(a)(i) What are allosteric enzymes ? Explain in detail on the allosteric enzyme modulation. 8 (a)(ii) Describe the process of…

(a)(i) Allosteric enzymes and their modulation

Definition: allosteric enzymes are regulatory enzymes, usually made of several subunits, whose activity is changed by modulators (effectors). These bind non-covalently at an allosteric (regulatory) site separate from the active (catalytic) site. Binding changes the enzyme's shape and so its affinity for substrate or its catalytic rate. They usually catalyse the first committed step of a pathway, so the pathway can be switched on or off quickly (Monod, Changeux and Jacob, 1963).

Features

• Kinetics: they do not follow Michaelis–Menten kinetics. A plot of rate against substrate concentration is sigmoidal, not hyperbolic, because substrate binding is cooperative: binding at one subunit raises the affinity of the others. The substrate concentration giving half-maximal rate is called K₀.₅, not Kₘ. The sigmoid shape makes the enzyme very sensitive to small changes in substrate concentration near K₀.₅, like a switch.

Types of modulation:

• Homotropic: the substrate itself is the modulator.
• Heterotropic: a different molecule is the modulator.
• Positive modulators (activators) raise affinity or rate; negative modulators (inhibitors) lower them.
• In K-type enzymes the modulator changes K₀.₅; in V-type enzymes it changes Vmax.
• Feedback inhibition: the end-product of a pathway inhibits its first enzyme.

Models of modulation

Concerted (symmetry) model (Monod, Wyman and Changeux, 1965): all subunits exist together in either a low-affinity T (tense) state or a high-affinity R (relaxed) state.

• Substrate and activators bind preferentially to R and shift the equilibrium towards R, moving the curve to the left.
• Inhibitors bind T and shift it to the right, making the curve more sigmoid.
• Sequential model (Koshland, Némethy and Filmer, 1966): each subunit changes shape as it binds a ligand, and passes the change to its neighbours. This model also allows negative cooperativity.

Examples

• Aspartate transcarbamoylase of E. coli: the classic example, inhibited by CTP (feedback) and activated by ATP.
• Phosphofructokinase: inhibited by ATP and citrate, activated by AMP (and in plants by Pi).
• ADP-glucose pyrophosphorylase (starch synthesis in chloroplasts): activated by 3-PGA and inhibited by Pi. Starch is thus made when photosynthesis is active.
• PEP carboxylase in C4 and CAM plants: activated by glucose-6-phosphate and inhibited by malate. It is also regulated by phosphorylation.
• Aspartate kinase: feedback-inhibited by lysine and threonine.

Significance: allosteric enzymes provide fast, reversible, sensitive control of metabolic flux, matching supply to demand without making new protein.

(a)(ii) Gluconeogenesis and its significance

Gluconeogenesis is the synthesis of glucose, or in plants of sucrose and other hexoses, from non-carbohydrate precursors: lactate, pyruvate, glycerol, glucogenic amino acids, and (in plants and microbes) acetyl-CoA from fatty acids via the glyoxylate cycle. It uses the seven reversible steps of glycolysis in reverse, but must bypass the three irreversible steps:

1. Pyruvate → phosphoenolpyruvate (PEP).

• In animals: pyruvate carboxylase (in mitochondria; biotin, ATP) makes oxaloacetate, and then PEP carboxykinase (PEPCK) makes PEP (using GTP in animals, ATP in plants).
• In germinating oilseeds, oxaloacetate comes directly from malate made by the glyoxylate cycle.

2. Fructose-1,6-bisphosphate → fructose-6-phosphate: by fructose-1,6-bisphosphatase (FBPase). This is the key control point. It is inhibited by fructose-2,6-bisphosphate and AMP.
3. Glucose-6-phosphate → glucose: by glucose-6-phosphatase, in animal liver and kidney. Plants instead convert hexose phosphates to UDP-glucose and then to sucrose (sucrose-phosphate synthase), the transport sugar.

Energy cost: making one glucose from 2 pyruvate uses 6 high-energy phosphates (4 ATP + 2 GTP in animals) and 2 NADH. It is more costly than the 2 ATP gained by glycolysis, so the two pathways are reciprocally regulated to avoid a futile cycle.

Significance

In plants:

• In germinating oil seeds (castor, sunflower, groundnut), stored triacylglycerol is broken down by β-oxidation in glyoxysomes. The glyoxylate cycle (isocitrate lyase, malate synthase) and gluconeogenesis then turn it into sucrose for the growing seedling, until it becomes photosynthetic. Seeds deficient in these enzymes establish poorly.
• It also recycles carbon from amino acids and lipids in senescing leaves, and is used by some CAM plants (PEPCK type) to regenerate carbohydrate from stored acids.

In animals:

• It keeps blood glucose up during fasting, since brain and red blood cells depend on glucose.
• It clears lactate from exercising muscle through the Cori cycle (muscle lactate → liver glucose → muscle) and alanine through the glucose–alanine cycle.
• It is regulated by glucagon and cortisol (which stimulate it) and insulin (which inhibits it).

(b) Environmental management and control of pollution

Environmental management is the planning, implementation and monitoring of human activities so as to use resources sustainably and keep pollution within the carrying capacity of air, water and land.

Its tools

Policy and law:

• the Environment (Protection) Act 1986;
• the Water Act 1974 and Air Act 1981 (enforced by the Central and State Pollution Control Boards);
• the National Green Tribunal (2010).

Assessment tools:

• Environmental Impact Assessment (EIA Notification 2006) before projects;
• environmental audits and ISO 14001 environmental management systems in industry (plan–do–check–act);
• life-cycle assessment.

Economic instruments:

• the "polluter pays" principle, cess and taxes;
• extended producer responsibility (EPR);
• emission trading (Surat's particulate-matter trading scheme, launched in 2019, was the world's first for particulates).
• Monitoring and public participation: continuous emission monitoring, public hearings, and awareness.

Control measures

1. Air pollution

At source:

• cleaner fuels (CNG for Delhi's public transport after the 2002 Supreme Court order; LPG under Ujjwala);
• BS-VI vehicle norms (from April 2020);
• electric mobility;
• renewable energy (target of 500 GW non-fossil capacity by 2030).

Industrial controls:

• cyclones, electrostatic precipitators, bag filters and wet scrubbers for particles;
• flue-gas desulphurisation for SO₂ in thermal plants;
• catalytic converters and low-NOx burners.

Programmes and agriculture:

• the National Clean Air Programme (2019), which targets a 40% cut in PM10 in non-attainment cities by 2025–26;
• the Graded Response Action Plan in Delhi-NCR;
• alternatives to stubble burning (Happy Seeder, bio-decomposer sprays, use of straw as fuel).
• Green belts: vegetation that absorbs dust and pollutants (for example Azadirachta, Ficus, Polyalthia).

2. Water pollution

• Sewage treatment plants (primary, secondary and tertiary), and common effluent treatment plants for industrial clusters.
• Zero liquid discharge for highly polluting industries.
• Namami Gange (2014) and the National River Conservation Plan.
• Reducing fertiliser and pesticide runoff: soil health cards, integrated pest management, organic and natural farming.
• Protecting wetlands (Wetland Rules 2017).
• Biological methods: constructed wetlands, oxidation ponds, and root-zone treatment with reeds (Phragmites, Typha).

3. Solid and hazardous waste

• The Solid Waste Management Rules 2016: segregation at source, composting, biomethanation, waste-to-energy and scientific landfills.
• The Plastic Waste Management Rules, with EPR and a ban on identified single-use plastics from 1 July 2022.
• The E-waste Rules 2022, the Bio-medical Waste Rules 2016, and hazardous waste rules.
• The 3R/5R hierarchy (reduce, reuse, recycle, recover, and finally dispose); Swachh Bharat Mission.

4. Soil and land

• Remediation of contaminated sites.
• Bioremediation: oil-eating bacteria such as TERI's "Oilzapper" for oil spills; microbes that break down pesticides.
• Phytoremediation: Brassica juncea for lead, Pteris vittata for arsenic, vetiver for landfill leachate.

5. Noise: the Noise Pollution Rules 2000, with zone-wise limits.

6. Biotechnology and cleaner production: biofilters and biosensors for monitoring, biodegradable plastics, bio-pesticides, and enzyme-based industrial processes that use fewer chemicals.

The key idea: effective management combines prevention at source (cleaner technology, efficient resource use) with treatment, strict enforcement, economic incentives and community participation. Local examples include Indore's waste-segregation model, and pollution control boards publishing data for public scrutiny. End-of-pipe treatment alone is not enough.

(c) Mutualism and commensalism

Both are positive interspecific interactions, but they differ in who benefits.

Definition:

• Mutualism (+,+): an interaction in which both species benefit.
• Commensalism (+,0): one species (the commensal) benefits, while the other (the host) is neither helped nor harmed.

Benefit:

• Mutualism: both partners gain food, protection, dispersal or pollination.
• Commensalism: only the commensal gains, usually shelter, support, transport or food scraps.

Dependence:

• Mutualism: can be obligate (neither survives alone, as in fig and fig wasp) or facultative.
• Commensalism: the host is independent; the commensal may or may not depend on the host.

Physiological or chemical contact:

• Mutualism: often intimate, with exchange of nutrients or signals (nodules, mycorrhizae).
• Commensalism: usually no exchange of nutrients; often only physical association.

Evolution:

• Mutualism: coevolution is common (fig–wasp, yucca–moth).
• Commensalism: coevolution is little or none on the host's side.

Examples of mutualism

1. ***Rhizobium* and legumes** (for example pea, Pisum sativum; soybean): the bacteria in root nodules fix N₂ into ammonia for the plant, and receive carbohydrates and a low-oxygen home (leghaemoglobin).
2. Mycorrhizae: arbuscular mycorrhizal fungi (*Glomus*/*Rhizophagus*) in roots of about 80% of land plants, such as maize and wheat. The fungal hyphae bring phosphate and water; the plant supplies up to about 20% of its photosynthate.

Other examples: lichens (fungus + alga), fig and fig wasp, Yucca and yucca moth, and ants living in Acacia thorns (Vachellia cornigera).

Examples of commensalism

1. Epiphytes: orchids such as Vanda and Dendrobium, and Spanish moss (Tillandsia), grow on branches of mango, sal or oak. They get support and light but take no water or nutrients from the host; they have velamen roots and absorb from rain and air.
2. Epizoochory: hooked fruits of Xanthium and Achyranthes stick to the fur of passing animals and are dispersed, with no effect on the animal.

Other examples: the cattle egret feeding on insects disturbed by grazing cattle, and barnacles attached to whales.

Ecological note: the line between the two is not fixed. Heavy epiphyte loads can break branches, and "commensals" such as egrets may also remove ticks and become mutualists. Mutualisms can also turn parasitic when conditions change, as when mycorrhizae cost more than they give in phosphorus-rich soils. Both interactions shape community structure, nutrient cycling and biodiversity. Mutualists like rhizobia and mycorrhizae are also exploited in agriculture as biofertilisers.