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HIV protease in a complex with the protease inhibitor ritonavir

Enzyme inhibitors are molecules that bind to enzymes and decrease their activity. Since blocking an enzyme's activity can kill a pathogen or correct a metabolic imbalance, many drugs are enzyme inhibitors. Inhibitor binding is either reversible or irreversible. Irreversible inhibitors usually react with the enzyme and change it chemically. These inhibitors modify key amino acid residues needed for enzymatic activity. In contrast, reversible inhibitors bind non-covalently and different types of inhibition are produced depending on whether these inhibitors bind the enzyme, the enzyme-substrate complex, or both. Their discovery and improvement is an active area of research in biochemistry and pharmacology. A medicinal enzyme inhibitor is often judged by its specificity (its lack of binding to other proteins) and its potency (its dissociation constant, which indicates the concentration needed to inhibit the enzyme). A high specificity and potency ensure that a drug will have few side effects and thus low toxicity. Enzyme inhibitors also occur naturally and are involved in the regulation of metabolism. (more...)

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Petrified wood

Petrified wood, such as this sample found in Petrified Forest National Park, Arizona, United States, is a fossil wood where all the organic materials have been replaced with minerals, while retaining the original structure of the wood. The petrifaction process occurs underground, when wood becomes buried under sediment. Mineral-rich water flowing through the sediment deposits minerals in the plant's cells and as the lignin and cellulose decay, a stone cast forms in its place.

Photo credit: Daniel Schwen
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