Protein is generally not used as a primary energy source due to its importance in other bodily functions. Using protein for energy routinely could lead to serious health issues.
However, in extreme energy deprivation, proteins can be utilized for energy.
Proteins must be digested and absorbed to provide a reservoir of amino acids required for protein synthesis by the cell.
Proteolysis (Protein Breakdown)
Begins in the stomach with pepsin.
Continues in the pancreas with pancreatic proteases:
Trypsin
Chymotrypsin
Carboxypeptidases A and B
These are secreted as zymogens (inactive precursors).
Protein digestion is completed by small intestinal brush border enzymes:
Dipeptidase
Aminopeptidase
Main end products of protein digestion:
Amino acids
Dipeptides
Tripeptides
Absorption of amino acids and small peptides through the luminal membrane:
Accomplished by secondary active transport linked to sodium.
At the basal membrane:
Simple and facilitated diffusion transports amino acids into the bloodstream.
Amino Acid Transport
Major transport mechanisms are involved in moving amino acids across the luminal and basal membranes of intestinal cells.
Protein as Energy Source
Protein obtained from the diet or the body (during prolonged fasting or starvation) can be used as an energy source.
Body protein is catabolized primarily in muscle and liver.
Amino acids released from proteins usually lose their amino group through transamination or deamination.
The remaining carbon skeleton can be used for energy.
Amino Acid Classification
Amino acids are classified by their ability to turn into specific metabolic intermediates.
Glucogenic amino acids: All except leucine and lysine can be converted into glucose through gluconeogenesis.
Ketogenic amino acids: Leucine and lysine, as well as isoleucine, phenylalanine, threonine, tryptophan, and tyrosine (which are also glucogenic), can be converted into acetyl CoA and ketone bodies.
Nitrogen Excretion
Amino groups removed by transamination or deamination constitute a potential toxin (ammonia).
The urea cycle occurs in the liver and is the body's primary way of removing excess nitrogen from the body.
Urea Cycle
The MCAT is unlikely to test the steps and intermediates directly but is provided for reference.
The fate of each amino acid's side chain depends on its chemistry.
Basic amino acid side chains feed into the urea cycle.
Other side chains act like the carbon skeleton and produce energy through gluconeogenesis or ketone production.
Conclusion
The lecture has reviewed all of the vital metabolic processes of the cell.
Overview of the chapter contents included dietary lipids and different ways that lipids are metabolized in the cell, lipid transport in blood and lymphatic fluid, the mobilization of lipids from adipocytes also the structure, synthesis, and breakdown of fatty acids required to address the energy needs of the cell.
Importance of ketone bodies and how they are utilized by the cell during periods of starvation and the digestion and metabolism of proteins and amino acids were also reviewed.
Integration of Metabolism
Metabolism of different macromolecules does not occur in isolation. Acetyl CoA produced in fatty acid oxidation regulates the pyruvate dehydrogenase complex and pyruvate carboxylase to create a shift in carbohydrate metabolism from glycolysis and the citric acid cycle to gluconeogenesis.