Bke2 Biochemistry Lectures

Lecture 25: Physiological aspects on metabolism
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Abstract:
Muscle metabolism
Skeletal muscle consists of long, thin muscle fibres, each of which is a single, unusually large cell formed by fusion of many separate cells. A muscle cell contains many parallel myofribrils. The myofribils are built up of two kinds of interacting protein filaments, the thick and thin filament. Thick and thin filaments slide past each other during contraction and the muscle can be shortened as much as a third of its original length as it contracts. Energy for muscle contraction comes from the hydrolysis of ATP to ADP and inorganic phosphate.

The skeletal muscles consist of different types of muscle fibres. The dominating fibre type varies depending on the type of muscle. The muscle fibres are divided in three main groups related to their metabolic characteristics. 1) slow oxidative (SO), 2) fast oxidative glycolytic (FOG) and 3) fast glycolytic (FG).

Beside the common metabolic pathways as glycolysis and the citric acid cycle, the muscle uses creatine phosphate to phosphorylate ADP to ATP during periods of muscular exertion.

Physical training can change the enzymatic activities in the different metabolic pathways. An increased enzyme activity in fat uptake, b-oxidation and in the citric acid cycle is achieved by continuous sub maximal training. Maximal training (anaerobic training) increases the enzyme activities for production of lactic acid, and for breaking down glycogen and glucose.

Acetonemia
In dairy cows, acetonemia may occur during the first six to eight weeks of the lactation period. The clinical signs of acetonemia are low blood glucose concentration, high levels of ketone bodies, decreased intake of feed and loss of weight. The reason why acetonemia appears relatively often in cows with high milk yield is primarily the large demand of glucose for synthesis of lactose by the mammary gland in relation to the supply of glucose.

b-oxidation as a way to survive
Many animals depend on fat stores for energy during hibernation or dormancy, during migratory periods, and in other situations involving radical metabolic adjustments. The grizzly bear for example, which goes into a continuos state of dormancy for periods of seven months, uses body fat as it sole fuel during hibernation. The oxidation of fat yields sufficient energy for maintaining body temperature, for active synthesis of amino acids and proteins, and for other energy-requiring activities, such as membrane transport. Fat oxidation also releases large amounts of water, which replenishes water loss during breathing.

Urea formed during the degradation of amino acids is reabsorbed and recycled by the bear, the amino groups being used to make new amino acids for maintaining body proteins.

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