Question

Rubisco has a very low turnover number, about \(3 \mathrm{CO}_{2}\) per second. What might this low number tell us about the evolution of rubisco?

Short answer

Rubisco evolved in CO₂-rich conditions where high catalytic speed was not necessary.

Step-by-step solution

Understanding Turnover Number

The turnover number of an enzyme indicates how many substrate molecules one enzyme molecule can convert into product per second. For rubisco, this number is about 3 CO₂ molecules per second, which is considered very low compared to other enzymes.

Implications of Low Turnover Number

A low turnover number implies that rubisco works relatively slowly. This could suggest that there was less selective pressure for rubisco to be highly efficient in its catalytic activity during its evolutionary history.

Environmental Conditions

Consider the environmental conditions in which rubisco evolved. It likely evolved in environments where CO₂ was abundant. Thus, the need for a highly efficient enzyme, working at high speeds, would not have been critical.

Evolutionary Perspective

Given all these points, rubisco's low turnover number indicates that it has evolved under conditions where the need for rapid CO₂ fixation was not a significant driving factor. The enzyme may have adapted to function well enough to meet the needs of the organism without requiring rapid catalytic activity.

Key concepts

Turnover Number

The turnover number of an enzyme specifies the number of substrate molecules one enzyme molecule can convert to product per second. For rubisco, this value is particularly low—about 3 CO₂ molecules per second. While other enzymes can catalyze thousands of reactions per second, rubisco's low turnover number suggests it operates more slowly. This tells us something important about the enzyme's efficiency and function within the cell.

Enzyme Efficiency

Enzyme efficiency isn't solely dependent on the turnover number. While rubisco's turnover number is low, its overall efficiency is influenced by other factors such as substrate availability and enzyme concentration. Despite its slow catalytic rate, rubisco's efficiency might be optimized for the specific cellular and environmental contexts in which it functions. This shows that enzyme efficiency is a nuanced concept and must be interpreted within its specific biological scenario.

Evolutionary Adaptation

Rubisco likely evolved in ancient environments rich in CO₂. Back then, there was less evolutionary pressure for the enzyme to work at high speeds. As a result, rubisco adapted to be 'good enough' to meet the organism's needs without requiring swift catalytic activity. Over millions of years, this evolutionary adaptation has persisted, suggesting that rapid CO₂ fixation was not a significant evolutionary driver for rubisco.

CO₂ Fixation

Rubisco's primary role is to fix CO₂ during photosynthesis. Despite its low turnover number, rubisco carries out this crucial function effectively enough to sustain plant life. The enzyme helps convert CO₂ into organic molecules that plants use to grow. This process is fundamental to the carbon cycle and supports life on Earth by contributing to the growth of plants, which are at the base of the food chain.