Affinity label
Affinity labels are a class of enzyme inhibitors that covalently bind to their target causing its inactivation. The hallmark of an affinity label is the use of a targeting moiety to specifically and reversibly deliver a weakly reactive group to the enzyme that irreversibly binds to an amino acid residue. The targeting portion of the label often resembles the enzyme's natural substrate so that a similar mode of noncovalent binding is used prior to the covalent linkage. Their usefulness in medicine can be limited by the specificity of the first noncovalent binding step whereas indiscriminate action can be utilized for purposes such as affinity labeling - a technique for the validation of substrate-specific binding of compounds. These labels are not limited to enzymes but may also be designed to react with antibodies or ribozymes although this usage is less common. Although proteins such as hemoglobin do not have an active site, binding pockets can be exploited for their affinity and thus be labeled. Classifications Affinity labels can be broken down into three distinct categories based on their reactive groups and mode of delivery. Classical affinity labels This category encompasses the simplest approach of coupling an electrophile with low intrinsic reactivity