Past exam of the biochemistry course of the University of Cambridge / 2025 / Paper 4 / Section III

Engineering EPA or DHA production requires reconstruction of a coordinated lipid pathway, not simply adding an omega-3 desaturase. Conventional higher-plant oils already contain shorter omega-3 fatty acids such as alpha-linolenic acid; the missing trait is substantial synthesis and storage of particular long-chain products such as eicosapentaenoic acid and docosahexaenoic acid. The engineering problem includes enzyme compatibility, substrate trafficking, storage and fitness.

A representative aerobic route converts an 18-carbon precursor into a 20-carbon product through desaturation, elongation and a further desaturation. In shorthand, with $n$ denoting the omega family,A suitable elongation and $\Delta 4$ desaturation can extend this route to $22:6n-3$. Other organisms use different routes, so this sequence is a design choice rather than a universal algal pathway. Because elongation adds two carbons at the carboxyl end, the omega family can remain unchanged even though the carboxyl-based double-bond positions change. Gene selection must specify the actual substrate and product, not just an enzyme name.

The first obstacle is metabolic flux through a multi-enzyme pathway. An introduced fatty acid desaturase needs the correct substrate, electron supply, oxygen availability, membrane environment and intracellular location. A fatty acid elongase must work with the host's elongation machinery and access an appropriate acyl substrate. Early transgenic plant experiments demonstrated production of long-chain polyunsaturated products, establishing biochemical feasibility.^[ref] Accumulation of a product proves that a route can operate, but neither high transgene expression nor appearance of an intermediate establishes an adequate final yield.

A major problem is the substrate dichotomy between lipid-linked desaturation and acyl-CoA-dependent elongation. An introduced desaturase may act on a chain in phosphatidylcholine, whereas the next elongase requires that chain in the acyl-CoA pool. Poor exchange creates intermediate accumulation despite active enzymes. Experiments in transgenic oilseeds identified constraints involving desaturases, elongases and acyl transfer between these pools.^[ref] Possible engineering responses include choosing enzymes with compatible substrate-carrier preferences, improving acyl editing, or using an alternative order of reactions. These are different hypotheses and should be tested by measuring the actual molecular pools rather than only total fatty-acid percentages.

The pathway must also reach a useful destination. An unusual long-chain fatty acid made in a membrane lipid may fail to enter seed triacylglycerol. Host acyltransferases can discriminate among chain lengths and double-bond patterns, so storage can become limiting after biosynthesis is improved. Conversely, forcing too much unusual lipid into an essential membrane can disturb membrane protein function and seed development. Seed-specific expression and efficient transfer into lipid droplets can limit effects on vegetative tissues, but successful storage still requires coordination with the normal timing of seed oil accumulation.

Camelina sativa provided a proof of substantial engineered storage: different transgenic designs accumulated high levels of EPA or a combination of EPA and DHA in seed oils.^[ref] This shows that evolutionary absence is not a fundamental chemical prohibition. It does not establish that every variety, environment or production scale will give the same yield. Measure oil mass per seed, seed number, germination and field performance as well as fatty-acid composition. An illustrative crop yielding $1\mathrm{t}$ of oil per hectare at a target fraction of $0.20$ supplies $0.20\mathrm{t}$ of the target; raising that fraction to $0.25$ while halving oil yield gives only $0.125\mathrm{t}$. Fractional enrichment and agronomic productivity are different objectives.

Multi-gene expression creates additional constraints. Relative enzyme amounts matter, transgenes can segregate or become silenced, and excessive activity upstream can deplete precursors needed elsewhere. Stable inheritance and developmental timing must be verified over generations. Analyze independent lines and segregating controls, quantify enzyme activity where possible, and distinguish the intended product from structurally related intermediates. stable isotope labeling can reveal whether carbon is reaching a blocked intermediate or being diverted to another lipid class. Whole-seed measurements can conceal tissue-specific storage and transport limitations.

Evolutionary constraints have several levels. Closely related enzymes can differ substantially in substrate preference, while distantly related algal and plant proteins may require different electron partners or targeting signals. Sequence homology therefore guides selection but does not prove compatibility. Pathways have evolved together with their membrane composition, growth temperature and ecological niche; transferring one set of catalysts leaves the receiving plant's regulatory network and lipid-handling enzymes largely intact. Appropriate localization and balanced carrier pools may be as important as catalytic sequence changes.

Fitness costs are also plausible. Long-chain polyunsaturation can change membrane packing and increase susceptibility to lipid peroxidation, while synthesis consumes carbon and reducing power. A pathway beneficial in a marine microorganism may provide little reproductive advantage in a terrestrial seed plant, or may impose costs under heat, drought or oxidative stress. These are testable evolutionary hypotheses, not an established single explanation for the absence of EPA and DHA from conventional crop oils. Compare engineered and control plants across environments, measure oxidation products and fertility, and test whether storage-specific targeting reduces the cost.

The successful trait must combine a compatible biosynthetic route, exchange between lipid carriers, selective storage and acceptable reproductive performance. Evolution helps identify likely incompatibilities and tradeoffs, but the existence of productive engineered oilseeds shows why absence in an ancestral crop should not be mistaken for impossibility.

cellulose is hard to saccharify chiefly because enzymes must attack a densely organized, poorly accessible solid substrate. Its resistance is useful when a plant needs a durable wall or an industry needs a persistent fibre; conversion to soluble sugars requires overcoming that organization without losing too much carbohydrate or damaging the catalysts.

cellulose is a polymer of glucose joined by $\beta$-1,4 glycosidic bonds. Extended chains associate into cellulose microfibrils, with close packing, hydrogen bonds and other intermolecular interactions stabilizing ordered regions. Many chains are buried inside the fibril, so a soluble enzyme cannot access every bond. A long chain also provides relatively few ends per unit mass for an enzyme that attacks processively from an end. The glycosidic bond is chemically hydrolyzable; physical access and disruption of chain association create much of the difficulty under mild conditions.

In a plant wall, the problem is compounded by lignin, hemicellulose and the larger wall architecture. lignin can obstruct access and adsorb enzymes nonproductively, while hemicelluloses can cover cellulose surfaces. Fibre drying and aggregation can reduce accessible area further. Crystallinity alone is therefore an incomplete predictor: pore size, surface accessibility, degree of polymerization, chemical substitutions and the distribution of noncellulosic material can all affect a measured rate. Compare substrates with characterized composition rather than attributing every rate difference to one crystallinity index.

Microbes use complementary enzyme activities. An endoglucanase cleaves accessible internal bonds, creating new ends. A cellobiohydrolase can bind an exposed chain and release short products during successive cleavage steps. A beta-glucosidase converts cellobiose and related soluble products into glucose, reducing product accumulation that can inhibit other enzymes. carbohydrate-binding modules enrich enzymes at an insoluble surface, but adsorption must be productive: binding very tightly to an inaccessible site can lower effective turnover. Cooperation can therefore produce more sugar than the same amounts of isolated enzymes acting independently.

Some bacteria organize multiple activities into a cellulosome, using a scaffold and specific protein–protein recognition to retain enzymes near one another and the substrate. Other microorganisms secrete mixtures of free enzymes. The expected advantage of a scaffold is local cooperation and reduced loss, but it can also constrain diffusion or enzyme access. Test synergy with matched total enzyme amounts and controlled component ratios; compare an intact assembly with binding-defective or scaffold-free components. Activity on a soluble cellulose derivative does not establish equal performance on a crystalline microfibril.

Oxidative enzymes provide another route into the solid. Fungal copper-dependent polysaccharide monooxygenases were shown to cleave cellulose oxidatively and produce oxidized products, giving a chemical mechanism that complements hydrolysis.^[ref] lytic polysaccharide monooxygenases can disrupt otherwise resistant surface chains and create additional entry points for hydrolases. Their activity cannot be inferred simply from a fall in turbidity: identify oxidized oligosaccharides and distinguish them from hydrolytic products and nonspecific oxidative damage.

The oxidant and reducing conditions matter. Experiments using enzyme assays, isotope tracing and product analysis showed efficient peroxide-dependent activity in LPMO systems, highlighting hydrogen peroxide as a relevant cosubstrate rather than assuming that dissolved oxygen alone describes the working chemistry.^[ref] Too much peroxide or reducing activity can inactivate the catalyst or create damaging side reactions. Controls should include copper loading, no-enzyme and inactive-enzyme samples, controlled peroxide supply and assays for residual activity. An apparent enhancement from a crude extract can reflect several coupled redox enzymes rather than one isolated activity.

Microbial ecology adds further cooperation. enzymes exposing cellulose may act alongside hemicellulose-degrading activities, and metabolism of released sugars can prevent local product inhibition. In a mixed community, sugar release and sugar consumption occur simultaneously; disappearance of cellulose is not the same as recovery of fermentable monomers. Separate biomass growth, solubilization and product identity when assigning the contribution of a particular organism. An isolate with weak performance alone may still supply a useful complementary activity in a consortium.

Industrial saccharification often begins with biomass pretreatment to improve accessibility: mechanical size reduction, hydrothermal treatment, acid or alkaline processing, or other fractionation methods. The appropriate method depends on feedstock and intended coproducts. Pretreatment can expose cellulose and remove barriers, but can also degrade sugars, generate fermentation inhibitors or produce enzyme-binding surfaces. A primary pretreatment study connected changes in plant cell-wall structure and accessibility to enzyme digestibility, illustrating why the physical state of the substrate must be characterized.^[ref] This is not a universal ranking of pretreatments for every biomass.

A process then combines complementary cellulases, beta-glucosidase and, where useful, oxidative activities under compatible pH and temperature. High-solids operation increases potential product concentration but creates mixing, viscosity and mass-transfer constraints. enzymes that work rapidly on dilute model substrates may behave differently in that environment. Product removal, enzyme recycling or simultaneous fermentation can help, but each introduces new constraints on compatibility and stability. Evaluate complete material and energy balances, including pretreatment, enzyme production and recovery, rather than only initial hydrolysis rate.

There is a simple stoichiometric check. A dry anhydroglucose residue in cellulose has molar mass approximately $162\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$; complete hydrolysis incorporates water to give glucose at approximately $180\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$. Thus an ideal glucose yield from pure dry cellulose isA mass yield slightly above one is therefore chemically possible; actual biomass composition, incomplete conversion, soluble oligomers and microbial consumption must be accounted for. Microbes overcome recalcitrance by combining surface access, complementary cleavage chemistries and local cooperation. Industry exploits both sides of the same property: durable cellulose for materials, and controlled deconstruction when soluble sugars or modified fibres are the desired products.

Photosynthetic exoelectrogenesis is measurable electron export, but the complete physiological export pathway is not established as one universal mechanism. Experiments must distinguish the intracellular source of reducing power, passage across the cell envelope and the final electrochemical reaction at an external acceptor. A light-dependent current alone does not identify all three steps.

In oxygenic photosynthesis, photosystem II oxidizes water and supplies electrons to a transport chain. plastoquinone, a cytochrome complex, photosystem I and downstream carriers connect charge separation to production of NADPH. In cyanobacteria, respiratory and photosynthetic electron-transfer pathways can share components; reducing power can therefore reach the relevant pools from both light reactions and metabolism of stored carbon. In a eukaryotic microalga, chloroplast and cell-surface membranes introduce additional compartment barriers. Electron export by one organism should not be assigned automatically to the architecture of another.

A biophotovoltaic device couples such cells to an electrode. At an anode, oxidation of an exported reductant or a cell-associated redox component transfers electrons into the external circuit. At least three broad mechanisms are possible: direct transfer through cell-associated components at the interface; soluble endogenous redox mediators; or added artificial mediators that access internal or surface-accessible carriers. A fourth practical possibility is release from damaged cells. These routes may coexist, and their relative contributions depend on electrode material, potential, growth state and preparation.

Evidence for a soluble route includes current from separated cell medium, loss of activity after removal of a candidate reductant and restoration by an appropriate add-back. In a cyanobacterial bioelectrochemical system, extracellular NADPH accumulated under illumination and device operation, its removal suppressed current, and addition of NADP+ enhanced current generation.^[ref] Those observations support a mediated route in the tested system. They do not prove that freely released NADPH is the principal mediator in every cyanobacterium or electrode arrangement. The process releasing it, its physiological relevance and possible leakage must be tested separately.

Cell-envelope experiments help locate a barrier. Comparing cyanobacteria with systematically removed outer layers showed that the periplasm and bacterial outer membrane strongly influence the photocurrent response.^[ref] Greater current after permeabilization can indicate that a previously internal reductant or carrier has become accessible; it does not establish that intact cells normally export through the same route. Measure viability, photosynthetic competence and leakage of cytoplasmic markers, normalize to active cell material, and compare the time dependence. A damaged-cell preparation can make a useful bioelectrode while being a poor model of normal physiology.

Artificial mediators provide mechanistic probes but can create a new pathway. A membrane-impermeant acceptor such as ferricyanide reports reduction at an accessible site, whereas a membrane-permeant quinone may intercept electrons deeper in the photosynthetic apparatus. Ultrafast spectroscopic experiments showed that an added quinone can extract charge on very short timescales from photosynthetic reaction-center systems in living cells.^[ref] This changes what is physically accessible for engineered extraction. It does not demonstrate that unmodified microorganisms use that compound or that the complete external current follows the same rapid timescale: intracellular interception and extracellular transport remain distinct steps.

Photosynthetic inhibitors and mutants can constrain the electron source. If inhibition of photosystem II decreases photocurrent, that supports a contribution from water-derived electron flow under those conditions. Persistence of current in darkness can reflect oxidation of stored carbon or previously reduced mediator rather than a failure of the light-reaction model. Conversely, acute inhibition can change the redox balance of the shared chain and secondary metabolism, so a decrease does not uniquely identify the terminal export carrier. Use more than one perturbation, independently confirm target inhibition and monitor oxygen evolution or another photosynthetic readout.

A strong electrochemical design includes an abiotic electrode under identical illumination, cell-free medium, killed-cell controls and fresh versus conditioned medium. Control electrode polarization and distinguish sustained faradaic current from capacitive transients when switching light on and off. Compare a washed cell pellet with its separated medium, test molecular-size fractions, and identify active molecules analytically. An added compound that increases current might be an electron shuttle, a nutrient, a membrane perturbant or an antioxidant; reversible redox behavior and mass balance are needed to classify it as a mediator.

Charge accounting is especially useful. For integrated faradaic charge $Q$,where $F$ is the Faraday constant. A two-electron oxidation of one reduced NADP molecule can supply at most $2F$ coulombs per mole for a single pass. Thus $1\mathrm{nmol}$ can supply about $0.193\mathrm{mC}$ before regeneration. If a measured charge greatly exceeds that amount, the reductant must be replenished, recycled, underestimated or supplemented by another source. This is an illustrative bookkeeping test, not a measured mediator concentration in any cited device.

It is also important to distinguish extraction efficiency from cell performance. A larger photocurrent after physiological stress may reflect diversion from carbon fixation, altered permeability or increased damage. Measure biomass production, carbon balance and sustained current over time. A useful engineered route must remain compatible with sufficient energy conservation and repair, while an ecological explanation requires evidence that extracellular acceptors are encountered and beneficial outside an electrode experiment.

Photosynthetic and respiratory reducing pools can supply current through condition-dependent export routes. The envelope gates accessibility, and mediators can connect internal redox chemistry to an electrode. Native carriers and export steps still require identification in each system.