12 Glycolysis

12.1 Introduction

Glycolysis is the decomposition of glucose by hydrolysis. Most of the glucose in muscle is stored as glycogen. Thus , glycolysis is also the decomposition of glycogen.  Glycolysis is supremely important in understanding how muscles are converted to meat. Glycolysis normally causes an increase in acidity (= decrease in pH) as muscles are converted to meat.  Too much or too little acid - and the meat is ruined.

IF you have not taken a course in biochemistry - concentrate on section 12.2 to 12.5.  If you are bored by yet another repetition of glycolysis have a look at sections 12.6 onwards.

12.2 Living muscle

12.3 Glycogen

Structure of a glucose unit with its carbon atoms numbered is shown below.

These are joined by straight 1-4 linkages or branched 1-6 linkages to form glycogen.

The image below shows a transverse section of pork stained by the periodic acid-Schiff reaction for glycogen. All the myofibres started with glycogen, but most myofibres have depleted their glycogen during the conversion of muscle to meat. Note how some of the myofibres have a central core of glycogen. 

12.4 After exsanguination

While the structure of glycogen and the mechanism of glycogenolysis are important in understanding the conversion of muscles to meat, the remainder of the pathway by which glycogen is anaerobically oxidized to lactate need only be covered in general principle. The most important question to be answered is why lactate is produced under anaerobic conditions, yet hardly at all under aerobic conditions.
LDH adds hydrogen to pyruvate to produce lactic acid and exists in a number of isoenzymes separable electrophoretically by their net electrical charge. If LDH-1 is prevalent, it facilitates aerobic metabolism, where possible, since it is inhibited by pyruvate and lactate. LDH-5 is not inhibited by high levels of lactate and pyruvate, and it facilitates anaerobic metabolism. LDH-1 is typical of cardiac muscle while LDH-5 is typical of skeletal muscles, particularly those adapted for anaerobic conditions during contraction. In skeletal muscles, the ratio of LDH-1 to LDH-5 corresponds to the dominant activity pattern of a muscle. For example, muscles capable of sustained activity and which only use aerobic metabolism have high LDH-1. LDH-5 is the dominant isoenzyme in skeletal muscles from slaughter-weight pigs.

12.5 Regulation of glycogenolysis

There are two conflicting requirements complicating the activation of phosphorylase. Firstly, since phosphorylase initiates the release of considerable amounts of chemical energy, there must be safeguards to prevent its uncontrolled activity. In stress-susceptible pigs, for example, the uncontrolled activity of anaerobic glycolysis may lead to excessive heat production and to a level of acidity that may soon prove fatal. The conflicting requirement is vast amounts of phosphorylase spread through the muscle mass must be rapidly activated by relatively small amounts of adrenaline. The adrenaline activation of severely frightened animals often is called the "fight or flight" response: neither of these responses is likely to be of much survival value if the anaerobic energy supply to body muscles is delayed.

The conflicting demands for fail-safe but rapid activation are satisfied by two particular features of the activation system.

  1. The conversion of phosphorylase b to phosphorylase a is inhibited locally in each myofibre by high concentrations of ATP and glucose-6-phosphate (an intermediate in the conversion of glycogen to lactate). Thus, if the energy released by phosphorylase is not rapidly consumed, the energy release system shuts down. If the energy is used, however, AMP and phosphate further enhance the activation of phosphorylase.
  2. To enable the rapid activation of phosphorylase throughout the musculature, the relatively small amounts of adrenaline arriving at the muscle initiates a series of biochemical changes functioning as an amplifier. A small input leads to a large output.

Remember the sarcoplasmic reticulum?

When animals require energy anaerobically during normal activity, phosphorylase b kinase is activated by calcium ions released from the sarcoplasmic reticulum - normally the trigger for muscle contraction in living muscle. Glycogen granules are closely related to the sarcoplasmic reticulum and to glycogenolytic enzymes as part of a structural complex. The activation system linking muscle contraction to glycogenolysis is short lived to avoid the continuous use and depletion of glycogen reserves. Glycogen is a rapidly available energy source for both brief muscle activity and the early stages of sustained activity. Phosphorylase activity is curtailed by phosphatase, which dephosphorylates phosphorylase a and phosphorylase b kinase a, when muscle activity ceases or an animal recovers from fright. Many features of the system for activating phosphorylase are shared with the activation system for glycogen synthesis, but the shared features are opposite in effect. Thus, the myofibre does not attempt to synthesize new glycogen at the same time that it is breaking it down, and vice versa.

Newborn pigs often have difficulty maintaining their blood glucose levels, and may die of hypoglycaemia after even short periods of starvation. Given they have high glycogen levels at birth, this is not easy to explain, although stored glycogen may simply be inadequate in amount if milk is not available. Despite an adult predisposition to the excessive accumulation of adipose tissue, newborn pigs have very little adipose tissue and liberation of free fatty acids is severely limited. Most phosphorylase is in the relatively inactive b form just after birth. In adult pigs, hyperglycaemia readily occurs in response to exercise and adrenaline secretion.

12. 6 Fate of lactate in living animals

Meat animals may exhibit a considerable range in their general muscular and cardiovascular fitness. At one extreme, sheep and cattle may roam in search of feed and water for part of the year, coping with adverse conditions much like wild animals. At the other extreme, animals may be reared in close confinement, partly to prevent them wasting feed energy by converting it to muscular work. Pigs rarely exercised may have a low oxygen transport capacity whereas regular exercise reduces lactate production.

In living animals, the lactate produced by contracting muscles is removed by the circulatory system, but first it must travel from the myofibre into the interstitial space. Lactate release may be retarded in situations such as exhaustive exercise. A fraction of the lactate leaving a myofibre may be in the form of undissociated lactic acid, and this fraction may increase with a rise in external pH. In living animals, blood flow is increased in active muscles (hyperaemia). When hyperaemia occurs as a secondary response to hypoxia, it may be mediated by the release of adenosine (from AMP A + P) into the interstitial fluid between myofibres.

On arriving in the liver, lactate may be converted to glucose-6-phosphate, and then stored as glycogen or released as glucose back into the blood. Circulating glucose is available to muscles to be stored as glycogen or used directly as energy for contraction (Cori cycle). After exhaustive exercise, an animal may continue to consume oxygen at an increased rate for some time (oxygen debt). During this recovery period, some lactate may be completely oxidized to release enough energy for the remaining lactate to be converted to glucose (gluconeogenesis) in the liver. When an animal is exsanguinated, the blood can no longer perform its transport function in the Cori cycle, and lactate accumulates in the musculature. There are, however, a number of other possible fates for lactate in living animals.

Heart muscle receives its own blood supply from the coronary artery, straight from the aorta, and cardiac muscle may use circulating lactate as an energy source. The high oxygen concentration in the aorta usually allows complete oxidation of lactate by cardiac muscle. Any lactate remaining in the aorta may not all get pumped to the liver or kidneys to participate in the Cori cycle, because the aorta supplies other major arteries in addition to the hepatic and renal arteries. Much of the circulating lactate may pass through non-contracting muscles oxidizing lactate as an energy substrate, particularly if circulating fatty acids are unavailable. Resting muscles also may take up circulating lactate and reconvert it directly to glycogen, probably via a pathway which is independent of the mitochondrial Krebs' cycle (the evidence is in research papers even if not in your biochemistry book). Finally, also it is possible for all the lactate consumed during an oxygen debt period to be oxidized to carbon dioxide.

Contraction causes an initial drop in myofibre pH because of the hydrolysis of ATP, but this may be followed by an increase in pH from the breakdown of creatine phosphate (CP). CP acts as a short term store of energy since it has a phosphate  transferable to ADP  by the creatine phosphokinase (CPK). CP is the dominant carrier of energy from mitochondria to myofibrils. CPK and creatine are normally contained within myofibres, but may leak into the blood from damaged or diseased myofibres. In healthy muscle, creatine is slowly but continuously converted to creatinine. Creatinine is lost in the urine in approximate proportion to the muscle mass of the body. The production of lactate following muscle contraction may cause another drop in pH. The interstitial pH between muscle fibres and the pH at the muscle surface are profoundly affected by overall pH levels in the blood.

12.7 Effect of pH on water-holding

Further information

Structure and Development of Meat Animals and Poultry.  Pages 495-548.