Carotenoid biosynthesis

Carotenoids are a class of natural pigments synthesized by various organisms, including plants, algae, and photosynthetic bacteria. They are characterized by their vibrant yellow, orange, and red colors, which contribute significantly to the coloration of fruits and vegetables. Carotenoids play essential roles in photosynthesis and offer various health benefits, such as antioxidant properties and serving as precursors to vitamin A.

Biosynthetic pathway

Carotenoid biosynthesis occurs primarily in the plastids of plant cells, particularly within chloroplasts and chromoplasts. The biosynthetic pathway initiates with the condensation of two molecules of geranylgeranyl pyrophosphate (GGPP), a 20-carbon isoprenoid precursor. The key steps in this pathway are as follows:

1. Formation of phytoene: The enzyme phytoene synthase (PSY) catalyzes the condensation of two GGPP molecules to produce phytoene, a colorless carotenoid.
2. Desaturation to lycopene: Phytoene undergoes a series of desaturation reactions facilitated by enzymes such as phytoene desaturase (PDS) and ζ-carotene isomerase (Z-ISO), resulting in the formation of lycopene, a red carotenoid.
3. Cyclization to carotenoids: Lycopene is cyclized into various carotenoids, including α-carotene and β-carotene, through the action of lycopene cyclase (LCY), which catalyzes cyclization at the ends of the lycopene molecule.
4. Further modifications: Subsequent modifications, such as hydroxylation and oxidation, lead to the formation of xanthophylls (e.g., lutein and zeaxanthin) and other derivatives.

Key enzymes

Several enzymes play critical roles in the carotenoid biosynthetic pathway:

1. Phytoene synthase (PSY): Catalyzes the first committed step in carotenoid biosynthesis, converting GGPP into phytoene.
2. Phytoene desaturase (PDS): Introduces double bonds into phytoene, facilitating its conversion into lycopene.
3. Lycopene cyclase (LCY): Responsible for the cyclization of lycopene into α-carotene or β-carotene.
4. Carotenoid hydroxylases: Enzymes such as lutein epoxide cyclase (LUT) introduce hydroxyl groups into carotenoids, leading to the formation of xanthophylls.

Regulation

The regulation of carotenoid biosynthesis is influenced by various factors, including:

• Gene Expression: Many carotenoid biosynthetic genes are upregulated by light, enhancing the expression of PSY and subsequently increasing carotenoid production.

• Hormonal Regulation: Phytohormones such as auxins and abscisic acid modulate carotenoid biosynthesis. Notably, abscisic acid enhances carotenoid accumulation under stress conditions.

• Environmental Factors: Stressors like drought or pathogen attack can trigger carotenoid accumulation as a protective response, thereby enhancing plant resilience.

Significance

In plants

Carotenoids play roles in photosynthetic organisms by:

• Protecting chlorophyll from photodamage.
• Scavenging reactive oxygen species (ROS).
• Attracting pollinators and seed dispersers through their bright colors.

In human health

Carotenoids, especially provitamin A carotenoids such as β-carotene, are essential for human health. Their benefits include:

• Supporting vision, particularly in low-light conditions.
• Enhancing immune function.
• Contributing to skin health.
• Providing antioxidant properties that may reduce the risk of chronic diseases, including cardiovascular diseases and certain cancers.