Amino acid metabolism I and II week 4 Flashcards
How are amino acids used in the body?
- synthesis of body protein (i.e.muscle) as they are the building blocks of proteins
- synthesis of glucose and ketone bodies from the carbon atoms from AA
- synthesis of many biomolecules and N-containing compounds.
- production of fuel with CO2 as a by-product.
What are the essential aa?
PVT TIM HALL (“Private Tim Hall”) uses the first letter of each of the essential AA.
Phenylalanine
Valine
Threonine
Tryptophan
Isoleucine
Methionine
Histidine
Arginine
Leucine
Lysine
Note that arginine is synthesized but at an insufficient rate to meet the need during growth (is only essential during growth periods).
What are the 3 basic parts of an aa?
Amino acids consist of both amine and carboxyl functional groups on an α- keto acid which is the carbon skeleton.
T or F: In the alpha amino acids, the amino and carboxylate groups are attached to the same carbon atom (the α–carbon).
True.
From where do we obtain aa?
- dietary protein
- degradation of our own body proteins
- synthesis of nonsessential aa from simple intermediates of metabolism
Explain the degradation and absorption of dietary aa.
AA can be obtained in the diet from meat, dairy products, grains, beans. In a healthy individual, very little protein is lost in the feces (~5% daily) as AA absorption from protein breakdown is a very efficient process:
• HCl first denatures proteins in the stomach. Proteins are then broken down into large fragments by pepsin (which is secreted as the inactive proenzyme, pepsinogen; HCl activates pepsinogen to pepsin) in the stomach
- Further breakdown occurs in the small intestine at neutral pH via trypsin and chymotrypsin, which are produced by the pancreatic exocrine cells.
- Single AA are released from small peptides via amino and carboxypeptidases in the plasma membrane on the microvilli of the intestinal cells.
- The resulting AA are absorbed by the intestinal cells through specific AA transport systems.
Free AA are then transported through wall of the GI tract, into the bloodstream throughout the body and can be found in cells, blood and extracellular fluids. AA from this source can be found along side AA from the breakdown of body protein and from new synthesis of proteins in the AA pool. This requires transport systems to get the AA into the cells of the gut wall and into other cells.
What can cause deficiencies in panreatic secretion? What is the result (as it pertains to lipids and proteins)?
In skeletal muscle, amine groups are transferred to glutamate and pyruvate to form _____ and _____.
These molecules are transported to the liver and kidney. What modifications occur in these places and what is the purpose?
In muscle amine groups from the AA are transferred to:
• glutamate (to make glutamine)
• pyruvate to make alanine
- They are then transported to the liver or kidney.
- Urea is produced in the liver and ammonia (from glutamine) in the kidney.
-Carbon skeletons are used for energy or transported to the liver for gluconeogenesis
What 4 situations cause massive degradation of muscle protein? What is the first step in this process and where in the body does most of it occur under these circumstances?
Starvation, trauma, burns, septicemia cause massive degradation of muscle protein. The first step in the process is transamination which occurs almost exclusively in the muscle (under these circumstances).
How is protein concentration regulated?
For many proteins, regulation of synthesis determines the protein concentration in the cell (degradation plays a minor role). For other proteins, the rate of synthesis is constitutive and thus relatively constant, requiring selective degradation to control the cellular level.
In healthy adults, the total amount of protein in the body is constant. This is because the rate of synthesis is sufficient to replace degraded protein. Some proteins are short-lived while others, such as structural proteins like collagen are very stable and have half-lives of months to many years.
List the 2 major proteolytic systems responsible for the degradation of damaged or uneeded proteins.
- ubiquitin-proteasome pathway
- lysosomal proteolytic degradation
Explain the ubiquitin proteasome pathway. What is required for this pathway?
What is the pH optimum of proteases in the lysosome? Is energy required for protein degradation in lysosomes?
o Ubiquitin-proteasome proteolytic pathway:
- Energy (ATP) dependent
- Proteins selected for degradation by this mechanism are first covalently linked to ubiquitin (a small protein) which allows them to be recognized by a large proteolytic complex called a proteosome which functions like a garbage disposal. The proteosome cuts the protein into fragments that are then further degraded to AA which enter the AA pool. need multiple ubiquitin to form polyubiquitin tail. primarily for degradation of intracellular proteins.
oLysosomal protein degradation:
- Intracellular and extracellular proteins are degraded by a variety of proteases with an acidic pH optima (e.g. cathepsins) in the membrane-bound lysosomal compartment.
What is required for the transport of aa into cells?
Name one aa transport system and the aa it transports into cells.
- The concentration of free AA in the extracellular fluids is significantly lower than that within the cells. This gradient is maintained through active transport systems, driven by the hydrolysis of ATP, for movement of AA from the extracellular space into the cells.
- Several of these active transport systems have overlapping specificities for different AA.
- One transport system, for instance, is responsible for the uptake of cystine, ornithine, arginine, and lysine (COAL)]. Note that these all have 2 amino groups.
What is cystinuria? How common is it? What can result from this disorder?
Cystinuria is one of the most common inherited diseases (occurring in 1/7000 individuals) and the most common genetic error of AA transport. It is characterized by the precipitation of cystine to form kidney stones (calculi) thus blocking the urinary tract. Oral hydration is important in these individuals.
What is Hartnup’s disease?
What aa is most affected in this disease? What does this result in?
What is a possible treatment for Hartnup’s disease?
In Hartnup’s disease, there are defects in intestinal absorption and renal reabsorption of neutral AA (particularly tryptophan which is a precursor to serotonin, melatonin and niacin [nicotinamide, vitamin B3]) from the kidneys to the rest of the body, resulting in deficiencies in essential AA since they will not be absorbed from the diet. Skin and neuronal problems ensue (pellegra).
In these individuals, some AA have been found in excess within the urine. This is a potentially very serious condition, but it can be somewhat treated with vitamin B3 (niacin) (still doesn’t provide tryptophan). B3 deficiency causes pellagra.
What molecule can aa be attached to for transport into cells? What enzyme is required? Where in the body is this enzyme found?
What substrates are required? What is the energy expense?
γ-glutamate is also used to transport aa into cells. This requires that the aa is carried across the cell membrane attached to γ-carboxyl group of glutamate. The liver enzyme γ-glutamamyl transferase (GGT) transfers γ-glutamate from glutathione to the aa. This is an energy expensive transport since it costs 3 ATPs to reform each glutathione molecule.
How can serum GGT levels be used clinically?
Elevated serum levels of GGT often occur in intra- and posthepatic biliary obstructions, indicating cholestasis (bile can’t flow from the liver to the duodenum), liver disease, pancreatic cancer, alcoholism, increased aspirin intake, and congestive heart failure. Because of this non-specificity, its use as a test is controversial.
T or F: Many aa are also transported across cell membranes by symport or antiport mechanisms coupled to sodium transport.
True.
What is the purpose of removing nitrogen from aa?
What is the fxn of alpha amino groups on aa? Once removed, what cellular processes can the carbon skeletons be used for?
The α-amino group protects AAs from oxidative breakdown. However, removal of the α-amino group is necessary for producing the energy derived from the AA. Once removed, N can be incorporated into other compounds or excreted, with the carbon skeletons being used for gluconeogenesis, ketogenesis, or both.
What 2 steps/rxns are involved in removing nitrogen from aa? What 2 molecules are ultimately produced?
The steps involved in removing nitrogen, ultimately provide ammonia and aspartate (the 2 sources of nitrogen in urea) through:
- Transamination
- Oxidative Deamination
What is the first step in the catabolsim of most AAs?
What do aa’s yield when deaminated? What happens to these products?
The first step in the catabolism of most AAs is the transfer of their α-amino group to α-ketoglutarate. Amino acids (AA), when deaminated, yield α-keto acids that, directly or via other reactions, feed into the major metabolic pathways.
What is formed from the transfer of an amine group to alpha-ketoglutarate? What is this type of rxn called?
What are the only 2 aa that do not undergo this rxn?
What type of enzyme catalyzes this rxn?
The first step in the catabolism of most AA is the transfer of the amine group to α-ketoglutarate thus producing glutamate (this is a transamination reaction). Only threonine and lysine do not participate in transamination.
So, the products of transamination are:
• α-keto acid (derived from the original AA)
• glutamate
Catalyzed by aminotransferases
What is the obligate pair (required, always present) of transamination rxns? How are transamination rxns different for essential and non-essential aa?
What 2 things can be done with the glutamate that is produced in transamination rxns?
• The glutamate/α-ketoglutarate pair is always part of the transamination reaction. It is reversible for nonessential AA.
• For essential AA (which can not be synthesized) the reaction is unidirectional (deamination only) unless its α-keto acid is provided via therapy.
The glutamate produced by transamination can then be:
- Oxidatively deaminated
- Used as an amino group donor in the synthesis of nonessential AAs.
Where in the body are aminotransferases (transaminases) mostly located?
What are these enzymes named after?
In what organ do these enzymes primarily fxn?
Aminotransferases (transaminases) catalyze the transfer of an amino group from one carbon skeleton to another.
They are found in cells throughout the body, especially liver and kidney, and are specific for one (or perhaps a few) amino group donors. They are named after the specific amino group donor. Transamination occurs primarily in the liver.
List the 2 most important aminotransferases.
alanine aminotransferase (ALT)
aspartate aminotransferase (AST)
What does ALT catalyze? During AA catabolism, in what direction does this rxn go?
• Alanine aminotransferase (ALT)
o Catalyzes the transfer of the amino group of alanine to α-ketoglutarate, and forms pyruvate
o Reversible, but during AA catabolism it goes in the direction of glutamate synthesis.
pyruvate can then be used for energy generation
What rxn does AST catalyze? In AA metabolism, what direction does its rxn go?
• Aspartate aminotransferase (AST)
o In AA catabolism, AST transfers amino groups from glutamate to oxaloacetate, forming aspartate (used as a source of N in urea cycle).
o Reversible reaction.
Explain the alanine glucose cycle. What organ uses this cycle and why?
- Used primarily in skeletal muscle to eliminate N and replenish energy.
- During prolonged exercise, branched chain AA are released from muscle and their carbon backbones are used as an energy source, while their N is used to make alanine.
- Alanine brings C and N to the liver where it can transfer the NH3 to α ketoglutarate.
- So, in the liver, alanine can be regenerated to pyruvate.
- Pyruvate can then be diverted for gluconeogenesis.
- The newly formed glucose travels through the blood to the muscle.
What is the cofactor for aminotransferases? What else is this cofactor used for as it pertains to glutamate?
Pyridoxal phosphate (the functional form of vitamin B6) is the cofactor for aminotransferases. B6 held in active site of enzyme-site where amino group is transferred. It is also used to remove the carboxyl group from glutamate.
Equilibrium allows the rxns of aminotransferases to be bidrectional. Under what conditions are aa broken down? synthesized?
What do elevated levels of aminotransferase indicate? What can result in elevated levels of these enzymes in the blood?
What 2 aminotransferases are measured in disease?
In what diseases are these 2 aminotransferases elevated?
Aminotransferases = normally low in plasma. The levels found in the plasma are therefore representative what is released during normal cell turnover. An elevated level of an aminotransferase indidcated damage to cells containing high levels of these enzymes. Trauma or disease can result in cell lysis with the release of enzymes into the blood. The plasma levels of Alanine aminotransferase (ALT) and Aspartate aminotransferase (AST) are of particular interest in the diagnosis of liver disease. These 2 enzymes are elevated in nearly all liver diseases but are especially high in conditions causing extensive cell necrosis in the liver or in striated (cardiac or skeletal) muscle.
Under what cellular energy conditions would glutamate be formed from oxidative deamination? What conditions would glutamate DH work in the reverse manner (urea and alpha-ketoglutarate formation)? What is required/produced for these rxns?
Direction of reactions: depends on the relative concentrations of glutamate, α-ketoglutarate, and ammonia
In caloric restriction and low blood glucose, what direction does the rxn catalyzed by glutamate dehydrogenase go in and why?
In caloric restriction and low blood glucose, glutamate dehydrogenase activity is raised in order to increase the amount of α-ketoglutarate produced, which can be used to provide energy by being used in the TCA cycle to ultimately produce ATP.
What enzyme catalyzes the formation of glutamate from glutamine?
What enzyme catalyzes the formation of glutamine from glutamate? What is required for this rxn?
What is the intercellular glutamine cycle?
The overall pic of the breakdown and synthesis of AA includes transamination and oxidative deamination.
Explain how the following aa are derived and what enzymes are involved.
alanine
aspartate
asparagine
glutamate
glutamine
What aa are ketogenic? What is the definition of a ketogenic aa? What processes can they be used for?
Ketogenic AA: (leucine & lysine). Their carbon skeletons are degraded to acetyl-CoA or to acetoacetate which cannot yield net production of oxaloacetate, the precursor for gluconeogenesis. For each 2-C acetyl residue entering the TCA cycle, two carbons leave as CO2. Carbon skeletons of ketogenic AA can, however, be catabolized for energy in the TCA cycle, converted to ketone bodies or converted to fatty acids.
What aa are both glucogenic and ketogenic?
aromatic aa: tyrosine, phenylalanine, and tryptophan
What is the fate of protein consumed in excess of the body’s needs? (what molecules is excess protein metabolized to?)
What health issues may accompany excess protein intake? Why does this occur?
If protein is consumed in excess of the body’s needs, it is deaminated, with the resulting carbon skeletons being metabolized to provide pyruvate or acetyl CoA for fatty acid synthesis.
Excess intake of protein leads to elimination from the body as urinary nitrogen. It is often accompanied by increased urinary calcium thus increasing the risk of kidney stones and osteoporosis (alkaline salts are often mobilized to neutralize acid production therefore calcium will be taken from the bones by the action of osteoclasts).
What are the “stress amino acids”? Why are they known as stress amino acids?
What enzyme is required to break these aa down? What disease results if this enzyme is defective?
What are the sx of this disease? How is this disease treated?
Leucine, isoleucine, valine: Branched-chain AA. essential AA needed for building and maintaining muscle mass during times of physical stress and intense exercise. The BCAAs are occassionally referred to as the “stress amino acids” because they are utilized more rapidly during times of intense stress (I remember this by thinking “I LIV”).
Branched-chain α- ketodehydrogenase breaks down leucine, isoleucine, valine (after action by aminotransferases). In Maple syrup urine disease (branched chain ketoaciduria), α- ketodehydrogenase is defecitve. Leucine, lysine, and valine accumulate giving the infant a sweet smelling urine. Infants appear healthy at birth but if left untreated, suffer severe neurologic damage and eventually die. Sx include intellectual disabilities and poor myelination. Dietary restrictions are difficult bc these aa are required. Specialized protein preps containing substitutes and adjusted levels of the aa must be used in the diet.
What are the fxns of methionine? Is it essential or non-essential?
What molecule is methionine essential for the synthesis of?
How is S-adenosyl methionine (SAM) formed? What are the fxns of SAM?
- is an essential sulfur-containing AA.
- serves as a sulfur donor, a methyl donor, and a precursor for other sulfur amino acids
- acts as a lipotropic factor, mobilizing the movement of fat through the liver to prevent its accumulation.
- is required for the synthesis of creatine monohydrate, which is essential for energy production and muscle function.
- When an adenosyl group from ATP is transferred to the sulfur atom of methionine the result is S-adenosyl methionine (SAM). Sam is an important methylating agent. It contributes to the synthesis of brain neurotransmitters and to detoxification reactions, and has analgesic properties.
What effects can an excessive intake of methionine along with inadequate intake of B9 and B12 have? What diseases may this cause?
Excessive methionine intake (in egg whites, fish broccoli, potatoes, spinach, etc), with inadequate intake of folic acid (vitamin B-9) and vitamin B 12 (B9 and 12 deficiency can lead to megaloblastic anemia and neurologic issues), may increase homocysteine levels (an amino acid in the blood). Epidemiological studies have shown that too much homocysteine in the blood plasma is related to a higher risk of coronary heart disease, stroke and peripheral vascular disease.
What is the cause of homocystinuria? What sx do pts with this disease present with?
Homocystinuria is often due to a defect in cystathionine β synthase resulting in an increase in homocysteine and methionine. The patients present with dislocation of the lens of the eye, intellectual disabilities, and skeletal and neurological abnormalities.
What aa can phenylalanine be converted to? What enzyme is required for this? What molecules may be synthesized after conversion to this aa?
L-Phenylalanine
• is an essential AA that can be converted in the body into L-tyrosine (by phenylalanine hydroxylase (PAH) which can be converted into L-dopa, norepinephrine, and epinephrine which are critical to nervous system functioning.
What is PKU caused by? What is its inheritance pattern? In what race of people is PKU incidence the highest?
What molecule is detected in the urine in ppl with PKU?
If left untreated, what sx can PKU lead to?
How is PKU treated?
How is PKU diagnosed?
Phenylketonuria (PKU) is an autosomal recessive disorder (1/10,000 births among whites, in whom the incidence is highest) characterized by a deficiency in the liver enzyme phenylalanine hydroxylase (PAH), necessary to metabolize phenylalanine to tyrosine. When PAH is deficient, phenylalanine accumulates and is converted into phenylpyruvate (AKA phenylketone), which is detected in the urine.
Left untreated, phenylketonuria can cause problems with brain development, leading to progressive intellectual disabilities, brain damage and seizures. PKU is one of the few genetic diseases that can be controlled by diet (low phenylalanine & high tyrosine; also low protein diet). However, diet alone may not be enough to prevent all of the negative effects of Phe levels.
Optimal treatment involves lowering blood Phe levels to a safe range and monitoring diet and cognitive development. Lowering of Phe levels to a safe range may be achieved by a combination of a low Phe diet and medication. However, not all individuals with PKU respond to the medication. There is currently no cure for this condition. PKU is generally detected through newborn screening and diagnosed through genetic testing. Brain damage is irreversible if not detected early.
How is tyrosine metabolized in the adrenal meduall? What is required for this?
In the adrenal medulla dopamine is converted to norepinephrine which is finally converted to epinephrine (via phenylethanolamine M methyltransferase & SAM).
What is alkaptonuria? What is its inheritance pattern?
What is alkaptonuria caused by?
What diseases/conditions/symptoms result from alkaptonuria?
When in life is this disease detected?
Alkaptonuria (black urine disease – urine turns black when exposed to air)) was the first identified inborn error of metabolism. It is a rare genetic disorder of phenylalanine and tyrosine metabolism. It is an autosomal recessive condition due to a defect in the enzyme homogentisate oxygenase, which participates in the degradation of phenylalanine and tyrosine.
The result is a toxic tyrosine byproduct, homogentisic acid (alkapton) accumulating in the blood and excreted in urine (uria). Excessive homogentisic acid causes damage to cartilage (leading to osteoarthritis) and heart valves, precipitates as kidney stones.
Urine in an infant’s diaper may darken and can turn almost black after several hours. However, many people with this condition may not know they have it until mid-adulthood, ~ 40 yrs, when joint and other problems occur.
Progressive arthritis, particularly of the spine
• Symptoms may include:
• Darkening of the ear
• Dark spots on the white of the eye (sclera) and cornea
How is arginine conditionally essential? What aa is arginine synthesized from? What pathway is it part of?
Arginine is conditionally essential to humans in that it is synthesized from glutamate but at an insufficient rate during growth. Is part of urea cycle.
What process is aspartate essential for? What does it contribute to this pathway?
Aspartate:
• Along with NH3 provides the nitrogen for urea synthesis via the urea cycle.
What aa is serotonin derived from? Where in the body is serotonin found? What are the fxns of serotonin? What can serotonin be converted to?
Where is melatonin found? When is it primarily synthesized? What is its function?
Serotonin:
• is a neurotransmitter in the brain
• much of it is found in some lining cells of the gut, where it is used to regulate intestinal movements
• causes contraction of smooth muscle in arterioles and bronchioles
• can be converted to melatonin
Melatonin:
• is a sleep-inducing molecule present in the pineal gland and retina.
• is involved in circadian rhythms, being synthesized primarily at night.
What genetic disorders are included in newborn genetic screenings?
What are some of the most common aa metabolic disorders?
PKU ………………………..…80
Cystinuria………………………70
Hartnup’s disease……………40
Hyperprolinaemia……………20
Argininosuccinate aciduria…..4
What molecules is creatine synthesized from?
How is creatine converted to phosphocreatine? What enzyme catalyzes this rxn and what is required?
In what organs and during what times is phosphocreatine used?
What does high levels in the blood of the enzyme used to create phospocreatine indicate?
What is the amount of creatine in the body related to?
What is creatinine? How is creatinine ridded of by the body? In what disease do creatinine levels rise?
- Creatine is produced from glycine, arginine, and SAM
- An important energy store in skeletal muscle and in the brain, creatine phosphate (phosphocreatine) is used anaerobically to regenerate ATP from ADP in the first few seconds following an intense muscular or neuronal effort. In the process of its dephosphorylation, it forms creatine.
- Conversely, excess ATP can be used during a period of low effort to convert creatine to phosphocreatine.
o Both reactions are catalyzed by several creatine kinases, which have muscle (M), brain (B) subunits, or both (MB). Presence of creatine kinase (CK-MB, MB for muscle/brain) in plasma is indicative of tissue damage and is used in the diagnosis of myocardial infarction. Serum levels are elevated within hours of MI and remain elevated for at least 2 days.
- The amount of creatine in the body is related to muscle mass and a certain proportion is turned over every day.
- A very small % of creatine is converted to creatinine, which is a waste product with no function, produced at a fairly constant rate by the body (depending upon muscle mass and kidney function). Creatinine is filtered out of the blood by the kidneys (although a small amount is actively secreted by the kidneys into the urine) and there is little or no tubular reabsoption. Therefore if kidney filtration is deficient, blood levels of creatinine rise. In kidney failure, creatinine rises as does blood urea nitrogen (BUN).
