Lecture 17 - Skeletal Muscle Energy Metabolism Video 1 Muscle Energy Metabolism - Muscle is an energetically expensive tissue as it goes through a lot of ATP What are the major processes in a muscle cell that require ATP? 1. Cross-bridge cycle - Myosin hydrolyzes ATP and this is what provides energy for the power stroke 2. Pumping Ca2+ into the SR - To reduce cytoplasmic [Ca2+] allowing relaxation which requires ATP - SERCA is a ca2+ ATPase so it hydrolyzes ATP to provide the energy to pump ca2+ into the SR 3. Reestablishing ion gradients - Particularly at the sarcolemma also requires ATP so after APs Na+/K+ ATPase will hydrolyze ATP in order to pump Na+ out of cell and K+ into cell - During high-intensity activity, a muscle can consume its stores of ATP in just one or two seconds so this ATP is used up very rapidly How does the muscle replenish its ATP stores such that it can continue contracting? (can continue pumping Ca2+ into SR/continue maintain ion gradients) - Muscle cells must be able to produce sufficient ATP to meet this demand: vertebrate myofibers can produce ATP by hydrolysis of phosphocreatine, glycolysis, and oxidative phosphorylation. Another way vertebrate myofibers can produce ATP is by: Phosphocreatine · a molecule used by Vertebrates · Vertebrates use phosphocreatine as an energy store that can be used to rapidly replenish the supply of ATP during muscle activity - Phosphocreatine is not only used in the muscles but also to replenish the supply of ATP in the brain - In the human body, about 95% of creatine is found in skeletal muscle/mainly used in the skeletal muscle for the replenish the supply of ATP How does this work? - When concentrations of ATP are relatively high the enzyme creatine phosphokinase is going to hydrolyze ATP and take that phosphate and use it to phosphorylate creatine to form phosphocreatine - The enzyme creatine phosphokinase: if ATP is consumed in the muscle -> leads to a
decrease in the concentration of ATP and increase in the concentration of ADP which will induce creatine phosphokinase to catalyze the reaction in the opposite direction - Hydrolyze phosphocreatine -> remove that phosphate -> add it to ADP -> to produce ATP and creatine - As ATP levels decrease, creatine phosphokinase is going to replenish the supply of ATP by hydrolyzing phosphocreatine - The stores of ATP during during extreme high-intensity activity, a muscle can deplete its stores of phosphocreatine in about ten seconds = allows muscle cells to replenish the supply of ATP making that supply last longer than it would without phosphocreatine but 10 seconds is not enough to sustain muscle activity so we are also going to get glycolysis and oxidative phosphorylation producing ATP as well to help replenish the supply of ATP in an active muscle - Muscles are not able to store really high [ATP] for many reasons however they can store relatively higher [phosphocreatine] H2N CH3 1 N ATP ADP