Question 1 In mammals, body temperature can be regulated by uncoupling proteins

Question

Question 1

In mammals, body temperature can be regulated by uncoupling proteins which act by uncoupling electron

transport and oxidative phosphorylation. Two such uncoupling proteins have been identified which have been

named UCP1 and UPC2. UCP1 is believed to be involved in heat production, whereas UCP2 might be involved

in obesity in humans.

The mechanism of thermogenesis in plants is not as well understood. Several studies have been carried out

which indicate that thermogenesis is an important process in the developing plant. For example, the Eastern

skunk cabbage has the ability to maintain the temperature of the plant 15-35°C higher than ambient

temperatures during the months of February and March where ambient temperatures range from -15 to + 15°C.

Thermogenesis in the skunk cabbage is critical to the survival of the plant since the spadix tissue is not frost-

resistant. The skunk cabbage may be unique in its ability to carry out thermogenesis for longer than just a few

hours.

Thermoregulation has also been studied in lotus flowers. Investigators in Australia noted that the

temperature of the lotus flower receptacle rises one or two days before the flower opens. Temperatures fall only

after the flower is completely opened. The investigators speculated that heat production in the lotus flower

serves to enhance evaporation of the floral scent that attracts pollinating insects. Noting that many pollinating

insects require a thoracic temperature of 30° C or higher to initiate flight, the investigators hypothesized that

heat production in the lotus flower would assist the departure of the insects from the flower.

Although observations of temperature and oxygen utilization in the skunk cabbage and lotus flower

implicated an uncoupler, an uncoupling protein was not isolated until fairly recently. A research team in France

recently reported that they had identified an uncoupling protein in potatoes (Solanum tuberosum) that is

involved in thermoregulation in plants in a manner similar to the brown fat uncoupling proteins in mammals.

The uncoupling protein is named StUCP and its synthesis is induced by cold temperatures. The authors note that

heat production concomitant with a “burst of respiration” occurs during plant flowering and fruit ripening.

a.   (15 pts) Explain how uncoupling agents work and how an uncoupling agent acts to bring about an increase

in temperature.

An uncoupling agent is protein that is capable of disabling the proton gradient between the mitochondrial inner membrane space and the matrix. This then uncouples ETC and oxidative phosphorylation. When this occurs, the extra energy is transformed into heat.

b.   (10 pts) In the skunk cabbage, the site of thermogenesis is the spadix. However, the spadix tissue does not

store starch in its tissues. Instead, the spadix relies on the massive root system in the skunk cabbage which

stores appreciable quantities of starch. The large quantity of starch available may explain why the skunk

cabbage is able to carry out thermogenesis in weeks rather than hours. Why is starch required for

Thermogenesis?

Starch is important because it can be broken down to glucose. Glucose can then be broken down through glycolysis, Citric Acid Cycle and Electron Transport Chain/Oxidative Phosphorylation. These pathways will provide NADH and FADH2 that can be used for ETC.

c.   (15 pts) Oxygen consumption by the skunk cabbage increases as temperature decreases. The rate of oxygen

consumption nearly doubles with every 10° drop in ambient temperature. Similarly, in the lotus flower, the

receptacle temperature is maintained between 30°C and 35°C for a two-day period during flower opening.

The ambient air temperature during this two-day period may fluctuate between 10°C and 30°C. Oxygen

consumption was observed to decrease during the day when temperatures were close to 30°C. Explain the

biochemical mechanism for these observations.

Higher temperature = lower O2 consumption

This could be due to

d.   (15 pts) In the skunk cabbage, lotus flower, and the potato, thermogenesis occurs because of the action of an

2 uncoupling protein. The gene that codes for the uncoupling protein in potatoes was recently isolated. The

results from one of the experiments are shown in Figure 24.1. What is your interpretation of these results?

How is the production of the uncoupling protein regulated?

mRNA expression of the uncoupling protein is not present at 20 degrees Celsius from days 0-3. This means that at higher temperatures, the uncoupling protein is not produced as much (or at all in this case). Thus, the uncoupling protein must be involved in processes that occur at lower temperatures. The protein, as described earlier, deactivates the proton gradient which then creates heat from the extra energy, which can be seen here. When the plant is “warm” the protein is not needed. When the plant is “cold” the protein needs to disable the gradient and create heat.

e.   (10 pts) Given electron transport and Oxidative phosphorylation occur in the mitochondria and chloroplasts in plants, what effect, if any, would you expect an uncoupler to have on photosynthesis and why? (Note this is a hypothesis question – not a “find the answer in a reference paper” question.)

20°C 4°C Days: 0 2 3 0 2 3 Figure 24.1: Northern blot analysis of the uncoupling protein from potatoes at 4°C and 20°C. Northern blots detect levels of mRNA (based on Laloi, et al., 1997).

Explanation / Answer

Answer-a:

Uncoupling agents are the substances that uncouples the phosphorylation of the ADP from electron trasnport chain. As a result, it causes the dissociation of ATP sysnthesis by altering the transportation of electrons through cytochromes. This means that ETC continues to function normally and oxygen is also consumed normally but the inhibition of ADP phosphorylation takes place inn the presence of uncouplers.

In normal cases, that is, when the oxidative phosphorylation is coupled with electron transport chain; then, due to formation of proton gradient and ATP synthase activity, ATP is synthesized. However, in the presence of uncoupling proteins (such as 2,4-DNP, CCCP, etc.), they binds to the ATP synthase enzyme and does not allow the passage of protons across the mitochondrial membrane, due to which ATP is formed otherwise. But in the presence of uncoupling proteins, protons will now remain confined to matrix only, so, due to accumulation of more and more protons or H+ ions. However, energy lost in dissipating of proton gradient is not used by the body in doing the biochemical work, but it is dissipated as heat energy.

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