A period of time normally elapses between the slaughter of an animal and consumption of the meat. In practical terms, the carcass cools down and becomes stiffer or ‘sets’, the surface dries and the fat becomes firmer. With time, the texture and flavour of the lean improve.
These effects are accompanied by significant biochemical changes in the muscles: acidification, the development of rigor mortis and, later, the gradual resolution of rigor and tenderization of the meat by a process referred to as conditioning.
meet the demands of contracting muscle. Glycogen is then used to supplement the blood-borne metabolites.
Glycogen and glucose are broken down essentially by the same process. This involves the operation of three interrelated processes:
glycolysis, oxidative decarboxylation and oxidative phosphorylation.
These together lead to the complete oxidation of one molecule of glucose (C6H12O6) by six molecules of oxygen to six molecules of carbon dioxide (CO2) and six molecules of water (H2O). Overall, the reaction can be described as:
C6H12O6+ 6 O2→6 CO2+ 6 H2O.
In practice the whole process is extremely complex but only an understanding of the outline of the system is necessary for our current purposes. In glycolysis, a glucose molecule (or a glucose moiety from glycogen) containing six carbon atoms is broken down into two pyruvate molecules each containing three carbon atoms. The process generates either two or three ATP molecules and four hydrogen atoms carried as reduced nicotine adenine dinucleotide (NADH). The net yield of ATP differs between glucose and glycogen because the start of the glycolytic pathway requires that the hexose be phosphorylated. An ATP molecule is required to do this for glucose (the reaction is endergonic) but the phosphorylation of glycogen is exergonicand consequently an extra ATP is produced. The full pathway of glycolysis is shown in Fig.
5.1. In decarboxylation, the carbon atoms in the pyruvate are removed as carbon dioxide, in the process generating 20 hydrogens carried as NADH and on other carrier molecules (flavin adenine dinucleotide, FAD). This process operates as a cycle, known as the Krebs or tricarboxylic acid (TCA) cycle, the pyruvate first being converted to acetyl coenzyme A. It is at this point that fatty acids would enter the system, being first converted by !-oxidation to acetyl coenzyme A. The last process is oxidative phosphorylation. In this, the 24 hydrogens generated by glycolysis and oxidative decarboxylation are oxidized by molecular oxygen in the cytochrome system. For each pair of hydrogens, three ATP molecules are produced. Therefore, in total, breakdown of one glucose molecule produces a further 36 ATP in addition to those produced by glycolysis. A summary of the three processes involved in the production of ATP is given in Table 5.1.
Glycolysis takes place in the sarcoplasm; the enzymes which catalyse the other processes are located in the mitochondria. Operation of the whole system requires aerobic conditions – six oxygen molecules are needed to oxidize each glucose molecule. Under anaerobic conditions only the glycolytic part of the system can operate. Normally this only occurs during very heavy exercise. Under these conditions, the pyruvate is reduced by the accumulated NADH to form lactic acid, the reaction being catalysed by the enzyme lactate dehydrogenase. This
is what happens in a runner in a 100 m sprint race where the energy required by the leg muscles could not be produced oxidatively because of the inability of the blood to deliver sufficient oxygen to the muscles quickly enough. Instead, for every glycogen moiety, equivalent to a glucose molecule, two lactic acid molecules are produced:
(C6H10O5)n+ nH2O →2nC3H6O3.
glycogen lactic acid
This level of muscular activity could not be sustained for very long because the build-up of lactic acid in the blood would lower its pH to an unacceptable level.
Mobilization of glycogen when it is needed
It is important that muscle glycogen can be broken down to release energy for contraction very quickly if, for example, the animal needs to
Glucose Glycogen
Glucose 6-phosphate Glucose 1-phosphate Fructose 6-phosphate
Fructose 1,6-diphosphate
Glycerol 1,3-diphosphate Glyceric acid 1,3-diphosphate Glyceric acid 3-phosphate Phosphoenol pyruvic acid Pyruvic acid lactic acid
Glyceraldehyde 3-phosphate Dihydroxyacetone phosphate
Fig. 5.1. The glycolytic pathway.
Table 5.1. The production of ATP from the oxidation of glucose.
1. Glycolysis: 1 glucose →2 ATP + 4 H + 2 pyruvate 2. Oxidative decarboxylation: 2 pyruvate →20 H + 6 CO2
3. Oxidative phosphorylation: 24 H + 6 O2→36 ATP + 6 H2O
run away from a threat like a predator. The hormone adrenaline (epinephrine), secreted in response to an external stressor (e.g. fear) promotes glycogen breakdown (glycogenolysis) through a series of steps which result in activation of the enzyme catalysing the first stage in glycogenolysis. This enzyme is phosphorylase. It catalyses the break- down of glycogen to glucose-1-phosphate and is present in large amounts in muscle. It exists in two forms, an active form (phosphorylase a) and an inactive form (phosphorylase b). Conversion of phosphorylase b to phosphorylase a is by the enzyme protein kinase. Protein kinase is itself activated by cyclic adenosine monophosphate (cyclic AMP), produced when adenylate cyclase is converted into an active form by adrenaline. The overall chain of events leading to glycogen breakdown is therefore as summarized in Fig. 5.2.
Therefore, whenever glycogen is likely to be needed rapidly for energy production by glycolysis it is mobilized through the activation of phosphorylase and this can be through the secretion of adrenaline during a stressful episode. In non-stressful situations, phosphorylase b is activated by other mechanisms. These include increased concentrations of metabolites such as AMP and inorganic phosphate, and a decreased concentration of ATP. During normal muscle stimulation by the nervous system, calcium ions also promote the activation of phosphorylase.
Energy storage
ATP stores energy as an energy-rich phosphate bond. The energy is liberated by releasing the phosphate to give adenosine diphosphate
Fear
Adrenaline
Adenylate cyclase
Cyclic AMP
Protein kinase
Phosphorylase b
Phosphorylase a
Glycogen breakdown
Fig. 5.2. The promotion of glycogen breakdown by adrenaline.
(ADP). As mentioned in Chapter 3, the reaction is in effect a hydrolysis and is reversible:
ATP + H2O !"ADP + H3PO4.
This is a simplified outline of the reaction. For example, ATP is actually complexed to magnesium ions. ADP can itself be hydrolysed to adenosine monophosphate (AMP). Perhaps surprisingly, bearing in mind its pivotal role in the operation of muscles, the concentration of ATP in the tissue is very low – only enough for a few brief ‘twitches’ or contractions. A typical concentration would be 5–7 mmol kg"1 of muscle and this would support contraction for only a few seconds at most. However, the concentration is maintained effectively constant by the reaction:
CP + ADP !"C + ATP.
In this, creatine phosphate, or phosphocreatine (CP), reacts with ADP to form ATP and creatine (C). The reaction is reversible but the equilibrium is to the right at neutral pH so that, immediately ATP is removed, more is generated so long as CP is available. The reaction is catalysed by the enzyme creatine kinase (CK) which is abundant and active in muscle. Therefore, the ATP used in contraction is restored almost instantly. The levels of CP in muscle are much higher than those of ATP (Jeacocke, 1984), although they can drop during exhaus- tive exercise. They are replenished by a ‘reversal’ of the above reaction during a recovery period when ATP is generated from muscle glycogen, or from blood-borne metabolites, and the ATP:ADP ratio rises. The levels of glycogen in most muscles (10–20 mg g"1) are sufficient for many thousands of ‘twitches’. A full account of the biochemistry of energy production in muscles can be found in Newsholme and Start (1973).