As you are already aware, amino acids are the building blocks that are used to synthesize proteins. However, amino acids also serve other functions in the cell which are independent of proteins. In this section, we will focus on three aspects of this type of metabolism:
the conversion of amino acids to other biologically active molecules, and
The role of amino acids in the removal of ammonia,
the conversion of amino acids to neurotransmitters.
The Digestion of Proteins into Amino Acids
When proteins are ingested during a meal, they must be cleaved into individual amino acids, which are then absorbed. This process is partially completed in the stomach, and completed in the small intestine, as described below. Gastric digestion was first studied in the 1800's, when a French-Canadian fur trapper named Alexis St. Martin presented himself to doctors at the fort situated at Mackinaw Island, MI. St. Martin had sustained a shotgun wound to the abdomen, and when the wound healed, there was a hole which allowed direct observation of the inside of his stomach. Dr. William Beaumont was the post physician, and spent a great deal of time observing the digestion of various materials through this hole. Ultimately, St. Martin got tired of being a guinea pig, and he disappeared.
We now know that the digestion of peptides in the stomach is mediated by the peptidase pepsin. Pepsin is synthesized by parietal cells in the stomach as a proenzyme known as pepsinogen. When proteins enter the stomach, a peptide hormone known as gastrin is secreted from the gastric mucosa, stimulating the secretion of gastric fluid. This fluid contains pepsinogen, and at pH 1.5 pepsinogen is converted to its active form, pepsin. Pepsin cleaves peptides on the carboxyl side of the aromatic amino acids PHE, TYR and TRP.
When the bolus of food moves into the small intestine, another peptide hormone, secretin is released into the bloodstream. This hormone causes the intestinal mucosa to release bicarbonate, which acts to raise the pH of the bolus entering from the stomach. In addition, the hormone cholecystokinin is released into the bloodstream, which causes the secretion of pancreatic fluid into the intestine. This fluid contains the zymogens trypsinogen and chymotrysinogen, which are cleaved to produce trypsin and chymotrypsin, respectively. The peptidase enterokinase, also present in pancreatic fluid, processes trypsinogen to trypsin, and trypsin in turn converts chymotrypsinogen to the active form, chymotrypsin. Trypsin cleaves peptides on the carboxy side of the amino acids LYS and ARG, and chymotrypsin cleaves peptides on the carboxyl side of the aromatic amino acids PHE, TYR and TRP.
In addition to the enzymes mentioned above, pancreatic fluid contains three additional zymogens, procarboxypeptidases A and B and proelastase. Trypsin converts procarboxypeptidases A and B to carboxypeptidase A and B, which sequentially cleave amino acids from the carboxyl end of peptides. Trypsin also converts proelastase to elastase, which cleaves peptides on the carboxyl side of the amino acids GLY and ALA. It should be mentioned that the pancreatic cells contain a molecule known as pancreatic trypsin inhibitor, which prevents the premature cleavage of the zymogens mentioned above to their active forms. In a condition known as acute pancreatitis, these zymogens are prematurely converted to the active form inside the pancreatic cells, resulting in destruction of the pancreas which is painful and potentially life threatening.
There is a disease known as gluten-sensitive enteropathy which is caused by a lack of enzymes needed to cleave N-glutamyl peptides in the small intestine. This disease is also known as non-celiac sprue or adult celiac disease. These patients are sensitive to glutens, which are glutamine-containing peptides from wheat, oats, barley and rye. These individuals must maintain a gluten-free diet.
Once produced from proteins, intestinal amino acids are absorbed by active transport in the small intestine. These amino acids are delivered to the liver, where they undergo a variety of transformations. Approximately one third of the amino acids used by the body are exogenous (i.e. absorbed from food), while two thirds are endogenous (i.e. recycled from the turnover of body protein. The major fate of amino acids is protein synthesis, but as we shall see below, some amino acids are used for other biological functions. Interestingly, some peptides, such as botulism toxin, can be absorbed intact, and as such can mediate toxicity or allergic reactions.
The Metabolic Fate of Amino Acids
The synthesis of amino acids in mammals is complex, and beyond the scope of this document. Suffice it to say that some of the 20 amino acids can be synthesized, and as such are termed non-essential amino acids, while others must be derived from the diet, and are hence termed essential amino acids. A summary of the synthesis of amino acids and their precursors appears below.
Each amino acid has a specific aminotransferase enzyme, also known as a transaminase. As shown below, these enzymes convert an amino acid and an alpha-keto acid to a second amino acid and a second alpha-keto acid. In this freely reversible reaction, it appears that an R group is "switched" from one analogue to another. The transamination process requires pyridoxal phosphate. Using transamination, amino acids can enter carbohydrate metabolism through the Krebs Cycle. For example, ALA can be transaminated to pyruvate, GLU can be transaminated to alpha-ketoglutarate, and ASP can be transaminated to oxaloacetic acid. Two of these transaminases, SGOT (serum glutamate oxaloacetate transaminase) and SGPT (serum glutamate pyruvate transaminase) can be used to diagnose cardiac or hepatic damage, since they are released following cell damage in these tissues.
Amino acids can be catabolized to intermediates in the fatty acid oxidation and glucose metabolic pathways. The catabolic pathways are complex and beyond the scope of this course, and as such, these transformations are summarized below. As you can see, some amino acids can be converted to a form in which they can produce glucose through gluconeogenesis. The amino acids are termed glucogenic. Others can produce acetoacetyl CoA, and as such are precursors to the ketone bodies. These amino acids are known as ketogenic. The division between ketogenic and glucogenic amino acids is complex, and many can perform both functions.
There are a number of genetic enzyme defects which affect amino acid catabolism. An important example is the condition known as phenylketonuria, the biochemistry of which is shown below. Normally, a great deal of phenylalanine is converted in vivo to tyrosine, which is then used for protein synthesis or in the synthesis of neurotransmitters (see below). However, in patients with phenylketonuria, the enzyme that forms tyrosine, phenylalanine-4-monooxidase is lacking. In this case, a minor pathway takes over, producing large amounts of phenylpyravate. When phenylpyruvate is formed in excess, it enters the CNS, where it can cause mental retardation.The treatment for this syndrome is a low phenylalanine diet, and infants with this condition must be placed on a phenylalanine-free formula (Lofenelac¨). Adults with this syndrome must avoid aspartame (NutraSweet¨), since it contains phenylalanine. In the US, all infants are screened for this defect shortly after birth.
There are a large number of genetic enzyme deficiencies, some of which are benign, and others that are lethal. Some of the more common of these diseases appear below:
Amino acids may serve as precursors to a wide variety of cellular intermediates. Some of these processes will be mentioned but not discussed in detail, while others (which are more relevent to drug therapy) will be discussed in detail. The amino acid glycine is an important precursor of the porphyrins, which are constituents of hemoglobin and the cytochrome P450 family of enzymes. Metabolism of the heme portion of hemoglobin produces the water soluble molecule bilirubin, which binds to plasma albumins and is transported to the liver. In the liver, bilirubin is glucuronidated and excreted in the bile. When the secretion of bile is blocked, or when liver disease is present, bilirubin leaks into the blood, resulting in a yellowing of the skin and eyes. This condition is know as jaundice. The compound phosphocreatine, which is an energy storage form in muscle, is synthesized from glycine and arginine.
The oligopeptide known as glutathione is synthesized from glutamate, cysteine and glycine. Reduced glutathione (GSH) is an important molecule in the cell, since it protects cellular biomolecules from damage due to peroxides and other intermediates. It also functions to keep the sulfhydryl sidechains of proteins in their reduced for. As shown below, 2 molecules of glutathione react with the oxidized sulfhydryl or peroxide in the presence of glutathione peroxidase, affording the reduced product and oxidized glutathione, a disulfide dimer known as GSSH. The oxidized form must be converted back to GSH using the NADPH-dependent enzyme glutathione reductase.
Glutathione is also important in protecting cellular biomolecules agains damage from reactive metabolites of drugs and other compounds from the environment. This is a particular problem in the case of acute acetaminophen overdose, as shown below. When excessive amounts of acetaminophen are ingested, the normal hepatic pathway for metabolism and excretion of acetaminophen becomes saturated, and the mixed function oxidases convert it to a reactive metabolite called the arylating intermediate (below). Because it is present in large quantities, it reacts with all of the available glutathione until the supply is exhausted. Since there is no glutathione to protect the liver, the result is potentially severe hepatic damage.
Amino Acid Derived Neurotransmitters
There are a number of biologically important amines which are produced from amino acids by specific pyridoxal-phosphate-dependent amino acid decarboxylases. One of the most important examples of this is the synthesis of epinephrine and norepinephrine, shown in the figure below. The process begins with the hydroxylation of PHE by phenylalanine-4-monooxygenase, which produces TYR. TYR is then hydroxylated in the meta position by tyrosine hydroxylase, producing an intermediate known as L-DOPA. The enzyme dopa decarboxylase then produces dopamine, an important neurotransmitter in the CNS. Dopamine is then hydroxylated in the beta position (dopamine-beta-hydroxylase) to afford norepinephrine (also an important CNS neurotransmitter). S-adenosylmethionine- and phenylethanolamine-N-methyltransferase dependent N-methylation of norepinephrine then produces epinephrine, which acts as a neurotransmitter in the peripheral tissues.
There are two drugs which are currently marketed that interfere with the epinephrine pathway. In Parkinson's Disease, patients suffer from a decrease in dopamine in certain regions of the brain. Dopamine is unable to cross the blood-brain barrier, but its precursor, L-DOPA, does penetrate the CNS. Therefore, Parkinson's patients are given L-DOPA, which is converted to dopamine in the brain. To prevent the conversion of L-DOPA to dopamine in the peripheral tissues, L-DOPA is co-administered with carbidopa, an analogue which inhibits dopa decarboxylase in the periphery, but NOT in the CNS.
A second drug that interacts with the epinephrine pathway is alpha-methyldopa, which acts as a peripheral inhibitor of dopa decarboxylase, and is marketed as an antihypertensive agent. Interestingly, this compound is also a substrate for dopa decarboxylase, and is also able to cross the blood-brain barrier. Once in the CNS, it is converted to a false neurotransmitter, alpha-methylnorepinephrine, which blocks the action of norepinephrine in the CNS, lowering the patients blood pressure.
There are two enzymes which are responsible for metabolizing epinephrine and norepinephrine, with the goal of rendering the inactive in the synaptic cleft, as seen below. The first of these enzymes, monoamine oxidase (MAO), performs an oxidative deamination to yield the corresponding aldehyde, and then the carboxylic acid. In order for this reaction to occur, the compound must have two hydrogens in the alpha position; in fact, MOA metabolizes a number of other compounds, including adrenergic drugs. The second enzyme involved in epinephrine and norepinephrine metabolism is catechol-O-methyltransferase (COMT). COMT will only react with a catechol, which is a substituted benzene ring with a meta- and para-hydroxyl group. It always places a methyl on the meta hydroxyl group. The products of the MAO and COMT reactions are both inactive. In addition, both reactions can occur on a single molecule. For this reason, the major metabolite of epinephrine and norepinephrine is vanallylmandelic acid.
The Role of Amino Acids in Nitrogen Metabolism
