Past exam of the biochemistry and Molecular Biology course of the University of Cambridge / 2025 / Paper 1 / Section C

The liver uses glucose sensing chiefly to regulate fuel storage and production, while pancreatic beta cells convert glucose availability into an endocrine signal. The comparison therefore includes transport, metabolism and allosteric control; it should not identify one transporter as the complete sensor in either tissue.

glucokinase phosphorylates glucose to glucose-6-phosphate in both hepatocytes and pancreatic beta cells. Unlike many other hexokinases, it is not strongly inhibited by its product and operates over a higher glucose range. Its monomeric enzyme nevertheless shows a sigmoidal steady-state response to glucose, arising from its kinetic conformational behaviour rather than cooperative binding between several subunits. A glucose half-saturation concentration in the physiological millimolar range makes its flux responsive to changing blood glucose, whereas a low-affinity constant cannot alone establish the in vivo sensing step without considering transport and downstream metabolism.

In beta cells, glucose metabolism through glycolysis and mitochondrial oxidation increases the cellular ATP-to-ADP state. ATP-sensitive potassium channels then close: ATP inhibits the Kir6.2 pore, while Mg-nucleotide effects mediated through SUR1 additionally regulate opening. Reduced potassium conductance produces depolarization, opening voltage-gated calcium channels. Calcium entry triggers exocytosis of insulin-containing secretory granules. Thus the channel senses nucleotides downstream of glucose metabolism; it does not bind extracellular glucose as its initiating ligand. Metabolic amplification and other signals also regulate the amount secreted, so this triggering pathway is necessary context rather than a complete description of every phase of insulin secretion.

glucokinase helps set the glucose threshold for that response. An activating glucokinase mutation associated with familial hyperinsulinism provides evidence that shifting the enzyme's glucose responsiveness can shift whole-body insulin control.^[ref] To separate this interpretation from altered expression, the variant's kinetics should be compared with the normal enzyme and its effect on secretion assessed at matched expression. Inhibiting the potassium channel downstream can bypass part of the sensing chain, whereas preventing calcium entry tests the final electrical-to-secretory coupling.

Glucose transport must be discussed with species in mind. GLUT2 is the prominent transporter in the classical rodent beta-cell model, but human beta cells use predominantly GLUT1 rather than sharing the same abundance of GLUT2. Direct comparison of human and rat preparations found much lower GLUT2 abundance in human cells while glucokinase retained a comparable influence on glucose utilization.^[ref] A transport step with capacity greater than phosphorylation can allow intracellular glucose availability to track the external supply, leaving glucokinase as a major metabolic control point. These observations argue against presenting rodent GLUT2 expression as a universal human sensing mechanism.

In the liver, GLUT2 supports rapid bidirectional facilitated transport. Hepatic glucokinase traps abundant glucose as glucose-6-phosphate, supplying glycogenesis and other metabolic pathways after feeding. glucokinase regulatory protein adds tissue-specific control: at low glucose it binds and sequesters glucokinase in the nucleus; increased glucose promotes release and cytoplasmic use. fructose-6-phosphate favours the inhibitory interaction, while fructose-1-phosphate antagonizes it, coordinating glucose trapping with nutrient context. Hormonal regulation of glucokinase expression adds a slower level of control. Unlike the beta-cell response, the immediate output is a change in hepatic metabolic flux, not release of insulin.

The liver also has an important direct glucose sensor in glycogen phosphorylase. Binding of glucose to liver glycogen phosphorylase a favours an inactive conformation and its dephosphorylation by protein phosphatase 1. Loss of phosphorylase-a activity switches off glycogenolysis and relieves its inhibitory influence on glycogen-synthase phosphatase activity, helping activate glycogen synthase. Glucose administration experiments observed phosphorylase inactivation preceding synthase activation, supporting an ordered switch rather than simultaneous independent responses.^[ref] This direct allosteric route is distinct from glucokinase-mediated metabolic sensing and complements insulin signalling. Together they suit an organ that must stop exporting and begin storing glucose when supply rises, while beta-cell sensing coordinates that transition across multiple tissues.

In vivo muscle NMR reveals energy-buffer use, phosphate chemistry and recovery kinetics without repeatedly removing tissue. It measures visible metabolites, not metabolic flux automatically. A particularly informative experiment is localized ${}^{31}\mathrm{P}$ nuclear magnetic resonance, with a limb positioned in a magnet and a radiofrequency coil over the exercising muscle. Repeated spectra can be collected at rest, during a defined workload and throughout recovery. Localization and synchronized contraction reduce motion artefacts and contamination from inactive tissue.

The resting ${}^{31}\mathrm{P}$ spectrum resolves phosphocreatine, inorganic phosphate and the phosphate resonances of ATP, together with some other phosphorus-containing metabolites. Peak areas, after correcting for excitation and relaxation effects, report relative quantities of NMR-visible pools. An external reference or another justified calibration is needed for absolute concentrations. Large phosphocreatine stores relative to ATP illustrate a buffering system, but a resting spectrum alone does not reveal how fast ATP is synthesized or consumed.

During exercise, creatine kinase buffers ATP throughConsequently phosphocreatine often falls and inorganic phosphate rises while ATP remains comparatively stable during moderate exercise. Stable ATP does not mean little ATP turnover: it can mean rapid replenishment matching rapid consumption. Serial spectra relate changes in the phosphocreatine-to-phosphate ratio to workload and fatigue. Gated human-muscle experiments directly demonstrated such workload-dependent energetic responses.^[ref]

The chemical shift of inorganic phosphate relative to phosphocreatine depends on its protonation state and therefore permits an estimate of intracellular pH. With appropriate calibrated acid and base endpoint shifts, a commonly used relation has the formThe observed shift must lie between the calibration endpoints, and the relevant calibration depends on experimental conditions. Exercise-related acidification and its recovery can therefore be followed alongside the energy-buffer changes. A resolved pair of phosphate signals may indicate metabolically distinct compartments; fitting one average shift to a mixed signal can conceal heterogeneity. A pH fall is not simply a direct lactate measurement, because proton production, consumption, buffering and export all contribute.

If creatine kinase is close to equilibrium, measured ATP and phosphocreatine, together with estimates of total creatine, pH and free magnesium, can constrain free cytosolic ADP. This is an inference from an equilibrium model rather than a direct ADP peak measurement. The corresponding ATP hydrolysis free energy depends on metabolite activities through $\Delta G=\Delta G^{\circ \prime }+RT\ln ([\mathrm{ADP}][P_i]/[\mathrm{ATP}])$ at the specified biochemical conditions. Concentration ratios and a consistent standard-state convention are essential when using this expression quantitatively.

After exercise, oxidative ATP production replenishes phosphocreatine. Recovery is often summarized by $\mathrm{PCr}(t)={\mathrm{PCr}}_{\mathrm{end}}+\Delta \mathrm{PCr}(1-e^{-t/\tau })$, making the initial recovery rate $\Delta \mathrm{PCr}/\tau$. With adequate oxygen delivery and suitable pH conditions, recovery kinetics provide an index of oxidative capacity. Slow recovery may instead reflect impaired perfusion, severe acidification or continuing ATP demand, so it does not uniquely identify a mitochondrial enzyme defect. Work-matched comparisons, adequate rest, reproducible exercise and independent perfusion or oxygenation measurements strengthen interpretation.

Related experiments extend the information available. ${}^1\mathrm{H}$ spectroscopy can detect additional metabolites or lipid pools, while ${}^{13}\mathrm{C}$ experiments can follow labelled substrate incorporation or glycogen under suitable sensitivity conditions. Magnetization-transfer experiments can probe exchange, but exchange rates need not equal net ATP-production flux. The strength of the method is the time-resolved connection between muscle function and intracellular chemistry, with explicit limits from spatial averaging, sensitivity, calibration and the biochemical model used to interpret each signal.

Photoprotection must regulate both the total excitation arriving at reaction centres and its distribution between the two photosystems. When light absorption exceeds the capacity for productive electron transfer and carbon metabolism, excited chlorophyll and reduced electron carriers persist. Triplet chlorophyll can generate singlet oxygen, while inappropriate electron transfer to oxygen can generate other reactive oxygen species. These can damage pigments, lipids and proteins, including photosystem reaction centres. Excess light and unbalanced excitation are related but distinct problems.

The fastest major response to excess light in many plants is energy-dependent non-photochemical quenching. Electron transport builds a proton gradient across the thylakoid membrane, acidifying the lumen. Protonation-dependent changes involving PsbS and antenna proteins favour harmless dissipation of excitation as heat. The xanthophyll cycle contributes: under lumen acidification, violaxanthin de-epoxidase converts violaxanthin through antheraxanthin towards zeaxanthin, altering antenna quenching properties. In dimmer conditions these changes can reverse. Quenching therefore reduces the excitation reaching photosystem II before every captured photon becomes a potentially damaging charge-separation event.

An Arabidopsis mutant lacking normal PsbS-dependent behaviour showed a major loss of rapidly inducible quenching despite retaining light-harvesting capability; this separates a protective antenna-control function from the ability to absorb light.^[ref] A convincing interpretation compares absorbed light, pigment content, baseline photosynthetic competence and restoration by complementation. Reduced chlorophyll fluorescence alone is not sufficient evidence of protective heat dissipation: photochemical use of excitation and irreversible damage can also change fluorescence. Saturating-pulse measurements help separate photochemical and non-photochemical components.

photosynthetic control at the cytochrome b6f complex provides another response to lumen acidification, slowing electron delivery towards photosystem I and limiting excessive reduction of its acceptor side. cyclic electron flow around photosystem I returns electrons to the intersystem chain, supporting proton-gradient formation and ATP production without net NADPH production. This can adjust the ATP-to-reductant supply and reinforce quenching; it is not simply a way of making more NADPH. Mutant analysis linking PGR5-dependent electron-flow regulation to photoprotection supports the connection between electron allocation and protection.^[ref] Alternative electron sinks, including oxygen-reducing pathways, can relieve pressure under some conditions, but require antioxidant systems because oxygen chemistry can itself be damaging. photorespiration also consumes energy and reducing capacity in appropriate conditions. These routes differ across photosynthetic lineages and are not interchangeable universal safety valves.

Unequal excitation requires redistribution. Preferential excitation of photosystem II reduces the plastoquinone pool, signalling an imbalance in electron supply to photosystem I. In plant photosynthetic state transitions, activation of the STN7 kinase promotes phosphorylation of a mobile fraction of LHCII, decreasing excitation delivery to photosystem II and increasing its functional association with photosystem I. When the plastoquinone pool becomes more oxidized, dephosphorylation favours the reverse allocation. This balances excitation rather than dissipating all of it. A kinase-deficient Arabidopsis mutant defective in state transitions demonstrates the requirement for STN7 in this response.^[ref]

State transitions can be tested by switching light preferentially exciting the two photosystems, measuring reversible fluorescence changes and antenna phosphorylation, and comparing wild type, mutant and complemented material. The same experiment should assess electron-transfer capacity so that loss of a state-transition signal is not confused with nonspecific photosynthetic failure. Different algae can use different antenna organizations, and cyanobacterial redistribution need not follow the plant LHCII mechanism. If photosystem I is preferentially excited, the reverse redox signal favours a greater share of antenna excitation reaching photosystem II.

Other short-term protections include carotenoid quenching of triplet chlorophyll or singlet oxygen, antioxidant removal of reactive products, and chloroplast avoidance movements that reduce absorbed light. Replacement of damaged D1 protein repairs photosystem II on a slower short-term timescale; it compensates for damage rather than preventing the original photochemical injury. Rapid quenching, reversible antenna redistribution, electron-flow control and repair therefore address different failure points. A protected organism keeps excitation, electron flow and metabolic demand sufficiently coordinated to avoid prolonged damaging states, rather than merely maximizing light capture.