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Spec Mapping — OCR H420 Module 6.2.1 — Cloning and biotechnology, content statements covering the immobilisation of enzymes by adsorption, covalent bonding, entrapment in gels or fibres, and membrane separation; named industrial examples including high-fructose corn syrup production, semi-synthetic penicillin production, lactose-free milk and pure L-amino acid manufacture; and the advantages and disadvantages of immobilised enzymes compared with free enzymes in solution (refer to the official OCR H420 specification document for exact wording). This lesson closes Module 6.2.1 and links forward to Module 6.3 (ecosystems and populations) only synoptically — though it has direct practical relevance to PAG-7-style biotechnology investigations.
Immobilised enzymes are enzymes that have been physically or chemically attached to an inert support so that they are held in place during a reaction. This allows reuse of expensive enzymes, simplifies product purification and improves enzyme stability. OCR A-Level Biology A specification 6.2.1 requires you to explain the methods of enzyme immobilisation, name examples of industrial use and evaluate the advantages and disadvantages compared with free enzymes.
The concept of using a heterogeneous biological catalyst rather than a homogeneous one dates from the early twentieth century, but the first commercially successful immobilised-enzyme process was Tanabe Seiyaku's aminoacylase column for L-amino acid resolution, launched in Osaka in 1969. The principles are essentially the same as those of any heterogeneous chemical catalyst: the catalyst is fixed in place, the substrate flows through, and the product emerges separated from the catalyst. Industrial-scale enzyme immobilisation has grown into a multi-billion-pound sector — high-fructose corn syrup alone, produced via immobilised glucose isomerase, accounts for over 10 million tonnes per year globally and underpins much of the soft-drinks and processed-foods industry. The OCR specification rewards candidates who can map the four standard immobilisation methods onto specific industrial examples and explain why each method suits each case.
Key Definitions:
- Immobilised enzyme — an enzyme attached to or entrapped within an insoluble support, allowing it to be reused.
- Adsorption — binding to a surface by weak forces (hydrogen bonds, hydrophobic interactions, van der Waals).
- Covalent bonding — formation of strong chemical bonds between enzyme and support.
- Entrapment — trapping the enzyme inside a gel or fibre without direct chemical bonding.
- Membrane separation — keeping enzyme on one side of a selective membrane through which substrate and product can pass.
Enzymes are powerful biological catalysts used in industry to make products ranging from high-fructose corn syrup to penicillin antibiotics. They are typically far more efficient than non-biological catalysts (rate enhancements of 106 to 1017 relative to uncatalysed reactions are routine), they operate under mild conditions (aqueous, near-neutral pH, ambient temperature), and they are stereospecific — properties that recommend them for sustainable industrial chemistry under the "green chemistry" framework. Free enzymes, however, have problems:
Immobilising the enzyme solves these problems: the enzyme stays on its support while substrate flows through, product leaves the reactor enzyme-free, and the same batch of enzyme can be used for weeks or months.
The enzyme is allowed to bind to the surface of an inert support such as clay, porous carbon, glass beads or a resin. Binding forces are weak (hydrogen bonds, hydrophobic interactions, van der Waals).
Advantages: simple, cheap, preserves enzyme activity well because the active site is usually unaffected. Disadvantages: enzyme can leak off the support ("leaching") with changes in pH, temperature or ionic strength.
The enzyme is chemically linked to the support (e.g. cellulose, sepharose, polyacrylamide) by strong covalent bonds. Cross-linking agents such as glutaraldehyde are often used.
Advantages: strong attachment, no leaching. Disadvantages: expensive; the chemical modification may partially denature the enzyme or block the active site.
The enzyme is trapped inside a gel (e.g. calcium alginate beads, silica gel) or hollow fibres. The gel is porous enough for substrate and product to diffuse in and out, but the enzyme molecules are too large to escape.
Advantages: the enzyme is not chemically modified, so its activity is preserved; simple to prepare. Disadvantages: diffusion of substrate and product through the gel is slow, limiting reaction rate; small enzymes can sometimes leak.
The enzyme is kept in solution on one side of a semi-permeable membrane. Substrate diffuses in and product diffuses out; the enzyme cannot cross the membrane.
Advantages: enzyme remains in solution, so activity is unaffected. Disadvantages: membranes can clog; equipment is more complex.
| Method | Attachment | Activity preserved | Leaching risk | Cost |
|---|---|---|---|---|
| Adsorption | Weak forces | High | High | Low |
| Covalent | Chemical bond | Medium | Very low | High |
| Entrapment | Gel matrix | High | Low | Low |
| Membrane | Physical barrier | Very high | None | High |
Glucose isomerase (xylose isomerase, EC 5.3.1.5) converts glucose to fructose. Because fructose is approximately 1.7 times sweeter than glucose on a weight basis, fructose syrups allow less sugar to be used in food and drink, cutting costs and calories per unit sweetness perceived. The enzyme is immobilised (usually by adsorption onto a polystyrene-divinylbenzene anion-exchange resin or by entrapment in DEAE-cellulose) in packed-bed reactors of typical industrial volume 1-10 m³, through which 60% w/w glucose syrup flows at ~60 °C, pH 7.5-8.0. The resulting high-fructose corn syrup (HFCS) — typically standardised to 42% fructose (HFCS-42) for general food use or 55% fructose (HFCS-55) for soft-drink sweetening to match sucrose sweetness — is used in soft drinks, breakfast cereals, baked goods and processed foods worldwide. Global production exceeds 10 million tonnes per year. Immobilisation is essential because free enzyme would contaminate the syrup, cost too much to replace, and would be lost in downstream processing. The catalytic half-life of immobilised glucose isomerase at industrial conditions is approximately 12-18 months, during which time substantial tonnage of HFCS is produced from each kilogram of immobilised enzyme — the economic case is overwhelming.
Penicillin acylase (penicillin amidase, EC 3.5.1.11) removes the side chain from penicillin G, producing 6-aminopenicillanic acid (6-APA), the universal building block from which all semi-synthetic β-lactam antibiotics are made. Chemists then attach different side chains to make semi-synthetic penicillins such as amoxicillin (broader Gram-negative spectrum), ampicillin (oral bioavailability), and oxacillin / methicillin (resistance to β-lactamase) — crucial in the fight against antibiotic-resistant bacteria. The enzyme is immobilised by covalent bonding on glutaraldehyde-activated carriers or by entrapment in polyacrylamide beads, in large packed-bed reactors of typical capacity 1-50 m³, and used continuously to produce tens of thousands of tonnes of 6-APA each year worldwide.
Lactase (β-galactosidase, EC 3.2.1.23) hydrolyses lactose (milk sugar) to glucose and galactose. About two-thirds of the world's adults are lactose intolerant because they lose lactase expression after weaning (the ancestral mammalian state; lactase persistence in adulthood is a derived trait that evolved independently in pastoralist populations in Europe, East Africa and parts of the Middle East over the last 8000 years, driven by strong positive selection on the lactase-persistence allele LCT). In lactose-intolerant individuals, undigested lactose reaches the colon and is fermented by gut microflora to short-chain fatty acids and gases, causing bloating, abdominal pain and diarrhoea.
Immobilised lactase is used commercially to produce lactose-free milk. Milk flows through a column containing lactase immobilised on beads (entrapment in alginate) or fibres (cellulose acetate); the lactose is hydrolysed essentially completely as the milk passes through, and the resulting glucose-and-galactose-containing milk can be consumed by lactose-intolerant individuals without symptoms. Brands include Lactaid (USA), Arla Lactofree (UK / Europe) and a₂ Milk-Free (Australasia).
A bonus effect: the product is sweeter than ordinary milk because glucose and galactose taste sweeter than lactose, allowing less added sugar in derived products (e.g. lactose-free yoghurt, ice cream).
A complementary approach uses lactase tablets that lactose-intolerant individuals take with meals — this is essentially the same enzyme used industrially, but consumed orally to operate transiently in the gut rather than upstream in a column reactor. The choice between in-process and at-table enzyme administration is a useful synoptic example of how the same enzymology can be deployed in two different industrial / clinical formats.
Aminoacylase from Aspergillus oryzae removes the acyl group from N-acyl-L-amino acids but not from the D-isomer, allowing chemists to separate the two isomers. This is important because only L-amino acids are biologically active; pure L-forms are needed for pharmaceuticals, nutritional supplements (intravenous nutrition formulations) and food (flavour enhancers such as monosodium L-glutamate and L-aspartate-derived aspartame).
The Tanabe Seiyaku 1969 column at Osaka is historically important: it was the first commercial immobilised-enzyme process anywhere in the world, and it remains the textbook case study of how enzymological enantio-selectivity can be turned into a continuous-flow industrial separation. Chemists racemise the unreacted D-isomer, recycle it back through the column, and achieve effectively quantitative conversion of starting racemic mixture to enantiopure L-amino acid. The enzyme is immobilised on DEAE-Sepharose resin via ionic adsorption, simple to set up and to replenish.
Glucoamylase (amyloglucosidase) from Aspergillus niger hydrolyses starch dextrins to glucose. Together with α-amylase, it is used to produce glucose syrups from corn and wheat starch. Immobilised glucoamylase simplifies production and reduces enzyme costs.
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