Name: factors that shape enzyme function
Uploaded: 2019-01-29T11:04:45.000Z
Duration: 4 min 35 s
Channel: JoVE

K12 Standards

Florida Department of Education

High School

Life Science

SC.912.L.18

SC.912.L.18.4

Enzyme Activity

欧洲探险家虽然具备充足的知识、技能和装备，但为何直到15世纪末才抵达新大陆？你可能会惊讶地发现，这个问题的答案与一类名为酶的小型生物分子有关，酶能够充当催化剂。

什么是催化剂？催化剂是一种能够使反应更快进行并减少所需能量的物质。如果绘制出非催化生物反应所需的能量图，结果会类似于这样。这个高度代表活化能，即反应进行所需的最低能量。酶作为催化剂，提供了一条活化能更低的替代反应途径。

那么，酶究竟是如何发挥作用的呢？大多数酶是蛋白质。其结构中与催化分子相互作用的部分称为活性位点，该位点只对特定的底物具有亲和力，而不会与其他底物结合。当正确的底物进入活性位点后，酶会改变其构象，以形成亲和力更高的状态。这种过渡态有助于反应的进行，并将反应物转化为产物。然而，酶对产物的亲和力较低，因此会释放产物并恢复至原始构象，准备重复相同的过程。这是一种分解代谢酶的实例，因为它将单一底物分解为多个产物。与此相反的是合成代谢酶，它将两个底物结合在一起，生成更大的产物。

酶的功能方式还有很多。例如，这种酶可将单一底物转变为新的构型；而另一种酶则可将一个底物的一部分转移到另一个底物上。有些酶无法独立催化反应，需要辅因子或辅酶才能催化其反应。

那么，你可能会问，这些内容与15世纪的航海有什么关系呢？当时，水手们无法获得含有维生素C的新鲜农产品，而维生素C是胶原蛋白合成所必需的辅酶。因此，水手们会患上坏血病——一种因缺乏维生素C而引起的疾病，且常常致命。他们通过腌制富含维生素C的水果和蔬菜来解决这一问题。腌制降低了食物的pH值，从而抑制了导致食物腐败的微生物酶的活性。因此，破坏微生物酶并保存这种必需辅酶，对长途航海的成功起到了关键作用。

在本实验中，您可以通过测定单位时间内产物的生成量（即反应速率），来定量分析不同条件对酶功能的影响。首先，您将在标准室温和中性pH条件下测定基础反应速率。反应初期，反应速率会逐渐增加，直至所有酶分子均被底物完全饱和；随后，由于酶持续进行底物结合、催化和产物释放的循环过程，催化反应将达到一个稳定水平。在确定基础反应速率后，可将其作为其他条件下反应速率的参照基准。本实验所使用的酶为辣根过氧化物酶（turnip peroxidase），您将评估该酶在不同温度和pH条件下的活性表现。

酶活性

<h4>生物催化剂</h4>

所有生物体都会持续进行大量的生化反应以维持其生命活动。大多数反应需要输入能量才能启动，这种能量称为活化能。催化剂是一类能够降低活化能的化学物质。尽管催化剂可促进化学反应的进行，但其本身在反应中不会被消耗。这意味着催化剂可以连续多次促进同一特定化学反应，从而提高该反应的速率。

酶是生物催化剂，能够加快生化反应的速度和效率。大多数酶是蛋白质，但某些核糖核酸分子也具有催化特性。每种酶只能与一种或少数几种特定的底物结合，因此酶的种类极为多样，各自具有不同的功能。酶所催化的特定反应取决于其活性位点，活性位点的形状与底物的形状高度匹配。当底物与酶结合后，酶的构象会发生轻微变化，从而促进化学反应更易进行，降低反应的活化能。反应完成后，酶释放产物并恢复至原始构象，从而可重复上述过程。

<h4>分解代谢酶与合成代谢酶</h4>

大多数酶促反应可分为分解代谢和合成代谢两类，这两类反应的功能相反。介导分解代谢反应的酶将较大的物质分解为多种产物。一个著名的分解代谢酶是乳糖酶，它可将乳制品中的双糖乳糖分解为单糖半乳糖和葡萄糖。人类在出生后的小肠中含有大量乳糖酶，但许多个体在幼儿期便失去了超过90%的乳糖酶活性1。这种减少会随着年龄增长而逐步降低消化乳糖的能力，这种现象被称为乳糖不耐受2。

合成酶将多种物质结合成单一产物。例如，DNA聚合酶通过连接核苷酸来合成DNA分子。类似地，糖原合酶将葡萄糖分子连接生成糖原。胰岛素调节糖原合酶的活性，因此胰岛素水平极低或缺乏的糖尿病患者若不接受胰岛素治疗，便无法合成糖原3。

<h4>环境因素</h4>

酶的活性中心具有特定的形状和电荷，与它所催化的化学反应相匹配，因此酶的活性会受到影响其活性中心的各种因素的调控。酶需要在特定条件下才能发挥最佳活性，而这些条件因其功能和所在位置的不同而有很大差异。其中重要的影响因素包括pH值和温度。例如，酶的活性中心通常含有酸性或碱性氨基酸。当pH偏离最适值时，活性中心的电荷状态会发生改变，从而阻碍底物与酶的有效结合。这一特性已被用于食品腌制保存数百年之久——通过降低腌制食品的pH值，有效抑制微生物中酶的活性。同样，每种酶都有其最适温度范围。当温度低于或高于该范围时，活性中心的构象会发生改变，导致其无法与底物有效结合。在大多数情况下，恢复至最适温度可使酶恢复原有的结构和功能；然而，过高的温度则会导致酶变性，即其结构发生不可逆的破坏。这些性质在食品安全中具有重要应用：我们通过充分加热肉类使有害细菌的酶变性以杀灭细菌，同时将大多数食品冷藏以减缓微生物的酶促反应速率。

嗜极生物，即存在于对大多数生命形式具有致命性的极端环境中的生物，拥有能够在这些极端条件下发挥作用的酶。例如，Thermus aquaticus 是一种存在于温泉和深海热液喷口中的细菌，其生存环境的温度足以使大多数蛋白质发生变性。T. aquaticus 中发现的 Taq DNA 聚合酶是一种耐热的 DNA 聚合酶，它的发现推动了聚合酶链式反应（PCR）技术的发展，该技术是实验室中广泛用于快速扩增 DNA 的方法。PCR 需要高温来解开待复制的 DNA 双螺旋结构，形成单链模板。在此高温条件下，Taq 聚合酶仍能保持稳定，而大多数酶则会降解。PCR 过程在高温（使 DNA 解链为单链）和较低温度（使聚合酶合成新的双链 DNA）之间循环进行。

<h4>影响酶活性的分子因素</h4>

除了特定的 pH 值和温度范围外，某些酶在其他分子或离子存在的条件下功能更佳。辅酶是能够结合到酶活性位点并参与催化反应但自身不被反应消耗的生物分子。辅酶通常负责转运电子、特定原子以及功能基团，因此可作为中间载体。B族维生素可作为多种生化反应的辅酶。例如，辅酶A（coA）是泛酸的活性形式，所有哺乳动物酶中有4%需要该辅酶4。辅因子通常被归类为酶发挥功能所必需的无机物质，例如金属离子。例如，所有DNA聚合酶在进行DNA复制以及切除错误掺入的核苷酸时，都需要Mg2+或Mn2+5。

还有一些分子并非酶发挥功能所必需，但能够改变酶的活性。能够增强酶活性的物质称为激活剂，而降低酶活性的物质则称为抑制剂。这些分子可以结合到酶的活性位点，也可以结合到酶上被称为变构位点的其他位置。有时这些分子会结合到活性位点中与底物相同的位置，从而与底物竞争结合酶，这类抑制称为竞争性抑制。另一些分子，特别是结合在变构位点的物质，可在不阻断底物与活性位点结合的情况下改变酶的功能，因此属于非竞争性抑制。此外，抑制剂的结合可能是可逆的，也可能是不可逆的。当相互作用为可逆时，一旦抑制剂脱离，酶便可恢复其功能；而当结合为不可逆时，抑制剂将永久性地结合在酶上，导致酶不可修复地失活，必须被替换。然而，酶抑制剂也有其重要用途。例如，抑制剂已被用作抗生素以抑制细菌生长，或作为化疗药物来抑制癌细胞的不受控分裂6。

<h4>测定酶功能</h4>

科学家研究不同条件对酶功能影响的方法之一是量化反应速率，即酶催化反应的速度。因此，较高的反应速率表明酶催化反应的效率更高，而较低的速率则意味着其工作效率较低。为了确定反应速率，可通过测量单位时间内生成的产物量来计算反应速度。该过程从反应开始时产物浓度呈线性增加的阶段起进行测量，直至酶被底物完全饱和，催化反应达到稳定水平。一旦确定了基础反应速率，便可将其作为对照或参照点，用于比较多种不同处理条件下的结果。我们可以将反应速率评估为相对值或绝对值。相对反应速率是一种比较性指标，用于显示两种或多种条件（例如单次实验中的不同处理组）之间的差异，但并不提供所测参数的具体数值。绝对反应速率则依赖于对反应进程的定量测量。理解相对与绝对反应速率之间差异的一个简单方法是类比赛车速度的测量：相对方法通过比较赛车在终点线的排名来判断快慢，而绝对速率则通过每小时英里数来评估并比较每辆赛车的实际速度。

由于酶的应用极为广泛，研究人员已开始探索创造和改造酶的方法以满足特定需求，这一领域被称为蛋白质工程。利用重组DNA技术，可以改变氨基酸序列，从而调整酶的构象或活性位点的亲和力。该技术已被用于开发针对多种疾病的高灵敏度诊断检测方法，同时也应用于生物修复和废物管理工具的研制7。

<h4>参考文献</h4>

<ol class="references">
	<li>Swagerty DL Jr, Walling AD, Klein RM. 乳糖不耐受。Am Fam Physician. 2002, Vol. 65, 9: 1845-50.</li>
	<li>Lomer MC, Parkes GC, Sanderson JD. 综述文章：临床实践中乳糖不耐受的认知误区与实际情况。Aliment Pharmacol Ther. 2008, Vol. 27, 2: 93-103.</li>
	<li>AR, Saltiel. 2型糖尿病分子发病机制及治疗的新视角。Cell. 2001, Vol. 104, 4: 517-29.</li>
	<li>DO, Kennedy. B族维生素与大脑：作用机制、剂量与疗效——一项综述。Nutrients. 2016, Vol. 8, 2: 68 doi:10.3390/nu8020068.</li>
	<li>Vashishtha AK, Wang J, Konigsberg WH. 不同二价阳离子改变DNA聚合酶的动力学与保真度。J Biol Chem. 2016, Vol. 291, 40: 20869-20875.</li>
	<li>Watson ID, Stewart MJ, Platt DJ. 抗微生物化学治疗中酶抑制剂的临床药代动力学。Clin Pharmacokinet. 1988, Vol. 15, 3: 133-64.</li>
	<li>Ahuja SK, Ferreira, GM, Moreira AR. 酶在环境领域中的应用。Crit Rev Biotech. 2004, Vols. 2-3, 125-54.</li>
</ol>

生物催化剂

所有生物体为了维持生命活动，会持续进行大量的生化反应。大多数这些反应需要……

Education

教育

JoVE Lab Manual

Biology

生物学

Cellular Processes

细胞结构与功能

factors that shape enzyme function

Enzymes are biological catalysts that help living things run biochemical reactions faster. They lower activation energy, which is the energy needed to start a reaction. Enzymes are not used up in the process, so they can keep working again and again. Most enzymes are proteins, although some ribonucleic acid molecules also have catalytic activity.

Each enzyme usually binds only one or a few specific substrates. This selectivity depends on the enzyme’s active site, which fits its substrate like a matching shape. When the substrate binds, the enzyme changes shape slightly and makes the reaction easier. After the products form, the enzyme releases them and returns to its original shape.

Enzyme reactions are often grouped as catabolic or anabolic. Catabolic enzymes break larger molecules into smaller products. Lactase is a catabolic enzyme that splits lactose in dairy products into galactose and glucose. Many people lose most of their lactase by early childhood, which can lead to lactose intolerance. Anabolic enzymes build larger molecules from smaller parts. DNA polymerase joins nucleotides to make DNA, and glycogen synthase links glucose molecules to form glycogen. Insulin helps regulate glycogen synthase, so people with little or no insulin may need treatment to make glycogen normally.

Enzyme activity depends on conditions such as pH and temperature. Many active sites contain acidic or basic amino acids, so changes in pH can alter charge and make substrate binding harder. Pickling uses this idea by lowering pH and slowing microbial enzyme activity. Temperature also matters because each enzyme has an optimal range. Low or moderate shifts may slow the enzyme, but high temperatures can denature it and cause lasting damage. Cooking meat and refrigerating food both reduce harmful microbial activity in this way. Extremophiles live in harsh environments and have enzymes that still work there. Thermus aquaticus is one example, and its Taq polymerase stays stable in the high heat used for PCR, a method that copies DNA by cycling between hot and cooler temperatures.

Some enzymes also need coenzymes or cofactors. Coenzymes are biomolecules that help the reaction and are not consumed. They often carry electrons, atoms, or functional groups, and many B vitamins serve this role. Coenzyme A, the active form of pantothenic acid, is needed by many mammalian enzymes. Cofactors are usually inorganic substances, such as ions. DNA polymerase, for example, requires Mg2+ or Mn2+ for DNA replication and for correcting some mistakes.

Other molecules can change enzyme activity without being required for the reaction. Activators increase activity, while inhibitors decrease it. These molecules may bind at the active site or at allosteric sites, which are different locations on the enzyme. Some inhibitors compete with the substrate for the active site, while others act in a non-competitive way by binding elsewhere. Inhibitor binding can also be reversible or irreversible. Reversible inhibitors can detach and let the enzyme work again, but irreversible inhibitors permanently damage the enzyme. Scientists use enzyme inhibitors as antibiotics and as chemotherapeutic agents to limit cancer cell division.

Scientists measure enzyme function by looking at reaction rate, or how fast product is made. A faster rate means the enzyme is working more efficiently. Reaction velocity is measured by tracking product concentration over time, usually from the start of the reaction until the enzyme becomes saturated with substrate and the rate levels off. Researchers use that baseline as a control when comparing different treatments. Reaction rates can be reported as relative or absolute values. Relative rates compare conditions, while absolute rates give a direct quantitative measure, much like comparing race rankings versus miles per hour.

Because enzymes are so useful, researchers also engineer them for special tasks. Protein engineering uses recombinant DNA technology to change amino acid sequences and adjust enzyme shape or active site binding. This approach has helped create sensitive diagnostic tests, as well as tools for bioremediation and waste management.

Factors That Shape Enzyme Function

photosynthesis

Photosynthesis: Light, Sugars, and Carbon

Photosynthesis turns sunlight into chemical energy in the form of glucose. This process powers most life on Earth by helping autotrophs, or primary producers, make food. The glucose they make can provide quick energy or be stored as starch for later use. Consumers depend on these producers for energy. Photosynthesis happens in many kinds of organisms. Cyanobacteria, some protists such as algae, and plants can all carry out this process. In eukaryotic cells, it takes place in the chloroplast.

cellular respiration

Cellular Energy From Glucose Breakdown

Cellular respiration is the process cells use to release usable energy from biomolecules such as sugars and lipids. Living things need a steady energy supply for growth, repair, movement, defense, and reproduction. Cells use that energy mainly by converting adenosine diphosphate (ADP) to adenosine triphosphate (ATP). Autotrophs and heterotrophs get the molecules for respiration in different ways. Autotrophs, such as plants, algae, and photosynthetic and chemosynthetic bacteria, build...

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Cell division is essential for growth, repair, and reproduction in living organisms. It lets single-celled organisms create a new organism and helps multicellular organisms make new cells for development and for replacing damaged cells. In multicellular species, cell division also makes reproductive cells needed for reproduction. Before a cell divides, it must copy its genetic material and split it into two daughter cells. This process is called the cell cycle. Although the timing can vary...

transpiration

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enzyme activity

Enzymes are biological catalysts that help living things run biochemical reactions faster. They lower activation energy, which is the energy needed to start a reaction. Enzymes are not used up in the process, so they can keep working again and again. Most enzymes are proteins, although some ribonucleic acid molecules also have catalytic activity. Each enzyme usually binds only one or a few specific substrates. This selectivity depends on the enzyme’s active site, which fits its substrate like...

cellular processes

Explore how pH, temperature, inhibitors, cofactors, and substrates affect enzyme function, plus examples like lactase, Taq polymerase, and PCR.

Factors That Shape Enzyme Function | JoVE K12

SC.912.L.18.11

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Biomolecules
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The DNA Helix

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RNA Structure

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Types of RNA

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SC.912.L.18.2

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Describe the structures of fatty acids, triglycerides, phospholipids, and steroids. Explain the functions of lipids in living organisms. Identify some reactions that fatty acids undergo. Relate the structure and function of cell membranes.

SC.912.L.18.3

Ligand Binding Sites

Cofactors and Coenzymes

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Introduction to Enzymes

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Enzymes and Activation Energy

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Introduction to Enzyme Kinetics

Enzyme kinetics studies the rates of biochemical reactions. Scientists monitor the reaction rates for a particular enzymatic reaction at various substrate ...

Turnover Number and Catalytic Efficiency

The turnover number of an enzyme is the maximum number of substrate molecules it can transform per unit time. Turnover numbers for most enzymes range from ...

Catalytically Perfect Enzymes

The theory of catalytically perfect enzymes was first proposed by W.J. Albery and J. R. Knowles in 1976. These enzymes catalyze biochemical reactions at ...

introduction-to-mechanisms-of-enzyme-catalysis

Introduction to Mechanisms of Enzyme Catalysis

For many years, scientists thought that enzyme-substrate binding took place in a simple &#34;lock-and-key&#34; fashion. This model stated that the enzyme ...

Ligand Binding and Linkage

Cooperative Allosteric Transitions

Ribozymes

Biological Catalysts

All living organisms continuously perform numerous biochemical reactions to sustain their presence. Most of these reactions require ...

Describe the structures of proteins and amino acids. Explain the functions of proteins in living organisms. Identify some reactions that amino acids undergo. Relate the structure and function of enzymes.

Chemiosmosis

Oxidative phosphorylation is a highly efficient process that generates large amounts of adenosine triphosphate (ATP), the basic unit of energy that drives ...

electron-transport-chain-complex-i-and-ii

Electron Transport Chain: Complex I and II

The mitochondrial electron transport chain (ETC) is the main energy generation system in the eukaryotic cells. However, mitochondria also produce ...

electron-transport-chain-complex-iii-and-iv

Electron Transport Chain: Complex III and IV

During the electron transport chain, electrons from NADH and FADH2 are first transferred to complexes I and II, respectively. These two complexes then ...

The Electron Transport Chain

The electron transport chain or oxidative phosphorylation is an exothermic process in which free energy released during electron transfer reactions is ...

the-supercomplexes-in-the-crista-membrane

The Supercomplexes in the Crista Membrane

The mitochondrial cristae membrane is the primary site for the oxidative phosphorylation (OXPHOS) process of energy conversion mediated through ...

ATP Synthase: Structure

ATP synthase or ATPase is among the most conserved proteins found in bacteria, mammals, and plants. This enzyme can catalyze a forward reaction in ...

ATP Synthase: Mechanism

In animals, the mitochondrial F1F0 ATP synthase is the key protein that synthesizes ATP molecules through a complex catalytic mechanism. While the nuclear ...

The ADP/ATP Carrier Protein

ADP/ATP carrier or AAC protein is the most abundant carrier protein in the inner mitochondrial membrane. It transports large quantities of ADP and ATP, ...

ATP Yield

Cellular respiration produces 30 - 32 ATP per glucose molecule. Although most of the ATP results from oxidative phosphorylation and the electron transport ...

Discuss the use of chemiosmotic gradients for ATP production in chloroplasts and mitochondria.

SC.912.L.18.5

Fermentation

Most eukaryotic organisms require oxygen to survive and function adequately. Such organisms produce large amounts of energy during aerobic respiration by ...

biofuels-producing-ethanol-from-cellulosic-material

Biofuels: Producing Ethanol from Cellulosic Material

Source: Laboratories of Margaret Workman and Kimberly Frye - Depaul University
In this experiment, cellulosic material (such as corn stalks, leaves, ...

Discuss the role of anaerobic respiration in living things and in human society.

SC.912.L.18.6

Photosynthesis

Autotrophs

Almost all living organisms on Earth depend on photosynthesis, which is the process that converts sunlight energy into a simple sugar called ...

What is Photosynthesis?

All living organisms on Earth are directly or indirectly dependent on photosynthesis. It is the only biological process that can capture energy from ...

The Anatomy of Chloroplasts

Green algae and plants, including green stems and unripe fruit, harbor specialized organelles called chloroplasts to carry out photosynthesis. They ...

Photosystems

Photosystems are multiprotein complexes that form the functional units of photosynthesis in plants, algae, and cyanobacteria. They are found embedded in ...

The Photochemical Reaction Center

Reaction centers are pigment-protein complexes that initiate energy conversion from photons to chemical entities. Therefore, photochemical reaction center ...

The Antenna Complex

Plants and other photosynthetic organisms comprise pigments capable of absorption of direct sunlight. These pigments are present in the reaction center - ...

the-z-scheme-of-electron-transport-in-photosynthesis

The Z-Scheme of Electron Transport in Photosynthesis

The light reactions of photosynthesis assume a linear flow of electrons from water to NADP+. During this process, light energy drives the splitting of ...

The Calvin Benson Cycle

Ribulose 1,5- bisphosphate carboxylase/oxygenase (RuBisCo) is a critical enzyme that catalyzes carbon dioxide assimilation during photosynthesis. However, ...

Identify the reactants, products, and basic functions of photosynthesis.

SC.912.L.18.7

Introduction to Cellular Respiration

Organisms harvest energy from food, but this energy cannot be directly used by cells. Cells convert the energy stored in nutrients into a more usable ...

What is Glycolysis?

Overview
Cells make energy by breaking down macromolecules. Cellular respiration is the biochemical process that converts &#34;food energy&#34; (from the ...

Glycolysis: Preparatory Phase

In cellular metabolism (the complete breakdown of glucose to extract energy),&#160; glycolysis is the first step. Glycolysis takes place in the cytoplasm ...

Glycolysis: Pay-off Phase

So far, glycolysis has cost the cell two ATP molecules and produced two small, three-carbon sugar molecules. These molecules will proceed through the ...

Fates of Pyruvate

Pyruvate is the end product of glycolysis, where glucose is oxidized to pyruvate, simultaneously reducing NAD+ to NADH. Two molecules of ATP are also ...

Role of Reduced Coenzymes NADH and FADH&#8322;

The energy released from the breakdown of the chemical bonds within nutrients can be stored either through the reduction of electron carriers or in the ...

The Citric Acid Cycle

The citric acid cycle, also known as the Krebs cycle or TCA cycle, consists of several energy-generating reactions that yield one ATP molecule, three NADH ...

The Citric Acid Cycle: Overview

In aerobic organisms, the citric acid cycle is the second stage of cellular respiration wherein molecules derived from the breakdown of carbohydrates, ...

The Citric Acid Cycle: Output

The citric acid cycle is termed an amphibolic pathway as it operates both anabolically and catabolically. The cyclic reactions balance the flux of the ...

Products of the Citric Acid Cycle

The cells of most organisms&#8212;including plants and animals&#8212;obtain usable energy through aerobic respiration, the oxygen-requiring version of ...

Identify the reactants, products, and basic functions of aerobic and anaerobic cellular respiration.

SC.912.L.18.8

A. All living things are composed of four basic categories of macromolecules and share the same basic needs for life.

B. Living organisms acquire the energy they need for life processes through various metabolic pathways (primarily photosynthesis and cellular respiration).

C. Chemical reactions in living things follow basic rules of chemistry and are usually regulated by enzymes.

D. The unique chemical properties of carbon and water make life on Earth possible.

Factors That Shape Enzyme Function

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