酶

Enzymes 酶 are globular proteins 球状蛋白质 — a type of protein 蛋白质 with a rounded, soluble shape. They are biological catalysts: they catalyse 催化 (speed up) the chemical reactions in living things, and they are not used up, so each enzyme works again and again.

Enzymes work in two places:

• intracellular 细胞内 enzymes work inside the cell 细胞 that made them. An example is catalase 过氧化氢酶, which breaks down harmful hydrogen peroxide.
• extracellular 细胞外 enzymes are secreted 分泌 (sent out) to work outside the cell. An example is amylase 淀粉酶, which is released into the gut to digest 消化 starch 淀粉.

Each enzyme has a special pocket called the active site 活性位点. The molecule it acts on is its substrate 底物. The substrate fits into the active site to form an enzyme–substrate complex 酶底物复合物. The reaction then happens, and the products 产物 leave, freeing the active site for the next substrate.

Specificity

An enzyme is specific: it usually works on only one substrate. This is because the shape of the active site is complementary 互补 to (fits) the shape of that substrate and no other. We call this specificity 专一性.

Two ideas explain how the substrate fits:

• the lock-and-key hypothesis 锁钥学说 — the active site is a fixed shape, and only a substrate with the matching shape fits, like a key in a lock.
• the induced-fit hypothesis 诱导契合学说 — the active site is not quite the right shape at first. When the substrate binds, the active site changes shape a little to wrap around it tightly. This idea fits the evidence better.

Lowering activation energy

Every reaction needs a small "push" of energy to start, called the activation energy 活化能. An enzyme lowers the activation energy. This lets the reaction go quickly at the cell's normal temperature 温度, instead of needing high heat.

You can follow an enzyme reaction in two ways:

• measure how fast product is made. With catalase, oxygen gas is a product, so you collect the gas and measure its volume over time.
• measure how fast substrate disappears. With amylase, you remove samples and use the iodine test; the blue-black colour fades as the starch is used up.

A colorimeter 比色计 makes this exact. It shines light through the tube and measures how much light is absorbed, so a colour change becomes a number you can plot.

The rate of reaction 反应速率 is steepest at the start (most substrate present), so the initial rate (the slope at time zero) is the fairest value to compare.

Worked example. In the first $20$ seconds of a catalase reaction, $16\ \text{cm}^3$ of oxygen is collected. Estimate the rate of reaction.

Because the reaction is fastest at the start, measuring over this short early interval gives a value close to the initial rate; averaging over a longer time would include the slower later stages and underestimate it.

Temperature

As temperature rises, molecules gain more kinetic energy 动能 and collide 碰撞 more often, so the rate rises. But above the optimum temperature 最适温度 the enzyme begins to denature 变性: the heat breaks the bonds holding its shape, so the active site changes and no longer fits the substrate. The rate then falls quickly.

pH

Each enzyme has an optimum pH. If the pH moves too far from it, the enzyme denatures and the rate drops. To study pH fairly, you keep it steady with a buffer solution 缓冲液.

Enzyme concentration

With plenty of substrate, more enzyme means more active sites, so the rate goes up in proportion to enzyme concentration.

Substrate concentration

At first, adding more substrate speeds the reaction. But once every active site is busy, adding more makes no difference — the rate levels off at a maximum.

Inhibitor concentration

An inhibitor 抑制剂 is a molecule that slows an enzyme. The more inhibitor present, the lower the rate.

The levelling-off rate, when all active sites are full, is the maximum rate, written $V_{\text{max}}$.

The Michaelis–Menten constant 米氏常数 ($K_{\text{m}}$) is the substrate concentration that gives half of $V_{\text{max}}$. It tells you about the enzyme's affinity 亲和力 (pulling power) for its substrate:

• a low $K_{\text{m}}$ means the enzyme reaches half-speed at a low substrate concentration, so it has a high affinity.
• a high $K_{\text{m}}$ means a low affinity.

So $K_{\text{m}}$ lets you compare how strongly different enzymes hold their substrates.

Some inhibitors are reversible 可逆: they can leave the enzyme again. There are two types.

An immobilised enzyme 固定化酶 is fixed in place — for example, trapped inside small beads of alginate 海藻酸盐 — instead of floating free in solution. The substrate solution flows past the beads.

A free enzyme usually works a little faster, because the substrate can reach it easily. But immobilised enzymes have big practical advantages:

• the enzyme is not washed away, so it can be used again and again.
• the product is pure — it is not mixed with enzyme.
• the enzyme is more stable, so it survives changes in temperature and pH better.
• the process can run continuously, with substrate flowing in and product flowing out.

Common mistakes.

• Saying a cold enzyme is denatured. At a low temperature the enzyme is only slow: molecules move and collide less. Its shape is unchanged, so the rate rises again when it is warmed. Only high temperature or extreme pH denatures it.
• Saying the active site has the same shape as the substrate. It has a complementary shape, so the two fit together like a key in a lock.
• Saying denaturing breaks the peptide bonds. Heat and pH break the hydrogen bonds and ionic bonds that hold the tertiary structure 三级结构. The chain of amino acids stays whole, but the active site loses its shape.
• Explaining the plateau of a rate–substrate graph as "the enzyme is used up". The enzyme is not used up; every active site is busy, so enzyme concentration now limits the rate.
• Saying a competitive inhibitor lowers $V_{\text{max}}$. With enough substrate the substrate out-competes the inhibitor, so the same $V_{\text{max}}$ is reached; it is $K_{\text{m}}$ that rises.
• Reading $K_{\text{m}}$ at $V_{\text{max}}$. $K_{\text{m}}$ is the substrate concentration at half $V_{\text{max}}$.
• Assuming every enzyme has an optimum of 37 °C and pH 7. Pepsin in the stomach works best near pH 2, and enzymes from bacteria in hot springs work best near 70 °C.

• Explain enzyme action with the induced-fit model (the active site moulds around the substrate) — now preferred to lock-and-key.
• Above the optimum the enzyme denatures (hydrogen bonds and tertiary structure break) — write "denatures", never "dies" or "is killed".
• Distinguish inhibitors: competitive binds the active site (overcome by more substrate, same $V_{max}$); non-competitive binds elsewhere (lower $V_{max}$).
• Compare rates using the initial rate (tangent at time zero) — the fairest measure, before substrate becomes limiting.