Question

Experimental evidence strongly suggests that the protein portions of cytochromes have evolved more slowly (as judged by the number of changes in amino acids per million years) than the protein portions of hemoglobin and myoglobin and even more slowly than hydrolytic enzymes. Suggest a reason why.

Short answer

Cytochromes evolve more slowly due to their critical role in energy production, making their function highly conserved.

Step-by-step solution

Identify the role of cytochromes

Cytochromes are essential in the electron transport chain, which is critical for cellular respiration and energy production.

Consider the importance of function conservation

Since cytochromes play a crucial role in such a vital process, maintaining their structure and function is essential for the survival of organisms.

Compare with hemoglobin and myoglobin

Hemoglobin and myoglobin are involved in oxygen transport and storage, which, while important, may be more adaptable without threatening immediate survival.

Compare with hydrolytic enzymes

Hydrolytic enzymes have diverse functions and can evolve more rapidly without severe consequences on basic cell functions.

Draw a conclusion

The slower evolution of cytochromes can be attributed to the critical and conserved nature of their function in energy production, compared to hemoglobin, myoglobin, and hydrolytic enzymes.

Key concepts

electron transport chain

The electron transport chain (ETC) is a series of protein complexes located in the inner membrane of mitochondria. Its primary role is to produce ATP, the energy currency of the cell, through a process called oxidative phosphorylation.
During cellular respiration, electrons are transferred from electron donors to acceptors like oxygen via these complexes.
As electrons flow through the chain, protons are pumped across the membrane, creating a proton gradient.
This gradient powers ATP synthase, the enzyme responsible for synthesizing ATP.
Cytochromes are key components in this chain, as they facilitate the transfer of electrons between complexes.
They are heme-containing proteins that undergo redox reactions, which is essential for the production of ATP.
Without the proper function of cytochromes, the ETC would fail, leading to a halt in ATP production and, consequently, cell death.

protein evolution

Protein evolution determines how protein sequences change over time, driven by the processes of mutation, natural selection, and genetic drift. Some proteins, like cytochromes, evolve more slowly because their function is integral to the survival of the organism.
If a critical function is disrupted, it could have lethal consequences.
In contrast, proteins such as hydrolytic enzymes evolve more rapidly due to their diverse functions and adaptability.
Each protein's rate of evolution is influenced by:

• Functional importance
• Impact of mutations on survival
• Constraints on structural stability

Certain amino acids within a protein are more critical for function, making them less likely to change.
For cytochromes, maintaining their exact structure is crucial due to their role in the ETC, leading to fewer permissible changes in their amino acid sequence.

cellular respiration

Cellular respiration is the process by which cells convert nutrients into energy in the form of ATP. It occurs in multiple stages, including glycolysis, the citric acid cycle, and the electron transport chain.
The ETC is the final stage and the most significant contributor to ATP production.
During cellular respiration:

• Glycolysis breaks down glucose into pyruvate, producing a small amount of ATP.
• The citric acid cycle further breaks down pyruvate, releasing electrons that are carried to the ETC.
• In the ETC, these electrons help create a proton gradient used by ATP synthase to produce ATP.

Cytochromes, as part of the ETC, are vital for this energy transformation.
Without efficient function in any stage, especially the ETC, cells would struggle to generate adequate energy, impacting overall organism survival.