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 crucial role in cellular energy production, leading to strong evolutionary pressure to conserve their structure and function.

Step-by-step solution

- Understanding Cytochromes

Cytochromes are proteins involved in the electron transport chain, crucial for cellular respiration and energy production in cells.

- Importance of Function

The function of cytochromes in energy production is critical for survival. Any significant mutations that alter their function could be detrimental to an organism's energy production and overall survival.

- Evolutionary Pressure

Due to their essential role, there is strong evolutionary pressure to maintain the structure and function of cytochromes. This leads to a lower rate of amino acid changes because mutations that significantly affect their function are likely to be selected against.

- Comparison with Hemoglobin, Myoglobin, and Hydrolytic Enzymes

Hemoglobin, myoglobin, and hydrolytic enzymes, while important, do not have as critical a role in immediate cellular energy production as cytochromes. This means there is less evolutionary pressure to conserve their exact amino acid sequences, allowing for more mutations over time.

- Conclusion

Because cytochromes are crucial for basic cellular function and survival, there is strong evolutionary pressure to preserve their structure, resulting in fewer amino acid changes compared to hemoglobin, myoglobin, and hydrolytic enzymes.

Key concepts

Protein Evolution

Proteins are essential molecules within living organisms, composed of chains of amino acids. Over time, the sequences of these amino acids can change due to genetic mutations. This process is known as protein evolution. Different proteins evolve at different rates depending on their functions and the evolutionary pressures they face. For example, structural proteins needed for basic cell functions evolve more slowly compared to those with less critical roles. This is because any detrimental change to a crucial protein can negatively impact the organism's survival.

Electron Transport Chain

The electron transport chain (ETC) is a series of protein complexes found in the inner mitochondrial membrane. It plays a crucial role in cellular respiration by transferring electrons from electron donors to electron acceptors via redox reactions. This process is coupled with the transfer of protons (H+ ions) across the mitochondrial membrane, creating a proton gradient. This gradient generates ATP - the energy currency of the cell. Cytochromes are integral components of the ETC, and any disruption in their function can significantly affect an organism's energy production.

Cellular Respiration

Cellular respiration is the process by which cells convert the chemical energy in nutrients into ATP. This occurs in several stages: glycolysis, the citric acid cycle, and oxidative phosphorylation. During oxidative phosphorylation, the electron transport chain plays a critical role. Cytochromes within the ETC facilitate the transfer of electrons, ultimately leading to the production of ATP. Effective cellular respiration is vital for cell survival and function, making the proteins in this pathway, such as cytochromes, extremely important.

Amino Acid Sequence Conservation

Amino acid sequence conservation refers to the phenomenon where specific sequences in proteins remain relatively unchanged throughout evolution. This occurs because these sequences are crucial for the protein's function and stability. In the case of cytochromes, their role in the electron transport chain is so vital that any significant changes in their amino acid sequence could be harmful. As a result, these proteins exhibit higher sequence conservation compared to others like hemoglobin or hydrolytic enzymes.

Evolutionary Pressure

Evolutionary pressure is the influence exerted by environmental and biological factors that affect an organism's survival and reproduction. Proteins involved in essential processes like cellular respiration face strong evolutionary pressure to maintain their structure and function. For cytochromes, the pressure is intense because any mutations disrupting their function can seriously impair energy production. Conversely, proteins less critical for immediate survival, such as hydrolytic enzymes or hemoglobin, face less stringent evolutionary pressure, allowing their amino acid sequences to evolve more rapidly.