Question

DNA photolyases convert the energy of light in the near-ultraviolet, or visible, region \((300-500 \mathrm{nm})\) into chemical energy to break the cyclobutane ring of pyrimidine dimers. In the absence of substrate, these photoreactivating enzymes do not absorb light of wavelengths longer than \(300 \mathrm{nm}\). Why is the substrateinduced absorption band advantageous?

Short answer

The substrate-induced absorption band allows photolyases to use a wider range of light, promoting efficient DNA repair in various light conditions.

Step-by-step solution

Understanding DNA Photolyases and Light Absorption

DNA photolyases are enzymes that use the energy from light to repair DNA damage, specifically pyrimidine dimers. These dimers form when two adjacent pyrimidine bases (usually thymine) bond together due to ultraviolet (UV) radiation, which can disrupt normal DNA function. Photolyases absorb light in the 300-500 nm range to perform their repair function.

Recognizing the Role of Wavelength in Photoreactivation

In their inactive form (without substrate), photolyases do not absorb light longer than 300 nm efficiently. This suggests that the active form, which can absorb longer wavelengths due to a substrate-induced absorption band, can utilize a broader range of light for energy.

Exploring the Advantage of a Broader Absorption Range

The substrate-induced absorption band allows photolyases to absorb light in the visible range (up to 500 nm), broadening the spectrum of usable light. Since sunlight includes considerable visible light but less UV light around 300 nm, this property enhances the enzyme's accessibility to energy from ambient light.

Concluding the Advantage of Substrate-Induced Absorption

By absorbing longer wavelengths, photolyases can function under a wider range of conditions, particularly in environments with limited UV exposure. This allows for efficient DNA repair even when UV light is not predominant, utilizing ambient light effectively for dimer repair.

Key concepts

Pyrimidine Dimers

Pyrimidine dimers are specific types of DNA damage caused by ultraviolet (UV) radiation. They occur when two adjacent pyrimidine bases—thymine and cytosine are the most common—become covalently bonded. This bond forms a cyclobutane ring, which is a stable but inappropriate linkage leading to distortion in the DNA structure. This distortion can affect DNA replication and transcription processes, potentially causing mutations that can lead to skin cancers and other genetic abnormalities. To put it simply, pyrimidine dimers essentially stick together sections of the DNA "ladder," interfering with the overall genetic message.

Repairing these dimers is crucial for maintaining DNA integrity and avoiding serious biological consequences. Photolyases, which are special enzymes, can reverse the formation of these dimers when activated by light. They break these inappropriate bonds, restoring the DNA to its original structure. It's like having a skilled repair crew that fixes the damage caused by too much sun exposure.

Light Absorption in Photoreactivation

Photoreactivation is the process by which photolyases use light energy to repair DNA. This biological mechanism is heavily dependent on the absorption of light within a specific range. Photolyases are designed to absorb light wavelengths between 300 nm and 500 nm. This range covers the near-ultraviolet and visible spectrum, effectively enabling them to harvest energy from an abundant natural resource: sunlight.

Interestingly, in their inactive state, without a bound DNA substrate, photolyases do not efficiently absorb light longer than 300 nm. However, upon binding with a pyrimidine dimer, these enzymes demonstrate an induced absorption band that allows them to utilize a wider range of light—up to 500 nm. This expansion is advantageous because it means the enzymes can draw power from a more substantial portion of the sunlight spectrum. Photoreactivation, therefore, transforms the light into the necessary energy to break the cyclobutane bonds of pyrimidine dimers.

Ultraviolet Radiation and DNA Repair

Ultraviolet (UV) radiation from the sun is a significant environmental factor that can cause DNA damage in living organisms. This type of radiation is high-energy light that most commonly results in the formation of pyrimidine dimers in DNA. Such dimers can be a serious threat if not repaired efficiently, emphasizing the critical role of the DNA repair mechanisms.

DNA repair systems, such as those involving photolyases, are nature's ingenious way of counteracting UV damage. Photolyases harness light energy, converting it into chemical energy to correct damage caused by UV radiation. Their ability to utilize light from both the UV and visible spectrum for reactivation is particularly important. Given that sunlight is more abundant in visible light than UV, the substrate-induced absorption band makes photolyases highly effective, ensuring that repair occurs even in low UV light conditions. This flexibility is key, as it allows organisms to maintain their DNA integrity across a variety of environmental settings.