Biosynthesis of Ethylene

Summary

This video explains the biosynthesis of ethylene in plants, focusing on its synthesis from the precursor molecule methionine. The process begins with methionine being converted into S-adenosylmethionine (SAM) by SAM synthetase, aided by ATP. The reaction then proceeds through a bifurcating pathway catalyzed by ACC synthase, resulting in the formation of ACC (amino cyclopropane carboxylate). ACC oxidase then catalyzes the final conversion to ethylene, with oxygen and ascorbic acid playing essential roles. The video also touches on the ethylene synthesis from linolenic acid, although this pathway is not observed in plants.

Takeaways

• 😀 Ethylene is a plant hormone whose biosynthesis is primarily derived from methionine.
• 😀 There are two reported precursors for ethylene: methionine (in plants) and linolenic acid (observed only in model systems).
• 😀 The linolenic acid pathway for ethylene synthesis does not naturally occur in plant tissues.
• 😀 Methionine is converted into S-adenosyl methionine (SAM) by the enzyme SAM synthetase, requiring ATP.
• 😀 SAM is then converted into ACC (1-aminocyclopropane-1-carboxylate) by ACC synthase.
• 😀 ACC synthase is a highly regulated, rate-limiting enzyme in the ethylene biosynthesis pathway.
• 😀 The side product of the ACC synthesis reaction is methylthioadenosine (MTA), which is recycled back to methionine via the Yang cycle.
• 😀 ACC is converted to ethylene through the action of ACC oxidase in the presence of oxygen.
• 😀 The Yang cycle maintains a steady pool of methionine, ensuring continuous ethylene production.
• 😀 Methionine-derived ethylene biosynthesis is the major pathway in plants, while the linolenic acid route is experimental and not naturally occurring.

Q & A

What are the two precursor molecules for ethylene biosynthesis mentioned in the video?

The two precursor molecules for ethylene biosynthesis mentioned are methionine and linolenic acid.

How is ethylene synthesized from methionine in plant tissues?

Ethylene is synthesized from methionine through a series of steps involving the conversion of methionine to S-adenosyl methionine (SAM), which then leads to the formation of 1-aminocyclopropane-1-carboxylate (ACC). ACC is eventually converted to ethylene by the enzyme ACC oxidase.

Why is the ACC synthase enzyme considered a rate-limiting step in ethylene biosynthesis?

ACC synthase is considered a rate-limiting enzyme because its activity is highly regulated and controls the speed at which ACC is produced, which directly impacts the overall rate of ethylene biosynthesis.

What is the Yang cycle and why is it important in the biosynthesis of ethylene?

The Yang cycle, also known as the methionine cycle, is a process where methylthioadenosine is recycled back to methionine. This cycle is important because it maintains the methionine pool, which is essential for continuous ethylene production in plants.

What happens to linolenic acid in the ethylene biosynthesis pathway?

Linolenic acid can break down to form ethylene, but this pathway has only been demonstrated in experimental model systems, not in plants. The breakdown of linolenic acid requires copper ions, oxygen, and ascorbic acid.

Is the linolenic acid pathway involved in ethylene biosynthesis in plants?

No, the linolenic acid pathway does not occur in plants. It has only been observed in model systems and there is no evidence to suggest that plants biosynthesize ethylene from linolenic acid.

What role does SAM synthetase play in the biosynthesis of ethylene?

SAM synthetase catalyzes the conversion of methionine to S-adenosyl methionine (SAM), a key intermediate in the ethylene biosynthesis pathway. This reaction requires energy from ATP.

What is the function of ACC oxidase in ethylene biosynthesis?

ACC oxidase catalyzes the final step in ethylene biosynthesis, where it converts ACC to ethylene in the presence of oxygen, completing the biosynthetic pathway.

What is the significance of methylthioadenosine in the ethylene biosynthesis pathway?

Methylthioadenosine is a byproduct of the conversion of SAM to ACC. It is recycled in the Yang cycle to regenerate methionine, ensuring that the methionine pool is maintained for continuous ethylene production.

What are the energy requirements for the conversion of methionine to SAM in ethylene biosynthesis?

The conversion of methionine to S-adenosyl methionine (SAM) requires energy in the form of ATP, which is used by the SAM synthetase enzyme to drive the reaction.