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#metabolism

27 public questions tagged with this topic.

Transgenic Plant -2

Beta-carotene is a precursor of:

Beta-carotene is tetraterpenoid C40 carotenoid consisting of two beta-ionone rings linked by eleven conjugated double bonds providing strong antioxidant scavenging singlet oxygen and light filtering. Most importantly it serves as provitamin A carotenoid because symmetrical cleavage at central 15,15' double bond by enzyme beta-carotene 15,15'-monooxygenase BCMO1 in intestinal enterocytes and hepatic stellate cells yields two molecules of retinaldehyde. Retinal can be reduced to retinol bound to retinol binding protein for systemic transport to retina for rhodopsin regeneration mediating scotopic vision, oxidized to retinoic acid activating gene transcription via nuclear receptors RAR and RXR heterodimers regulating embryonic development, epithelial differentiation, immune cell maturation, and spermatogenesis. Humans cannot synthesize carotenoid backbone requiring dietary intake. Vitamin A deficiency leads to xerophthalmia, night blindness, keratomalacia, impaired mucosal immunity increased measles mortality. Beta-carotene bioavailability enhanced by dietary lipids and food processing releasing crystalline aggregates, with conversion factor 12:1 to retinol activity equivalents in mixed diet.

Ref: Lodish Molecular Cell Biology Vitamins; NIH StatPearls Beta-carotene Vitamin A; Olson 1989.

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Microbial Physiology

The Archaea domain lacks which of the following metabolic processes?

Archaea exhibit remarkable metabolic diversity including methanogenesis converting hydrogen and carbon dioxide or acetate into methane via unique cofactors like methanofuran, tetrahydromethanopterin, and coenzyme M, anaerobic respiration using sulfur, nitrate, or ferric iron as electron acceptors, and oxidative phosphorylation driven by archaeal A-type ATP synthase related to vacuolar ATPases. Some halophiles like Halobacterium salinarum perform light-driven proton pumping via bacteriorhodopsin, a retinal-based phototrophy fundamentally different from chlorophyll-based photosynthesis. Chlorophyll-based oxygenic photosynthesis requiring photosystems I and II, chlorophyll a, and Calvin cycle enzymes for carbon fixation is characteristic of cyanobacteria and eukaryotic algae and plants, not of archaeal domain. No archaeon synthesizes chlorophyll or assembles photosystem reaction centers containing chlorophyll. Archaeal phototrophy relies on simple rhodopsins, not chlorin pigments. Hence absence of chlorophyll-based photosynthesis is defining metabolic gap, while methanogenesis remains unique archaeal contribution to global carbon cycle and greenhouse gas production in anaerobic environments such as wetlands and rumen. Photosynthetic bacteria use chlorophyll to harvest light, generating reducing power and ATP via electron transport chains, while archaeal bacteriorhodopsin pumps protons using retinal isomerization without producing reducing equivalents, representing functional convergence but chemically distinct phototrophy that does not fix carbon via Calvin cycle and therefore not counted as true chlorophyll photosynthesis.

Ref: Madigan et al., Brock Biology, Methanogenesis and Phototrophy; Thauer et al., Nature Rev Microbiol 2008, Methanogenic Pathways.

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