(Transcribed from Dr. Laville’s handout from Spring 2000 semester by Brian Buschman)
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Transaminases are the enzymes that remove or add amino groups from AAs. The two model transaminases that we look at the most are:
1) AST (aspartate transaminase) which removes the amine group from aspartate and place onto oxaloacetate to give aspartate.
2) ALT (alanine transaminase) which removes the amine group from alanine and places it into a-ketoglutarate glutamate.
The amino of glutamate can be converted into aspartate or oxidized to NH4+. When NH4+ is removed from aspartate it uses either NADP+ or NAD+ as a cofactor and generates either one NADPH or one NADH.
As discussed last semester AST is found in high levels in the erythrocytes, skeletal muscle, myocardium, kidney and liver. Disease (MI, viral hepatitis, etc) or damage associated with any of these organs will result in increased levels of these enzymes.
There is an essential coenzyme associated with transaminase action called pyridoxal phosphate which is a B12 derivative.
Nitrogen is removed from the body either by excretion of NH4+ (of which only a little happens) or by excretion of urea. Urea is made in the liver by the urea cycle. The cycle centers around the formation of carbamoyl phosphate to pick up NH4+ and the incorporation of aspartate to donate another amine group. Thus each molecule of urea (H2N-CO-NH2).
The rate determining step in the urea cycle is the production of carbamoyl phosphate in hepatocyte mitochondria. It is incorporated into ornithine to make citrulline which is transported to the cytosol where aspartate is added. As urea is removed it is converted back to ornithine which goes back into the mitochondria to start over.
The incorporation of NH4+ into carbamoyl phosphate uses 2ATP and the incorporation of aspartate into citrulline converts 1ATP into AMP (i.e. it uses 2 ATP). The removal of nitrogen from amino acids is an expensive process as it uses 4ATP compared with the 2-3 that are produced by the oxidation of the NADPH or NADH that is generated by the transaminase.
The synthesis of fumarate by the urea cycle links the urea cycle to the TCA cycle. On the other hand the formation of oxaloacetate in the TCA cycle links it to the urea cycle as it is converted to aspartate by ALT.
Deficiencies in the urea cycle enzymes lead to excessive NH4+ accumulation which results in neurological disorders. The body will try to get rid of some in the urine but may not be able to keep up. Treatment is with a low protein diet. You still need to make sure that the patient gets sufficient quantities of the essential amino acids. It’s better to give them supplements of the carbon backbones rather then the AAs themselves. The human body is able to synthesize essential amino acids if you take in the carbon backbone. It just uses the transaminases that break down the essential AAs to function in reverse to be able to form them.
It is also essential to keep NH4+ levels down because high levels will lead to reactions that will quickly deplete TCA cycle intermediated and again cause neurological problems (since the CNS functions by aerobic metabolism of glucose or ketone bodies).
One of the two types of amino acids are ketogenic which means that their carbon backbone is broken down into pyruvate, acetyl-CoA, acylacetate which are able to generate ketone bodies.
The other type of amino acids are those who’s breakdown products become TCA cycle intermediates and are able to eventually be funneled into gluconeogenesis and hence called glucogenic AAs. They are broken down into a-ketoglutarate, succinyl-CoA, fumarate or oxaloacetate.
Propionyl-CoA is an intermediate between may AA’s and succinyl-CoA. If propionyl-CoA carboxylase is deficient, you will end up with excess propionate and excess FAs. It’s coenzyme is biotin so biotin deficiency will cause buildup of propionate and FAs.
The condition methylmalonic acidemia is similar to propionyl-CoA carboxylase deficiency.
Both cystathioninuria and homocystinuria are caused by enzymes that are missing in the degradation of methionine to give succinyl-CoA or to form cystine. Treatment is with a low methionine diet but you must also give the patient cystine supplements. (Some nonessential AAs are formed from other AAs and if the pathway of the degradation of the given AA is blocked then a nonessential AA may become essential.)
Branched chain dehydrogenase deficiency is the cause of maple syrup urine disease. It blocks the pathways of leucine, isoleucine and valine breakdown. It not treated it can lead to mental retardation. Note that these AAs are broken down by different pathways but use the same enzyme in one of the steps in each.
Phenylketonuria (PKU) is caused by a shortage of phenylalanine hydroxylase. It is treated by lowering the dietary intake of phenylalanine in the diet. If not diagnosed and treated early after birth it can lead to neurological problems as the child develops because of the high levels of various phenylketones.
Phenylalanine is broken down by phenylalanine hydroxylase into tyrosine and tyrosine then either broken down to end at fumarate or is converted to melanin. If there is a shortage of tyrosine transaminase the patient has tyrosinemia. Because of the relationship between tyrosine and phenylalanine this condition is treated with a diet low in both phenylalanine and tyrosine.
Another disease related to phenylalanine is alcaptonuria which is from an homogentisate oxidase deficiency that blocks the normal breakdown pathway of tyrosine (and phenylalanine). The result is a buildup of a melanin like substrate that is excreted in the urine giving black pee.
Ten of the eleven nonessential amino acids can be synthesized in the body from glucose. Tyrosine needs phenylalanine in order to be synthesized. Because of this if a patient has PKU you must make sure the patient takes enough tyrosine in the diet to make up for lack of bodily production of tyrosine.
There is a deficiency in the transport of AAs across both the intestinal and the renal tubule membrane called Hartnup’s disease. Hartnup’s disease will cause a negative nitrogen balance because it both blocks intestinal absorption and renal reabsorption.
Similar to Hartnup’s is cystinuria which causes malabsorption of AAs into the tubules and again results in a negative nitrogen balance.
One very important bodily carrier of nitrogen is glucamine which is produced in astrocytes cells of the brain from glutamate (made from a-ketoglutarate) by glutamine synthase. It is significant because it’s a method that the brain uses to get rid of excess nitrogen (as excess nitrogen causes neural defects).
The glucamine is transported to the kidney where it is metabolized to release NH4+ and we all know what happens from there to the NH4+.
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