Name: substrate effects on enzyme rates
Uploaded: 2019-08-01T09:12:17.000Z
Duration: 1 min 19 s
Channel: JoVE

בתוך תא, תנאים סביבתיים שונים, כגון ריכוז סובסטרט, יכולים להשפיע רבות על הקצב שבו אנזימים מזרזים תגובות כימיות. את התהליך הזה, הנקרא קינטיקה אנזימטית, ניתן להציג באמצעות גרף, Y-שבו קצב התגובה בציר ה, והתנאי המשתנה, ריכוז הסובסטרט בדוגמה זו, X-בציר ה. קצב התגובה הוא הקצב שבו התוצר נוצר, והוא יכול להגיע למהירות Vmax מקסימלית המכונה ולהפוך למישורי כאשר כל האנזימים הזמינים נקשרו לסובסטרטים.לפיכך, קצב התגובה מוגבל על ידי ריכוז האנזימים הזמינים לקשירה. גרף זה יכול להציג גם את המהירות שבה עולה קצב התגובה. Km המשתנה מאופיין על ידי, Vmax-ריכוז הסובסטרט בחצי מה, ומייצג את זיקת האנזים או את נטייתו להיקשר לסובסטרט.נמוך Km לאנזים בעל יש זיקה גבוהה, מכיוון שהוא מגיע לקצב התגובה המרבי שלו בריכוז סובסטרט נמוך. גבוה Km באופן דומה תואם לזיקה נמוכה, מכיוון שהאנזים דורש ריכוז סובסטרט גבוה יותר כדי למקסם את קצב התגובה שלו.

ג'ונסון, קנת א. "מאה שנים של אנליזה קינטית אנזימטית, 1913 עד 2013". FEBS מכתבים 587, מס '17 (ספטמבר 2, 2013): 2753-66. [<a href="https://doi.org/10.1016/j.febslet.2013.07.012">מקור</a>]
סוארז, ר. ק., ג'.פ. סטייפלס, ג'.ר.ב. לייטון וטי.ג'י ווסט. "יחסים בין יכולות שטף אנזימטי ושיעורי שטף מטבולי: תגובות לא שיווי משקל בגליקוליזה של שרירים." הליכים של האקדמיה הלאומית למדעים של ארצות הברית 94, מס '13 (יוני 24, 1997): 7065-69. [<a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC21285/.">מקור</a>]

קינטיקה של אנזים

Enzyme Kinetics

<div style="direction:rtl">אנזימים מאיצים תגובות על ידי הורדת אנרגיית ההפעלה של המגיבים. המהירות שבה האנזים הופך מגיבים לתוצרים נקראת קצב התגובה. מספר גורמים משפיעים על קצב התגובה, כולל מספר המגיבים הזמינים. קינטיקה של האנזים היא המחקר שבודק כיצד אנזים משנה את קצב התגובה. מדענים חוקרים בדרך כלל קינטיקה של אנזים עם כמות קבועה של אנזים בסביבה המבוקרת של מבחנה. כאשר מוסיפים יותר מגיב, או סובסטרט, לכמות קבועה של אנזים, קצב התגובה עולה ככל שהאנזים יכול לייצר יותר תוצר. כתוצאה מכך, כאשר קצב התגובה מוצג בגרף כנגד ריכוז המצע, הגדלת המצע מגבירה את קצב התגובה. עם זאת, ברגע שכל האתרים הפעילים של האנזים תפוסים, קצב התגובה עולה. ריכוז המצע שבו מגיעים לקצב התגובה המרבי נקרא V_max. מספר מולקולות האנזים הנוכחיות מגביל את V_max. אם כמות האנזים גדלה, V_max עולה, אך להוספת עוד מצע אין השפעה. הגרף של קצב התגובה לעומת ריכוז המצע יכול לחשוף מאפיינים חשובים אחרים של הקינטיקה של האנזים. ריכוז המצע שבו קצב התגובה הוא במחצית הדרך ל-V_max (כלומר, 12V_max) נקרא קבוע מיכאליס-מנטן .(K_m) קבוע K_M הוא ייצוג של הזיקה בין אנזים למצע. אנזימים בעלי K_m נמוך יותר דורשים פחות מצע כדי להגיע ל-V_max ולכן יש להם זיקה גבוהה יותר למצע שלהם. מעניין שעבור אנזימים רבים, הערך של K_m קרוב מאוד לריכוז התאי של המצע. ליד K_m, שינויים קלים בריכוז המצע יכולים להשפיע באופן משמעותי על קצב התגובה, כך ששינויים קטנים בזמינות המצע התאי יכולים להשפיע על תפקוד של מסלול ביולוגי שלם. לא כל האנזימים מייצרים את גרף קצב המצע בצורת היפרבולית המכונה קינטיקה של מיכאליס-מנטן. קינטיקה של מיכאליס-מנטן מניחה שהאנזים מזרז מצע בודד. לאנזימים המווסתים מבחינה אלוסטרית יש מספר אתרים פעילים ונוטים לייצר גרף בצורת סיגמואיד כאשר קצב התגובה משורטט לעומת ריכוז הסובסטרט.</div>

אנזימים מאיצים תגובות על ידי הורדת אנרגיית ההפעלה של המגיבים. המהירות שבה האנזים הופך מגיבים למוצרים נקראת ...

Education

JoVE Core

Biology

ביולוגיה

Cellular Processes

תהליכים תאיים

Metabolism

חילוף חומרים

KEY TERMS AND CONCEPTS

מונחים ומושגים מרכזיים

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 more product. If the reaction rate is graphed against substrate concentration, the curve rises as substrate increases.

The rate does not keep rising forever. Once all of the enzyme's active sites are occupied, the reaction rate levels off. The substrate concentration at which the maximum rate is reached is called V max. V max is limited by the number of enzyme molecules present. If more enzyme is added, V max increases, but more substrate alone has no further effect.

A rate-versus-substrate graph can also show other features of enzyme behavior. The substrate concentration at which the reaction rate is halfway to V max is called the Michaelis constant, or K m. K m shows the affinity between an enzyme and its substrate. A lower K m means less substrate is needed to reach V max, so the enzyme has a higher affinity.

For many enzymes, K m is close to the substrate concentration in cells. Near K m, even small changes in substrate concentration can strongly affect the reaction rate. That means small shifts in cellular substrate availability can influence the activity of an entire biological pathway.

Not all enzymes follow Michaelis Menten kinetics. This model assumes that an enzyme acts on a single substrate. Enzymes regulated allosterically have multiple active sites and usually produce a sigmoid-shaped graph instead of a hyperbolic one when reaction rate is plotted against substrate concentration.

Substrate Effects on Enzyme Rates

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

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 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...

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 substrate concentration changes enzyme reaction rate, and how V max, K m, and allosteric enzymes shape enzyme kinetics in high school biology.

Substrate Effects on Enzyme Rates | JoVE K12

JoVE Core series brings biology to life through over 300 concise and easy-to-understand animated video lessons that explain key concepts in biology, plus more than 150 scientist-in-action videos that show actual research experiments conducted in today’s laboratories.

biology

myJoVE Corporation

Substrate Effects on Enzyme Rates

Description

Transcript

Tags