Regulatory Enzymes:- In Many Pathways a Regulated Step Is Catalyzed by an Allosteric Enzyme

In some multienzyme systems, the regulatory enzyme is specifically inhibited by the end product of the pathway whenever the concentration of the end product exceeds the cell’s requirements. When the regulatory enzyme re action is slowed, all subsequent enzymes operate at reduced rates as their substrates are depleted. The rate of production of the pathway’s end product is thereby brought into balance with the cell’s needs. This type of regulation is called feedback inhibition. Buildup of the end product ultimately slows the entire pathway. One of the first known examples of allosteric feedback inhibition was the bacterial enzyme system that catalyzes the conversion of L-threonine to L-isoleucine in five steps (Fig. 6–28). In this system, the first en zyme, threonine dehydratase, is inhibited by isoleucine, the product of the last reaction of the series. This is an example of heterotropic allosteric inhibition. Isoleucine is quite specific as an inhibitor. No other intermediate in this sequence inhibits threonine dehydratase, nor is any other enzyme in the sequence inhibited by isoleucine. Isoleucine binds not to the active site but to another specific site on the enzyme molecule, the regulatory site. This binding is noncovalent and readily reversible; if the isoleucine concentration decreases, the rate of threonine dehydration increases. Thus threonine dehydratase activity responds rapidly and reversibly to fluctuations in the cellular concentration of isoleucine.

FIGURE 6–28 Feedback inhibition. The conversion of L-threonine to L-isoleucine is catalyzed by a sequence of five enzymes (E1 to E5). Threonine dehydratase (E1) is specifically inhibited allosterically by L-isoleucine, the end product of the sequence, but not by any of the four intermediates (A to D). Feedback inhibition is indicated by the dashed feedback line and the symbol at the threonine dehydratase reaction arrow, a device used throughout this book.

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دار علوم نهج البلاغة تقيم محفلها الفكريّ الأسبوعي في ذي قار

العتبة العباسية المقدسة تناقش استعداداتها لحفل إزاحة الستار عن الطبعة الأولى من مصحف النجف الأشرف

شعبة الخطابة للتبليغ الحسيني تقيم ورشة بعنوان (مهارات الخطيب في التأثير والإقناع) في بغداد

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خطر خفي في أكياس الشاي يثير مخاوف صحية

نظام غذائي شائع يظهر نتائج واعدة لمرضى السكري

تقنية نانوية روسية تشخص النوبة القلبية خلال 6 دقائق فقط

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علماء يكتشفون انخفاض صلابة الألماس عند تحويله إلى مقياس نانوي

ثورة في علاج الصمم.. علاج جيني يحقق نتائج استثنائية في استعادة السمع

منهج جديد باستخدام الساعات الذرية يعيد طرح سؤال: ما طبيعة الزمن؟

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مواضيع ذات صلة

Regulatory Enzymes:- Multiple Phosphorylations Allow Exquisite Regulatory Control

Regulatory Enzymes:- Phosphoryl Groups Affect the Structure and Catalytic Activity of Proteins

Regulatory Enzymes:- Some Regulatory Enzymes Undergo Reversible Covalent Modification

Regulatory Enzymes: - The Kinetic Properties of Allosteric Enzymes Diverge from Michaelis-Menten Behavior

Regulatory Enzymes: - Allosteric Enzymes Undergo Conformational Changes in Response to Modulator Binding

Examples of Enzymatic Reactions:- Hexokinase Undergoes Induced Fit on Substrate Binding

Examples of Enzymatic Reactions:-The Chymotrypsin Mechanism Involves Acylation and Deacylation of a Ser Residue

Enzyme Kinetics as an Approach to Understanding Mechanism: -Enzyme Activity Depends on pH

Enzyme Kinetics as an Approach to Understanding Mechanism: -Enzymes Are Subject to Reversible or Irreversible Inhibition

Enzyme Kinetics as an Approach to Understanding Mechanism: -Pre–Steady State Kinetics Can Provide Evidence for Specific Reaction Steps

Enzyme Kinetics as an Approach to Understanding Mechanism:- Many Enzymes Catalyze Reactions with Two or More Substrates

Enzyme Kinetics as an Approach to Understanding Mechanism: -Kinetic Parameters Are Used to Compare Enzyme Activities