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Myophosphorylase
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<p>Myophosphorylase or glycogen phosphorylase, muscle associated (PYGM) is the muscle <a href="/facts/Isoform/oWmzfg4l">isoform</a> of the <a href="/facts/Enzyme/DSI1Fh7y">enzyme</a> <i><a href="/facts/Glycogen_phosphorylase/Hhep1hRQ">glycogen phosphorylase</a></i> and is encoded by the <i>PYGM</i> gene. This enzyme helps break down <a href="/facts/Glycogen/kWa8fPFE">glycogen</a> (a form of stored <a href="/facts/Carbohydrate/Geuuzxsf">carbohydrate</a>) into <a href="/facts/Glucose-1-phosphate/fYHcfFcs">glucose-1-phosphate</a> (not <a href="/facts/Glucose/oUSkoLCD">glucose</a>), so it can be used within the <a href="/facts/Muscle_cell/cP7Jea3q">muscle cell</a>. Mutations in this gene are associated with <a href="/facts/McArdle_disease/I1vk7MVr">McArdle disease</a> (GSD-V, myophosphorylase deficiency), a <a href="/facts/Glycogen_storage_disease/7SGTIKcp">glycogen storage disease</a> of muscle. </p><p>Myophosphorylase comes in two forms: form 'a' is phosphorylated by <a href="/facts/Phosphorylase_kinase/Hrd2kGst">phosphorylase kinase</a>, form 'b' is not phosphorylated. Form 'a' is de-phosphorylated into form 'b' by the enzyme <a href="/facts/Phosphoprotein_phosphatase/vrvXg4g7">phosphoprotein phosphatase</a>, which is activated by elevated insulin. </p><p>Both forms 'a' and 'b' of myophosphorylase have two <a href="/facts/Conformational_change/q23CgKgo">conformational</a> states: active (R or relaxed) and inactive (T or tense). When either form 'a' or 'b' are in the active state, then the enzyme converts glycogen into glucose-1-phosphate. </p><p>Myophosphorylase-b is <a href="/facts/Allosteric_regulation/alMRuyt9">allosterically</a> activated by elevated AMP within the cell, and allosterically inactivated by elevated ATP and/or glucose-6-phosphate. Myophosphorylase-a is active, unless allosterically inactivated by elevated glucose within the cell. In this way, myophosphorylase-a is the more active of the two forms as it will continue to convert glycogen into glucose-1-phosphate even with high levels of glycogen-6-phosphate and ATP. (See <a href="/facts/Glycogen_phosphorylase/Hhep1hRQ">Glycogen phosphorylase§Regulation</a>). </p>