Metabolism 101
By: Jamie Hale
The following article takes a look at the basic concepts applying to human metabolism.
Metabolism- sum of all the chemical changes occurring in a cell, a tissue, or the body.
Reactions occur through various pathways. In these pathways a cascade of events occur as one reaction results in another reaction. Most pathways can be classified as Catabolic( break down of complex molecules into simpler molecules) or Anabolic (complex products are formed from simple precursors). Each pathway is composed of multienzyme sequences, and each enzyme, in turn may exhibit important catalytic or regulatory features.
Catabolic pathways Catabolic reactions occur to capture chemical energy in the form of ATP from the degradation of energy rich fuel molecules. Catabolism also allows molecules in the diet or nutrient molecules stored in cells to be converted into building blocks needed for the synthesis of complex molecules. Catabolism provides the chemical energy necessary for the maintenance of the living cell. Generally, energy generation from degradation of complex molecules occurs in three stages.
Stages of Catabolism 1) complex molecules are broken down into simpler substances, proteins are degraded to amino acids, polysaccharides are degraded to monosaccharides, and triaglycerols to free fatty acids and glycerol
2) In the second stage, these diverse building blocks are further degraded to acetyl coa and a few other simpler molecules. Some energy is captured as ATP, but the amount is small compared with energy produced during third stage of catabolism
3) The tricarboxylic acid cycle is the final common pathway in the oxidation of fuel molecules such as acetyl CoA
Catabolic Hormones Because it is counterproductive to have anabolic and catabolic processes occurring in cells simultaneously, (this is why it is hard to gain muscle to a significant degree while losing fat, different processes) there are many signals that switch on anabolic processes while switching off catabolic processes and vice versa. Most of the known signals are hormones and the molecules involved in metabolism itself. Endocrinologists have traditionally classified many of the hormones as anabolic or catabolic.
Catabolic hormones Cortisol is a corticosteroid hormone that is involved in the response to stress; it increases blood pressure and blood sugar levels and suppresses the immune system. Synthetic cortisol, also known as hydrocortisone, is used as a drug mainly to fight allergies and inflammation.
Glucagon is a 29 amino acid polypeptide acting as an important hormone in carbohydrate metabolism. Glucagon helps maintain the level of glucose in the blood by binding to specific receptors on hepatocytes causing the liver to release glucose which is stored in the form of glycogen through a process known as glycogenolysis. As these stores become depleted, glucagon then encourages the liver to synthesize additional glucose by gluconeogenesis. This glucose is released into the bloodstream
Catecholamines are chemical compounds derived from the amino acid tyrosine that act as hormones or neurotransmitters. The most abundant catecholamines are epinephrine (adrenaline), norepinephrine (noradrenaline) and dopamine. They are produced mainly from the adrenal medulla and the postganglionic fibers of the sympathetic nervous system. Adrenaline acts as a neurotransmitter in the central nervous system and as a hormone in the blood circulation. Noradrenaline is primarily a neurotransmitter of the peripheral sympathetic nervous system but is also present in the blood (mostly through "spillover" from the synapses of the sympathetic system).
Anabolic Hormones Anabolic reactions combine small molecules, such as amino acids, to form complex molecules, such as proteins. Anabolic reactions require energy, which is generally provided by the breakdown of ATP, to ADP and Pi. Anabolism is divergent process in which a few biosynthetic precursors form a wide variety of polymeric or complex products.
Anabolic Hormones Growth hormone is a polypeptide hormone synthesized and secreted by the anterior pituitary gland which stimulates growth and cell reproduction in humans and other vertebrate animals. Stimulators of GH release include (among others) sleep, exercise, hypoglycemia, dietary protein, and estradiol. Inhibitors of GH secretion include dietary carbohydrate and glucocorticoids.
'Insulin-like growth factor 1' (IGF-1) is a protein hormone similar in molecular structure to insulin. IGF1 is produced by the liver and target tissues. Production is stimulated by growth hormone and retarded by undernutrition. Its primary action is mediated by binding to specific IGF receptors present on many cell types in many tissues. The signal is transduced by intracellular events. The effect is the promotion of cell growth and multiplication.
Insulin (Latin insula, "island", as it is produced in the Islets of Langerhans in the pancreas) is a polypeptide hormone that regulates carbohydrate metabolism. It is involved in many other functions including comprehensive nutrient storage. Refer to Insulin The Hormone http://www.maxcondition.com/page.php?24 for in depth info concerning insulin. Its concentration (more or less, presence or absence) has extremely widespread effects throughout the body.
Testosterone is a steroid hormone from the androgen group. Testosterone is secreted in the testes of men and the ovaries of women. In general, androgens promote protein synthesis and growth of those tissues with androgen receptors. Testosterone effects can be classified as virilizing (Masculinisation. Signs may include: increased body or facial hair, change in voice, clitoral enlargement and male-type baldness.and anabolic effects, although the distinction is somewhat artificial, as many of the effects can be considered both. Anabolic effects include growth of muscle mass , increased bone density and strength, and stimulation of height growth and bone maturation.
A sophisticated communication system regulates metabolic state. Other factors that influence metabolism are neurotransmitters, enzymes and nutrient availability. These in turn influence signals generated intra and intercellularly.
The information provided above is very basic in terms understanding metabolism. For an in depth look at metabolism (very technical ) look at Champe, P, Harvey, R, Ferrier, D (2005) Lippincotts Illustrated Reviews Biochemistry 3rd Edition. LWW. There are various other texts available as well.
References Champe, P, Harvey, R, Ferrier, D (2005) Lippincotts Illustrated Reviews Biochemistry 3rd Edition. LWW.
Gropper, S (2000). The Biochemistry of Human Nutrition. Wadsworth.
Wikpedia.Org www.wikpedia.org
Copyright 2005 Jamie Hale www.maxcondition.com
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