In outlining the process of glycolysis explain...

Question

In outlining the process of glycolysis explain (i) the difference in ATP yield when glycogen rather than glucose is used as the starting point and (ii) the production of lactic acid in muscle tissue.

Answer 1

well if one starts with glucose ....the first step of glycolysis  CONSUMES an ATP  because of the action of hexokinase. Another ATP is consumed at the rate-limiting step fructose 6-phosphate to fructose 1,6 bis phosphate via the enzyme phosphofructokinase.  After the aldolase step one has two 3 carbon units that can go threough the rest of glycolysis ..each 3 C fragment can yield 2 ATPs  sooo  you've invested 2 ATPs  and gotten back 2 X 2 ATPs  for a net of 2 ATPs.

When starting from glycogen, however, the breakdown of glycogen...via glycogenolysis  yields glucose -1-phophate which freely converts to glucose 6-phophate to go through glycolysis without the hexokinase step...so via glycolysis starting fron glycogen the yield is 3 ATPs.

In the step from glyceraldehyde -3 phophate to 1,3 bis-phosphoglycerate the aldehyde is oxidized to the carboxylic acid using the power of NAD+. The energy of this oxidation is conserved by phosphorylating the carboxylic acid making a phosphate ester ( high energy phosphate ). In order to do this reaction one needs a plentiful supply of NAD+. As you can see as the recation proceeds the NAD+ is being converted to NADH ( the gly-3p-----1, 3 bisphosphoglycerate step will not work with NADH). So it is imperative to quickly recycle the NAD+ by stripping the H- off of NADH and using it to reduce pyruvate  to lactate . The NAD+ is then available to continue to assist glycolysis and the lactate , although a deadend in the muscle can enter the blood , go to the liver, and be converted back to glucose by the Cori Cycle