Nicotinamide adenine dinucleotide phosphate, reduced form. This molecule is exactly like NADH but it contains an extra phosphate group on the ribose attached to the nicotinamide side of the molecule. Some enzymes require the presence of this phosphate group in order to bind this cofactor, others will only bind (and utilize) NADH. The body can thus turn various enzyme activities up and down by altering the ratio of NADH to NADPH. Generally, NADH-utilizing enzymes are involved in metabolism, while NADPH-utilizing enzymes are involved in catabolism. Both of these molecules carry reducing equivalents in the form of a hydride which is used to supply the energy needed for the reactions in which they're involved.

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