Enzymes
A-Level Biology Topic 3 13:52 English narration · English + 中文 subtitles burned in
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Inside every cell, thousands of chemical reactions happen every second — far faster than they ever could on their own.
在每一个细胞里，每一秒都有成千上万个化学反应发生——比它们自己发生要快得多。
The secret is a special kind of protein: an enzyme.
秘密是一种特殊的蛋白质：酶。
Each enzyme has a pocket, its active site, shaped to grip just one molecule.
每一种酶都有一个口袋，也就是它的活性位点，其形状只能抓住一种分子。
When the right partner drops in, like a key into its lock, the reaction fires.
当合适的搭档落进来，就像钥匙插进它的锁，反应便触发了。
Enzymes are the catalysts that make life possible.
酶是让生命成为可能的催化剂。
Let's see how they work.
我们来看看它们如何工作。
Enzymes are biological catalysts — they catalyse reactions, speeding them up, without being used up.
酶是生物催化剂——它们加速反应，自己却不被消耗。
In this lesson we meet the active site, compare lock-and-key with induced fit, see how enzymes lower the activation energy, measure their rate, and explore everything that changes it: temperature, pH, concentration, and inhibitors.
在这节课里，我们会认识活性位点， 比较锁钥学说与诱导契合，看酶如何降低活化能，测量它们的速率，并探索改变它的一切： 温度、pH、浓度和抑制剂。
Finally, we tie it up with Vmax, the Michaelis constant, and enzymes locked into beads.
最后，我们用最大速率、米氏常数，以及被锁进小珠里的酶来收尾。
Let's begin.
让我们开始吧。
An enzyme is a globular protein that acts as a catalyst.
酶是一种球状蛋白质，充当催化剂。
The molecule it works on is called the substrate, and it fits into the enzyme's active site to form an enzyme-substrate complex.
它作用的分子叫做底物，底物嵌入酶的活性位点，形成酶底物复合物。
Inside that complex the reaction happens, the products form and leave, and the active site is free again for the next substrate.
在这个复合物里，反应发生，产物形成并离开，活性位点又空出来，迎接下一个底物。
And here is the key idea: an enzyme is specific.
这里有个关键的想法：酶是专一的。
Its active site is shaped to match one substrate and no other — the two shapes are complementary.
它的活性位点只与一种底物形状相配，别无其他——两者的形状是互补的。
Enzymes work in two places.
酶在两个地方工作。
Intracellular enzymes work inside the cell that made them.
细胞内酶在制造它们的细胞内部工作。
Catalase is a classic example: it breaks down harmful hydrogen peroxide into water and oxygen before the peroxide can damage the cell.
过氧化氢酶是经典例子： 它把有害的过氧化氢分解成水和氧，免得过氧化物损伤细胞。
Extracellular enzymes are secreted — sent out — to work outside the cell.
细胞外酶被分泌出去—— 送到细胞外工作。
Amylase is released into the gut, where it digests starch into smaller sugars.
淀粉酶释放到肠道里，在那里消化淀粉，变成更小的糖。
Yeast is full of enzymes too: they ferment sugar and give off bubbles of carbon dioxide.
酵母里也满是酶：它们发酵糖，放出二氧化碳气泡。
Same idea — catalysts that never get used up, working again and again.
同一个想法——催化剂从不被消耗， 一次又一次地工作。
How exactly does the substrate fit?
底物究竟是怎么嵌合的？
Two ideas explain it.
有两种解释。
The older lock-and-key hypothesis says the active site is a fixed, rigid shape, and only a substrate with the matching shape can enter — just like a key in a lock.
较早的锁钥学说说，活性位点是固定、刚性的形状， 只有形状相配的底物才能进入——就像钥匙插进锁。
The newer induced-fit hypothesis says the active site is not quite right at first; when the substrate binds, the site changes shape a little, moulding tightly around it.
较新的诱导契合学说说，活性位点起初并不完全合适； 当底物结合时，位点会略微改变形状，紧紧包裹住它。
Induced fit matches the evidence better, so use it in your answers.
诱导契合更符合证据，所以答题时要用它。
Look at the two diagrams side by side.
并排看这两张图。
On the left, lock-and-key: a rigid notch, and only a matching triangular substrate can drop in.
左边是锁钥学说：一个刚性的凹口，只有相配的三角形底物才能落进去。
On the right, induced fit: the enzyme moulds around a rounded substrate and grips it tightly.
右边是诱导契合：酶包裹住圆形的底物，把它紧紧抓住。
Either way, the active site is complementary to its own substrate and not to others.
无论哪一种，活性位点都与自己的底物互补， 而不是与别的分子互补。
That complementary shape is why enzymes show specificity — one enzyme, usually one substrate.
正是这种互补的形状，让酶表现出专一性——一种酶，通常一种底物。
In an exam, prefer induced fit: say the active site is flexible and wraps around the substrate when it binds.
考试里优先用诱导契合：说活性位点是柔韧的，底物结合时会包裹住它。
Why does an enzyme speed things up?
酶为什么能加速反应？
Every reaction needs a small push of energy to get started — the activation energy.
每一个反应都需要一点点能量来启动——这就是活化能。
Without an enzyme, that energy barrier is high, so only a few molecules have enough to react.
没有酶时， 这个能量壁垒很高，所以只有少数分子有足够能量去反应。
An enzyme provides a different route with a much lower activation energy.
酶提供了一条不同的路径，活化能低得多。
Now far more molecules can get over the barrier, so the reaction runs quickly — and at the cell's normal temperature, with no need for fierce heat.
现在，多得多的分子能翻过这道壁垒，于是反应快速进行——而且在细胞的正常温度下，不需要猛烈的高温。
Here is the energy profile.
这就是能量曲线。
Both routes start at the same reactants and end at the same products.
两条路径都从同样的反应物出发，到同样的产物结束。
Without enzyme, the hump in the middle is high — that is the activation energy.
没有酶时，中间的峰很高—— 那就是活化能。
With enzyme, the hump is much lower.
有酶时，峰低得多。
More molecules have enough energy to clear the smaller barrier, so the reaction goes faster at body temperature.
更多分子有足够能量翻过这道较小的壁垒， 所以反应在体温下就能更快进行。
The enzyme does not change the energy of the reactants or products; it only lowers the push needed to get from one to the other.
酶并不改变反应物或产物的能量；它只是降低从一边走到另一边所需的推动力。
To measure how fast an enzyme works, you can follow the product forming or the substrate disappearing.
要测量酶工作的快慢，你可以跟踪产物的生成，或底物的消失。
Plot product against time, and you get a curve that is steepest at the start, when the most substrate is present, then levels off as substrate runs out.
把产物对时间作图，你会得到一条曲线， 它在开始时最陡，那时底物最多，随后随着底物耗尽而趋于平缓。
So the fairest value to compare is the initial rate — the slope of the tangent at time zero.
所以最公平的比较值是初始速率—— 时间零点处切线的斜率。
For example, if sixteen cubic centimetres of oxygen form in the first twenty seconds, the rate is sixteen over twenty — zero point eight cubic centimetres per second.
举个例子，如果最初二十秒内生成十六立方厘米的氧气，速率就是十六比二十—— 每秒零点八立方厘米。
In the lab you choose what to track.
在实验室里，你要选择跟踪什么。
With catalase, oxygen gas is a product, so you collect the gas and measure its volume over time.
用过氧化氢酶时，氧气是产物，所以你收集气体，测量体积随时间的变化。
With amylase, you watch the substrate disappear: take samples and add iodine.
用淀粉酶时，你观察底物的消失：取样并加入碘。
Starch with iodine goes blue-black; as the starch is used up, that colour fades.
淀粉遇碘变蓝黑；随着淀粉被用完，颜色逐渐褪去。
A colorimeter makes the colour change exact — it shines light through the tube and reports how much light is absorbed, so a fading colour becomes a number you can plot.
比色计让颜色变化变得精确——它让光穿过试管，报告有多少光被吸收，于是褪色变成可以作图的数字。
Either way, the curve of product against time is steepest at the start, then levels off.
无论哪一种，产物对时间的曲线都在开始时最陡，随后趋平。
Back to that worked example.
再看那个例题。
Sixteen cubic centimetres of oxygen in the first twenty seconds gives zero point eight cubic centimetres per second.
最初二十秒里十六立方厘米氧气，得到每秒零点八立方厘米。
Why measure over a short early interval?
为什么要在很短的早期区间测量？
Because the reaction is fastest at the start, when substrate is plentiful.
因为反应在开始时最快，那时底物充足。
Averaging over a longer time would fold in the slower later stages, when substrate is running out, and that would underestimate the true rate.
如果对更长的时间取平均，就会把后面更慢的阶段也算进去—— 那时底物快耗尽了——那样会低估真实速率。
The dashed tangent at time zero on the graph is the fairest single number for comparing conditions.
图上时间零点处的虚线切线，是比较不同条件时最公平的一个数。
Now the factors that change the rate, starting with temperature.
现在来看改变速率的因素，先从温度开始。
As it rises, molecules move faster and collide more often, so the rate climbs.
温度升高时，分子运动更快、碰撞更频繁，于是速率上升。
But only up to a point — the optimum temperature, where the rate peaks.
但只升到某个点——最适温度，速率在那里达到峰值。
Push higher, and the heat begins to break the bonds holding the enzyme's precise shape.
再往上，热开始破坏维系酶精确形状的键。
The active site distorts and no longer fits the substrate.
活性位点扭曲，不再与底物相配。
We say the enzyme denatures, and the rate falls off a cliff.
我们说酶变性了，速率随之骤降。
Read the temperature graph carefully.
仔细读这张温度图。
As temperature rises, molecules gain more kinetic energy and collide more often with the active site, so the rate rises.
温度升高时，分子获得更多动能，更频繁地与活性位点碰撞，于是速率上升。
At the optimum the collision rate is high and the enzyme still holds its shape.
在最适点，碰撞频率高，酶仍保持形状。
Above that, heat breaks the hydrogen bonds and other weak bonds that hold the tertiary structure.
超过之后，热会破坏维系三级结构的氢键和其他弱键。
The three-dimensional fold unravels, the active site changes shape, and substrate no longer fits.
三维折叠松开，活性位点改变形状，底物不再契合。
In the exam write denatures — never dies or is killed.
考试里写变性——绝不要写死了或被杀死。
Denaturation is a change of shape, not death.
变性是形状的改变，不是死亡。
Two more factors.
还有两个因素。
Each enzyme also has an optimum pH, where it works best.
每一种酶也有一个最适pH，在那里工作得最好。
Move the pH too far either way and the enzyme denatures, so the rate falls on both sides — a bell-shaped curve.
pH向任一方向偏离太远，酶就变性， 于是速率在两侧都下降——一条钟形曲线。
Substrate concentration behaves differently.
底物浓度则不同。
At first, more substrate means a faster rate.
起初，底物越多速率越快。
But once every active site is busy, adding more makes no difference — the rate levels off at a maximum.
但一旦每个活性位点都忙起来，再加也没有区别——速率在一个最大值处趋平。
And with plenty of substrate, more enzyme simply means more active sites, so the rate rises in proportion.
而在底物充足时，酶越多就意味着活性位点越多，于是速率成正比上升。
The pH graph is a bell: rate peaks at the optimum pH and falls away on both sides as the enzyme denatures.
pH图是钟形的：速率在最适pH达到峰值，两侧因酶变性而下降。
To study pH fairly in an experiment, you keep it steady with a buffer solution.
要在实验中公平地研究pH， 就要用缓冲液把它保持稳定。
A buffer resists changes in pH when small amounts of acid or alkali are added, so the only variable you change is the pH you chose, not a drifting value during the run.
缓冲液能抵抗酸碱少量加入时pH的变化， 于是你改变的唯一变量是你选定的pH，而不是实验过程中漂移的值。
Without a buffer, the reaction itself can shift the pH and spoil the comparison.
没有缓冲液，反应本身就可能改变pH，毁掉比较。
Hold two concentration stories apart.
把两种浓度的故事分开。
With plenty of substrate, enzyme concentration and rate rise together — more enzyme means more free active sites, so collisions form more complexes per second.
在底物充足时，酶浓度和速率一起上升——酶越多，空闲活性位点越多， 所以每秒形成的复合物更多。
Substrate concentration is different.
底物浓度则不同。
At low substrate, adding more speeds the reaction because free active sites are waiting.
底物低时，再加会加速反应，因为还有空闲位点在等。
Once every active site is occupied all the time, the rate plateaus at its maximum.
一旦每个活性位点都始终被占满，速率就在最大值处趋平。
Extra substrate sits idle until a product leaves and frees a site again.
多出来的底物闲着，直到产物离开、位点再次空出。
That plateau is Vmax, which we name next.
那个平台就是最大速率，我们接下来给它命名。
That levelling-off point, when every active site is full, is the maximum rate — Vmax.
那个趋平的点，也就是每个活性位点都被占满时，是最大速率——Vmax。
But there is a subtler measure.
但还有一个更微妙的量。
The Michaelis constant, Km, is the substrate concentration that gives exactly half of Vmax.
米氏常数Km，是恰好给出一半Vmax的底物浓度。
It reveals the enzyme's affinity — how strongly it grabs its substrate.
它揭示酶的亲和力——它抓住底物有多牢。
A low Km means the enzyme reaches half-speed at a low substrate concentration, so it has a high affinity.
低Km意味着酶在很低的底物浓度就达到半速，所以亲和力高。
A high Km means a low affinity.
高Km则意味着亲和力低。
So Km lets you compare how tightly different enzymes hold on.
所以Km让你比较不同的酶抓得有多紧。
On the Michaelis–Menten curve, rate climbs with substrate concentration and then flattens at Vmax when all active sites are full.
在米氏曲线上，速率随底物浓度上升，然后在所有活性位点占满时于最大速率处变平。
Half of Vmax is marked on the vertical axis; drop across to the curve and down to the substrate axis — that substrate concentration is Km.
一半最大速率标在纵轴上；横着走到曲线，再落到底物轴——那个底物浓度就是米氏常数。
A low Km sits far left: the enzyme is half-busy at a low substrate level, so affinity is high.
低米氏常数靠左：酶在很低底物水平就半忙，所以亲和力高。
A high Km sits further right: you need a lot of substrate before the enzyme is half-busy, so affinity is low.
高米氏常数更靠右： 需要很多底物酶才半忙，所以亲和力低。
Use Km to compare enzymes; use Vmax to compare how fast they can go when saturated.
用米氏常数比较酶抓得有多牢；用最大速率比较饱和时有多快。
Some molecules slow enzymes down — inhibitors.
有些分子会减慢酶——这就是抑制剂。
The reversible ones come in two types, and the exam loves the difference.
可逆的抑制剂有两种，而考试最爱考它们的区别。
A competitive inhibitor has a shape like the substrate, so it blocks the active site.
竞争性抑制剂的形状像底物，所以它堵住活性位点。
Add more substrate and you out-compete it, so Vmax is unchanged, but Km rises.
加更多底物就能把它竞争掉，于是Vmax不变，但Km上升。
A non-competitive inhibitor binds somewhere else, changing the active site's shape.
非竞争性抑制剂结合在别的地方，改变活性位点的形状。
Adding more substrate cannot help, so Vmax falls, while Km stays the same.
再加底物也无济于事，于是Vmax下降，而Km保持不变。
Three curves against substrate concentration tell the whole story.
三条对底物浓度的曲线讲完整个故事。
With no inhibitor, the rate climbs to Vmax.
没有抑制剂时，速率升到最大速率。
With a competitive inhibitor, the curve still reaches the same Vmax, but more slowly — you need higher substrate to out-compete the blocker, so Km rises.
有竞争性抑制剂时， 曲线仍到达同一个最大速率，但更慢——你需要更高的底物才能竞争掉阻挡者，所以米氏常数上升。
With a non-competitive inhibitor, the plateau is lower: Vmax falls because some enzymes are warped and never work, while Km is unchanged.
有非竞争性抑制剂时，平台更低：最大速率下降，因为有些酶被扭曲、永远不工作，而米氏常数不变。
And as a factor on its own: the more inhibitor you add, the lower the rate.
作为单独的因素：抑制剂加得越多，速率越低。
Reversible inhibitors can leave again; that is why more substrate can rescue competitive inhibition.
可逆抑制剂可以再离开；所以加更多底物能挽救竞争性抑制。
One last idea with big practical uses.
最后一个想法，有很大的实用价值。
An immobilised enzyme is fixed in place — often trapped inside tiny beads of alginate — instead of floating free.
固定化酶被固定在原地——常常被困在海藻酸盐的小珠里—— 而不是自由漂浮。
Substrate solution flows past, reacts inside the bead, and product flows out.
底物溶液流过，在珠子里反应，产物流出来。
A free enzyme works a little faster, but immobilised enzymes win on practicality: the enzyme is never washed away, so it is reused again and again; the product comes out pure; the enzyme is more stable to heat and pH; and the whole process can run continuously.
自由的酶工作稍快一些， 但固定化酶胜在实用：酶不会被冲走，所以能一次次重复使用；产物出来是纯净的； 酶对热和pH更稳定；而且整个过程可以连续运行。
In the diagram, enzymes stay trapped inside the bead.
图里，酶留在珠子内部。
Substrate flows in on one side, product flows out on the other, and the enzyme is never washed into the product stream.
底物从一侧流进，产物从另一侧流出，酶绝不会被冲进产物流。
That is why the product is pure — it is not mixed with free enzyme protein.
所以产物是纯净的——不与游离的酶蛋白混在一起。
Because the enzyme is held in place, the process can run continuously: substrate in, product out, beads reused for many batches.
因为酶被固定住，过程可以连续运行： 底物进、产物出，珠子用于许多批次。
Immobilisation also makes the enzyme more stable against swings in temperature and pH.
固定化也让酶对温度和pH的波动更稳定。
The trade-off is a slightly slower rate, because substrate must diffuse into the bead to reach the active sites.
代价是速率稍慢，因为底物必须扩散进珠子才能到达活性位点。
You already meet enzymes outside the textbook.
课本之外你已经遇到酶。
Biological washing powders contain enzymes that digest food and blood stains at low temperatures.
生物洗衣粉含有酶，能在低温下消化食物和血迹。
Proteases attack protein stains; lipases attack fats.
蛋白酶攻击蛋白质污渍；脂肪酶攻击油脂。
Because enzymes work at mild temperatures, the wash can stay cool and still remove stains — saving energy.
因为酶在温和温度下就能工作， 洗衣可以保持凉爽仍能去污——节省能量。
That is the same catalytic idea as catalase in a cell: speed up a useful reaction without being used up yourself.
这与细胞里过氧化氢酶的催化想法相同： 加速有用的反应，自己却不被消耗。
Before you go, four ways to keep your marks.
结束之前，四个保住分数的办法。
First, explain enzyme action with the induced-fit model — the active site moulds around the substrate.
第一，用诱导契合模型来解释酶的作用——活性位点包裹住底物。
Second, above the optimum the enzyme denatures — write denatures, never dies or is killed.
第二，超过最适点，酶会变性——写变性，绝不要写死了或被杀死。
Third, keep the inhibitors straight: competitive binds the active site and raises Km; non-competitive binds elsewhere and lowers Vmax.
第三，把抑制剂分清楚： 竞争性结合活性位点、抬高Km；非竞争性结合别处、降低Vmax。
Fourth, always compare rates using the initial rate, the tangent at time zero.
第四，永远用初始速率来比较—— 时间零点处的切线。