fate of proteins Flashcards
what are functions of protiens
- transport hydrophobic compounds in the blood
- cell adhesion molecules that attach cells to each other and to the extracellular matrix
- hormones that carry signals from one group of cells to another
- ion channels through lipid membranes
- enzymes that increase the rate of biochemical reactions
discuss protein metabolism after eating
Once an animal has digested the ingested proteins to amino acids in the stomach and small intestine, the amino acids are transported to the liver via the hepatic portal vein.
Amino acids can be used in the liver to either synthesis new proteins (biosynthesis) or it can be converted to energy sources via gluconeogenesis.
Amino acids will also enter the general circulation and go into tissues, where they can be utilised to synthesise new proteins.
how are proteins digested
Dietary proteins are cleaved to amino acids by proteases. Pepsin acts in the stomach, and the proteolytic enzymes produced by the pancreas (trypsin, chymotrypsin, elastase, and the carboxypeptidases) act in the lumen of the small intestine.
Aminopeptidases, dipeptidases and tripeptidases associated with the intestinal epithelial cells complete the conversion of dietary proteins to amino acids, which are absorbed into the intestinal epithelial cells and released into the hepatic portal vein.
discuss what happens amino acids in the liver
The amino acids derived from dietary proteins travel from the intestine to the liver in the hepatic portal vein. The liver uses amino acids for the synthesis of serum proteins as well as its own proteins, and for the biosynthesis of nitrogen-containing compounds that need amino acid presursors, such as the nonessential amino acids, heme, hormones, neurotransmitters, and purine and pyrimidine bases (e.g., adenine and cytosine in DNA).
The liver also may oxidize the amino acids to produce energy via the Krebs cycle in the fed state. This process occurs due to high levels of blood insulin, which is a result of elevated blood glucose levels after eating.
discuss what happens to amino acids in the circulation
Many of the amino acids will go into the peripheral circulation, where they can be used by other tissues for protein synthesis and various biosynthetic pathways, or oxidized for energy. Proteins undergo turnover; they are constantly being synthesized and degraded, especially in muscle tissue.
The amino acids released by protein breakdown enter the same pool of free amino acids in the blood as the amino acids from the diet. This free amino acid pool in the blood can be used by all cells to provide the right ratio of amino acids for protein synthesis or for biosynthesis of other compounds.
discuss how protein is matabolised in the fasting/starved state
Enter a Description for Accessibility
The fasting and starved state results in a decrease in the concentration of glucose in the blood stream. This decrease in blood glucose is detected by receptors in the pancreatic islet cells, which in turn increase the secretion of glucagon and inhibit the secretion of insulin. To increase blood glucose levels, the body will breakdown the glycogen stored to glucose and release it into the bloodstream.
However this is only a short term measure as there are limited glycogen stores. The body then looks to convert non-carbohydrates into glucose, through gluconeogenesis. One of the substrates which can be used for gluconeogenesis are amino acids. As fasting progresses, gluconeogenesis becomes increasingly more important as a source of blood glucose. After a few hours or so of fasting, liver glycogen stores are depleted and gluconeogenesis is the only source of blood glucose. The main substrate for gluconeogenesis is protein breakdown from skeletal muscles.
discuss protein metabolism in the brief fasting state
Lactate, glycerol, and amino acids serve as carbon sources for gluconeogenesis. Amino acids are supplied by muscle. Some go straight into the circulation to the liver, but others are partially oxidized and the nitrogen stored in the form of alanine and glutamine, which enter the blood.
In the kidney, glutamine releases ammonia into the urine and is converted to alanine and serine. In the cells of the gut, glutamine is converted to alanine. Alanine (the major gluconeogenic amino acid) and other amino acids enter the liver, where their nitrogen is converted to urea, which is excreted in the urine, and their carbons to glucose and ketone bodies, which are oxidized by various tissues for energy.
discuss protein metabolism in the prolonged fasting state
If the pattern of fuel utilization that occurs during a brief fast were to persist for an extended period, the body’s protein would be quite rapidly consumed to the point at which critical functions would be compromised. Fortunately, metabolic changes occur during prolonged fasting that conserve (spare) muscle protein by causing muscle protein turnover to decrease (blue dotted line).
After 3 to 5 days of fasting, when the body enters the starved state, muscle decreases its use of ketone bodies and depends mainly on fatty acids for its fuel. The liver, however, continues to convert fatty acids to ketone bodies. The result is that the concentration of ketone bodies rises in the blood (blue solid line).
The amino acid pool, produced by the breakdown of protein, continues to serve as a major source of carbon for gluconeogenesis. A fraction of this amino acid pool is also being used for biosynthetic functions (e.g., synthesis of heme and neurotransmitters) and new protein synthesis, processes that must continue during fasting. However, as a result of the decreased rate of gluconeogenesis during prolonged fasting, protein is “spared”; less protein is degraded to supply amino acids for gluconeogenesis. While converting amino acid carbon to glucose in gluconeogenesis, the liver also converts the nitrogen of these amino acids to urea. Consequently, because glucose production decreases during prolonged fasting compared with early fasting, urea production also decreases.
