure of useful energy that can be taken from a chemical reaction to do work. It is written as G and named for scientist Gibbs, who discovered it. This energy is what remains after entropy is considered. A reaction can either take in energy or release it. Reactions that absorb energy are called endergonic reactions. Reactions that release energy are called exergonic reactions. The direction of energy flow helps show which type of reaction is happening. Plants carry...

activation energy

How Catalysts Lower Reaction Barriers

Activation energy is the minimum energy a chemical reaction needs to keep going. When the activation energy is higher, the reaction usually happens more slowly. Adding heat can speed the reaction because molecules move faster and collide more often. As reactants collide, bonds break and the reaction moves through an uphill phase. That step creates the transition state, which is an unstable, high-energy state of the reactants. After that, new bonds form, products appear, and free energy is...

hydrolysis of atp

ATP Breakdown and Energy Release

ATP breakdown releases energy for cell work. Adenosine triphosphate, or ATP, can lose one or two phosphate groups when water is added. This hydrolysis reaction is exergonic, which means it releases energy that the cell can use. When one phosphate group is removed, ATP becomes ADP, or adenosine diphosphate. Inorganic phosphate is also released. ADP can then be hydrolyzed again to form AMP, or adenosine monophosphate, after a second phosphate group is removed. ATP has a clear structure. It is...

phosphorylation

Phosphorylation and Protein Control

Phosphorylation helps control how proteins work in cells. It is the addition or removal of phosphate groups, and it is the most common chemical change that regulates cellular processes. These changes can alter a protein’s structure, activity, stability, location inside the cell, and interactions with other proteins. During phosphorylation, protein kinases move the terminal phosphate group from ATP to a specific amino acid side chain on a target protein. The amino acids most often...

induced-fit model

How Enzymes Adjust to Their Substrates

Enzymes speed up many chemical reactions in cells. They are biological catalysts, so they increase reaction speed without being consumed or permanently changed. They do this by lowering the activation energy needed to turn reactants into products. Enzymes are proteins that usually work by binding to a substrate, which is the reactant molecule they act on. They show substrate specificity, meaning they bind only certain substrates. This specificity depends mainly on the shape and chemical...

enzyme kinetics

Substrate Effects on Enzyme Rates

Enzyme kinetics examines how enzymes change the rate of a reaction. Enzymes speed reactions by lowering the activation energy of the reactants. The reaction rate is the speed at which an enzyme turns reactants into products, and it can be affected by several factors, including the number of available reactants. Scientists often study enzyme kinetics in a test tube with a fixed amount of enzyme. When more substrate, or reactant, is added, the reaction rate increases because the enzyme can make...

enzyme inhibition

Competitive vs Noncompetitive Inhibitors

Enzyme inhibitors are molecules that reduce enzyme activity by binding to enzymes. Cells use these inhibitors to help regulate normal enzyme function. Drugs and toxins can also block enzymes and change how they work. Some inhibitors bind at the active site, which is the part of the enzyme where the substrate normally fits. Competitive inhibitors do this by occupying the active site and preventing the substrate from binding. If the substrate concentration becomes high enough, it can outcompete...

feedback inhibition

How Cells Control Product Buildup

Cells use enzymes to run biochemical reactions and make the products they need. These reactions are happening constantly, and enzymes speed them up so they happen fast enough to matter. Enzyme activity also has to be regulated. Cells need the right amounts of starting materials, intermediate metabolites, and final products. If these substances build up too much or drop too low, the cell can be harmed. That is why cells use control systems to keep metabolic levels balanced. One important...

allosteric regulation

Allosteric control changes enzyme speed when a molecule binds to a site other than the active site. That alternate binding spot is called an allosteric site. An enzyme can have more than one allosteric site. This type of control can raise or lower enzyme activity. Many enzymes with allosteric control are metabolic enzymes. They help with the breakdown or the building of specific molecules in cells. In allosteric inhibition, the effector molecule causes a shape change that lowers the enzyme's...

cofactors and coenzymes

Enzyme Helpers: Metals, Vitamins, and More

Enzyme helpers are needed for many enzymes to work properly. These helpers fall into two groups: cofactors and coenzymes. Cofactors are metallic ions, while coenzymes are non-protein organic molecules. Both can be tightly bound to an enzyme or attach only when the substrate binds. Cofactors are common in mature proteins, appearing in about 30% of them. They are often added as the enzyme folds and help with catalytic activity. Magnesium is one important cofactor, and it is needed by more than...

key terms and concepts

Learn how allosteric activators and inhibitors change enzyme shape, affect reaction rate, and control feedback in metabolic pathways.

Allosteric Control of Enzyme Speed | JoVE K12

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Allosteric Control of Enzyme Speed

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