📚 Immobilising Enzymes | 固定化酶
Enzymes are remarkable biological catalysts with unmatched specificity and efficiency, but their industrial use has long been limited by the difficulty of recovering them from reaction mixtures. Immobilisation – the process of attaching or confining enzymes to a solid support – solves this problem, enabling enzymes to be reused and integrated into continuous processes. In this Cambridge A-Level topic, we explore how enzymes are immobilised, why it matters, and where these technologies appear in real-world bioprocessing and diagnostics.
酶是特异性极强、效率极高的生物催化剂，但由于难以从反应混合物中分离回收，它们在工业应用上长期受限。固定化——将酶附着或束缚在固相载体上的技术——解决了这一难题，使酶能够重复使用并整合到连续生产流程中。在这篇剑桥A-Level的选题文章中，我们将深入探讨酶如何被固定化、其意义何在，以及这些技术在现实生物加工与诊断中的具体应用。
1. What Is Enzyme Immobilisation? | 什么是酶固定化？
Enzyme immobilisation refers to the technique of restricting enzyme molecules to a defined region of space, usually by attachment to an inert, insoluble carrier or by entrapment within a matrix, while retaining their catalytic activity. The enzyme is thus converted from a free, soluble form into a heterogeneous catalyst that can be easily separated from the product stream.
酶固定化是指将酶分子限制在特定空间区域的技术，通常是通过附着在惰性不溶性载体上或包埋于基质中，并同时保留其催化活性。这样，酶就从未固定的溶解态转变为非均相催化剂，可以方便地与产物流分离。
The term was first applied to industrial processes in the 1960s, and since then immobilised enzymes have become essential in biotechnology. Unlike free enzymes, immobilised enzymes can be packed into columns or reactors and used repeatedly, drastically reducing costs.
该术语最早于20世纪60年代用于工业过程，此后固定化酶便成为生物技术中不可或缺的一部分。与游离酶不同，固定化酶可以装填到柱式或反应器中反复使用，从而大幅降低成本。
2. Why Immobilise Enzymes? | 为什么要固定化酶？
Free enzymes present several practical challenges: they are expensive to purify, they lose activity under harsh conditions, and after a single use they contaminate the product, requiring costly separation steps. Immobilisation addresses these issues directly by converting the enzyme into a reusable, stabilised form.
游离酶在实际应用中面临诸多挑战：纯化成本高，在苛刻条件下易失活，一次性使用后还会污染产物，需要昂贵的分离步骤。固定化技术直接将酶转变为可重复使用且更稳定的形态，从而解决了这些问题。
Key motivations include: improved enzyme stability against temperature and pH changes; ease of recovery from reaction media; possibility of continuous operation; reduced downstream contamination; and the ability to fine-tune reaction kinetics through controlled diffusion environments.
主要动机包括：提高酶对温度和pH变化的耐受性；便于从反应介质中回收；可进行连续化操作；减少下游产物污染；并可通过可控的扩散环境微调反应动力学。
3. Adsorption: Binding by Physical Forces | 吸附法：通过物理力结合
Adsorption is the simplest immobilisation method, relying on weak, non-covalent interactions such as van der Waals forces, hydrogen bonds and hydrophobic interactions to attach enzyme molecules to the surface of a support like activated charcoal, silica gel or glass beads. The support is soaked in an enzyme solution, and after washing, the enzyme remains adsorbed.
吸附法是最简单的固定化方法，依靠范德华力、氢键和疏水相互作用等弱非共价键，将酶分子附着在活性炭、硅胶或玻璃珠等载体表面。载体浸泡于酶溶液中，洗涤后酶即被吸附保留。
The major advantage is that the enzyme’s active site is rarely distorted because no chemical modification occurs. However, the binding is weak and sensitive to changes in pH, ionic strength and temperature, which can cause the enzyme to leak from the support. This technique is often used where gentle conditions are essential, such as in some diagnostic strips.
其最大优势在于酶的活性中心很少因化学修饰而变形。但由于结合力弱，对pH、离子强度和温度变化敏感，容易造成酶从载体上脱落。该方法常用于需要温和条件的场合，例如某些诊断试纸。
4. Covalent Bonding: Stable and Permanent Attachment | 共价结合法：稳定持久的附着
Covalent bonding involves forming strong, permanent chemical links between functional groups on the enzyme’s surface (such as —NH₂, —COOH, —SH) and activated groups on the carrier. Carriers like agarose, polyacrylamide or cellulose are chemically treated to create reactive sites that react with the enzyme.
共价结合法是在酶表面官能团（如—NH₂、—COOH、—SH）与载体上活化基团之间形成牢固永久的化学连接。常用的载体包括琼脂糖、聚丙烯酰胺或纤维素，经化学处理后产生可与酶反应的活性位点。
This method provides excellent stability and virtually no enzyme leakage, making it suitable for continuous industrial processes. The drawback is that covalent modification can alter the enzyme’s three-dimensional conformation, potentially reducing catalytic activity. It also requires toxic coupling reagents and careful control of reaction conditions.
此法稳定性极佳，几乎不发生酶泄漏，非常适合连续工业生产。缺点在于共价修饰可能改变酶的三维构象，从而降低催化活性；且需要使用有毒的偶联试剂，并严格控制反应条件。
5. Cross-linking: Enzyme Aggregates Without a Carrier | 交联法：无载体的酶聚集体
Cross-linking joins enzyme molecules together into large, insoluble aggregates using bifunctional reagents such as glutaraldehyde. No separate solid support is required; the enzyme molecules become their own matrix. The resulting cross-linked enzyme aggregates (CLEAs) or cross-linked enzyme crystals (CLECs) are robust and stable.
交联法是利用戊二醛等双功能试剂将酶分子彼此连接，形成大型不溶性聚集体。无需单独的固相载体，酶分子自身即构成基质。由此获得的交联酶聚集体（CLEAs）或交联酶晶体（CLECs）十分坚固和稳定。
The method yields high enzyme loading per unit volume but often suffers from significant loss of activity because the cross-linking agent can penetrate the active site. It is less common as a standalone method and is frequently combined with adsorption or entrapment to improve performance.
这种方法的单位体积酶载量很高，但由于交联剂可能侵入活性中心，常导致活性大幅丧失。它单独使用的情况较少，通常与吸附法或包埋法联用以提升性能。
6. Entrapment and Encapsulation: Physical Confinement | 包埋与微胶囊化：物理约束
Entrapment physically confines enzymes within a porous polymer network such as polyacrylamide gel, alginate beads or silica sol-gel. The substrate and product molecules can diffuse freely through the pores, but the enzyme is too large to escape. Encapsulation goes a step further by enclosing enzymes inside semi-permeable microcapsules, often made of cellulose nitrate or polyamide.
包埋法是将酶物理束缚于多孔聚合物网络中，如聚丙烯酰胺凝胶、藻酸盐珠或硅溶胶–凝胶。底物和产物分子可自由通过孔隙扩散，而大分子的酶则无法逸出。微胶囊化进一步将酶包裹在半透膜微囊内，常用硝酸纤维素或聚酰胺制成。
The enzyme retains its native state and activity because no chemical modification is involved. Mass transfer limitations, however, can reduce the observed reaction rate, as substrates must diffuse through the gel or membrane. This method is popular for producing immobilised cells or for delicate enzymes that cannot tolerate harsh coupling chemistry.
由于不涉及化学修饰，酶保持天然状态和活性。然而，传质阻力会降低表观反应速率，因为底物必须扩散通过凝胶或膜。这种方法常用于生产固定化细胞或对苛刻偶联化学敏感的酶。
7. Comparing Immobilisation Techniques | 固定化技术的比较
Choosing the right immobilisation strategy depends on the enzyme, the intended application and economic factors. The table below summarises the key characteristics of each method.
选择合适的固定化策略取决于酶的特性、目标应用及经济因素。下表概括了各方法的主要特征。
| Method / 方法 | Binding force / 结合力 | Leakage risk / 泄漏风险 | Activity retention / 活性保持 |
|---|---|---|---|
| Adsorption / 吸附法 | Weak (van der Waals, H-bonds) | High | High |
| Covalent bonding / 共价结合法 | Strong (covalent bonds) | Very low | Moderate–low |
| Cross-linking / 交联法 | Strong (covalent cross-links) | Very low | Moderate–low |
| Entrapment / 包埋法 | Physical confinement | Low | High |
8. Advantages of Using Immobilised Enzymes | 固定化酶的优势
The most celebrated advantage is reusability: immobilised enzymes can be recovered by simple filtration or sedimentation and used for multiple reaction cycles, cutting enzyme costs dramatically. This makes high-cost, high-purity enzymes economically viable for large-scale processes.
最显著的优势是可重复使用性：固定化酶可通过简单的过滤或沉降方式回收，经过多次反应循环使用，大幅降低酶成本。这使得高成本、高纯度的酶在大规模工艺中具备经济可行性。
Immobilisation often enhances thermal and operational stability. Many enzymes become less prone to denaturation and can tolerate wider ranges of pH and temperature. Moreover, the product stream remains enzyme-free, simplifying downstream purification and reducing allergenicity risks in food products.
固定化通常可增强热稳定性和操作稳定性。许多酶变得更不易变性，能耐受更宽的pH和温度范围。此外，产物流中不含酶，简化了下游纯化步骤并降低了食品中的致敏风险。
Continuous bioprocessing becomes possible through packed-bed or fluidised-bed reactors, where substrate flows constantly over immobilised enzyme columns, yielding a continuous output of product. This design is fundamental to modern biomanufacturing.
通过填充床或流化床反应器，可进行连续生物加工，使底物持续流过固定化酶柱，不断生成产物。这种设计是现代生物制造的基础。
9. Limitations and Challenges | 局限与挑战
Immobilisation is not without drawbacks. The catalytic efficiency can decrease due to steric hindrance, where the carrier physically obstructs access to the active site, or due to conformational changes induced by the immobilisation chemistry. The apparent Kₘ (Michaelis constant) may increase, indicating reduced substrate affinity.
固定化并非没有缺点。催化效率可能因空间位阻（载体物理上阻碍活性中心接近）或固定化化学反应引起的构象变化而下降。表观Kₘ（米氏常数）可能升高，表明底物亲和力降低。
Mass transfer limitations are common in entrapment systems, where the diffusion of substrate into the matrix and product out becomes rate-limiting. This can make the observed reaction velocity lower than that of the free enzyme at the same bulk substrate concentration.
传质限制在包埋系统中很常见，底物扩散进入基质和产物扩散出来的过程成为限速步骤。这可能导致在相同表观底物浓度下，观测到的反应速率低于游离酶。
Additionally, immobilisation procedures require careful optimisation and may involve hazardous chemicals. Scale-up can be technically demanding, and fouling or microbial contamination of the columns can reduce long-term reliability.
此外，固定化工艺需要精心优化，可能涉及危险化学品。放大规模在技术上要求很高，而且柱体的结垢或微生物污染可能降低长期可靠性。
10. Industrial Application: Lactose-free Milk | 工业应用：无乳糖牛奶
A classic example of enzyme immobilisation is the production of lactose-free milk using the enzyme lactase (β-galactosidase). Lactose-intolerant individuals lack sufficient lactase to break down milk sugar. Immobilising lactase onto cellulose or alginate beads allows milk to be passed through a column reactor where lactose is hydrolysed into glucose and galactose.
酶固定化的经典案例是利用乳糖酶（β-半乳糖苷酶）生产无乳糖牛奶。乳糖不耐受者缺乏足够的乳糖酶来分解乳糖。将乳糖酶固定在纤维素或藻酸盐珠上，可使牛奶流经柱式反应器，令乳糖水解为葡萄糖和半乳糖。
Lactose + H₂O → glucose + galactose
This continuous process is used by dairy companies worldwide. The immobilised lactase remains active for weeks or months, producing a constant stream of lactose-free product without contaminating the milk with enzyme protein, which also improves the taste and shelf life.
全球乳品企业已采用此连续工艺。固定化乳糖酶可保持活性数周至数月，源源不断地产出无乳糖产品，且不会用酶蛋白污染牛奶，同时改善了口感与保质期。
11. Immobilised Enzymes in Biosensors | 生物传感器中的固定化酶
Immobilised enzymes are at the heart of modern biosensors. For instance, glucose oxidase is immobilised onto a platinum electrode via entrapment in a polyacrylamide gel or by covalent attachment. When glucose in a blood sample diffuses into the enzyme layer, it is oxidised:
固定化酶是现代生物传感器的核心。例如，葡萄糖氧化酶通过聚丙烯酰胺凝胶包埋或共价连接的方式固定在铂电极上。当血样中的葡萄糖扩散进入酶层，即被氧化：
Glucose + O₂ + H₂O → gluconic acid + H₂O₂
The resulting hydrogen peroxide is detected amperometrically, generating an electrical signal proportional to glucose concentration. The immobilised enzyme retains stability and activity during repeated measurements, making continuous glucose monitoring feasible for diabetic patients.
生成的过氧化氢通过安培法检测，产生的电信号与葡萄糖浓度成正比。固定化酶在重复测量中保持稳定性和活性，使糖尿病患者的连续血糖监测成为可能。
12. Effect on Enzyme Kinetics and Efficiency | 对酶动力学与效率的影响
When an enzyme is immobilised, its kinetic parameters often change. The Michaelis constant (Kₘ) frequently increases due to electrostatic repulsion between charged support and substrate, or due to diffusion limitations, requiring higher substrate concentrations to reach ½ Vₘₐₓ. The maximum velocity (Vₘₐₓ) may decrease if a significant fraction of enzyme molecules are inactivated during immobilisation or are poorly oriented.
酶被固定化后，其动力学参数常发生变化。由于带电载体与底物的静电排斥或扩散限制，米氏常数Kₘ通常增大，需要更高的底物浓度才能达到½ Vₘₐₓ。若固定化过程中大部分酶分子失活或取向不佳，最大反应速度Vₘₐₓ可能降低。
However, in some cases, the microenvironment created by the carrier can increase the local substrate concentration, effectively lowering the Kₘ. Enzymes sensitive to product inhibition may also benefit from the rapid removal of product in a flow reactor, appearing more efficient than free enzymes under operational conditions.
但某些情况下，载体创造的微环境可增加局部底物浓度，从而有效降低Kₘ。对产物抑制敏感的酶还可能因流动反应器中产物被迅速移除而看似比游离酶更高效。
Understanding these kinetic shifts is vital for designing industrial reactors. Parameters such as pH optimum and temperature optimum may also shift, and the apparent activation energy can differ from the intrinsic value due to mass transfer effects.
理解这些动力学变化对于设计工业反应器至关重要。最适pH和最适温度等参数也可能漂移，表观活化能会因传质效应而与固有值不同。
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