Amino acid Metabolism Flashcards
Give an overview of amino acid breakdown?
10-15% of metabolic energy in animals comes from breakdown of amino acids
Amino acids can be degraded into one of seven common metabolic intermediates
Some are compounds that can be metabolised to CO2 and H2O
Some are compounds that can be used in gluconeogenesis
What are the two types of amino acids that can be broken down?
Glucogenic - amino acids that can be degraded to glucose precursors
E.g. Pyruvate, a-Ketoglutarate, Succinyl-CoA, Fumarate and Oxaloacetate
Ketogenic - amino acids that can be degraded to precursors of fatty acids and ketone bodies
E.g. Acetyl-CoA and Acetoacetate
Which amino acids are special in their break down?
Only lysine and leucine is exclusively ketogenic as acetyl-CoA cannot be converted to oxaloacetate or pyruvate so can yield no net synthesis of carbohydrates
Therefore they can’t convert fats to glucose, as all Cs are lost as CO2
Which amino acids are broken down into pyruvate?
Alanine - is transaminated and catalysed by alanine transferase
Serine - catalysed by serine dehydratase (PLP enzyme) and releases NH3
Glycine - is 1st converted to serine using serine hydroxymethyltransferase (PLP enzyme)
Theronine - converted to glycine using SHMT as above
Cysteine - the sulfhydryl group is released as either H2S, SO3 2- or SCN-
How does serine dehydratase work?
It breaks the α-carbon-H bond
Then a carbanion breaks down, leading to the loss of sidechain OH on the serine R-group
Aminoacrylate is formed and released, spontaneously hydrolyses to pyruvate and ammonia
Which amino acids are broken down in oxaloacetate?
Asparagine - using l-asparaginase and water this releases ammonium
Aspartate - using aminotransferase in a transamination reaction
Which amino acids are broken down into a-ketoglutarate?
Arginine, glutamine, histidine and proline are all degraded into glutamate
Glutamate is then deaminated to a-ketoglutarate using GDH, releasing NH3
Which amino acids are broken down into succinyl Co-A?
Methionine, Threonine, Isoleucine and Valine
They are all degraded to propionyl-CoA, which is then conveted into succinyl-CoA (using biotin and coenzyme B12)
What other molecule can be used as C1 carriers to transfer to another molecule (cofactors)?
Tetrahydrofolates (THF)
It is a 6-methylpterin derivative, linked to a p-aminobenzoic acid and a glutamate residue
Structurally similar to flavins (e.g. FAD)
What can be used as an inhibitor to THF?
Sulfonamindes - antibiotics that are structural analogs of the p-aminobenzoic acid
What is phenylalanine degraded to?
Fumarate and Acetoacetate
This uses 6 enzymes including the cofactor tetrahydrobiopterin (a THF)
What is a disease related to the breakdown of phenylalanine?
Phenylketonuria - PKU, deficiency of phenylalanine hydroxylase
Known as an inborn error - inherited disease that causes mental retardation
PKU leads to a build up of phenylalanine in the blood
Excess phenylalanine is converted into phenylpyruvate via transamination
People with PKU have lighter hair and skin than their siblings for example
This is due to having excess phenylalanine, it inhibits tyrosine hydroxylation and dihydroxyphenylalanine (L-DOPA) is a precursor for synthesis of melanin
How can PKU be helped?
Follow a low-phenylalanine diet (low protein)
Supplements can be taken to provide the necessary amino acids
However this is hard as for example some sweeteners e.g. Aspartame = aspartylphenylalanine methyl ester, which contains phenylalanine
Kuvan treats PKU
Sapropterin – synthetic form of tetrahydrobiopterin can be used to maximise remaining PH activity in these patients, allowing phenylalanine to be turned into tyrosine
How many essential amino acids? What are they?
Only 9/20 are essential - they must be obtained in the diet
Methionine, Valine, Histidine, Leucine, Phenylalanine, Threonine, Isoleucine, Lysine and Tryptophan
Pneumonic - Many Very Happy Little Pigs Take Iced Lemon Tea
What may be considered as an essentail amino acid?
Arginine may sometimes be listed as an essential amino acid as during pregnancy and childhood it is essential for growth - as at these times the body can’t make enough to maintain an increased growth rate
What are some issues for vegetarians?
Vegetarians can struggle to acquire all essential amino acids
Cysteine, methionine and tryptophan are most commonly lacking in plant proteins
Vegetarians needs to eat a range of combinations of protein
What are the amino acid precursors and the amino acids they synthesise in amino acid biosynthesis?
Pyruvate - alanine and valine&leucine (these 2 are only produced by this path in plants/bacteria)
A-ketoglutarate -> Glutamate - glutamine, proline and arginine
Oxaloacetate -> aspartate - arginine (in humans); methionine, threonine -> isoleucine and lysine (these 4 are just produced within plants/bacteria)
3-phosphoglycerate -> serine - glycine and cysteine
PEP + erythrose-4-phosphate - tryptophan, phenylalanine and tyrosine
How does 3-phosphoglycerate synthesise serine?
The first reaction converts to a ketone, uses transamination and then hydrolysis to get to serine
The enzymes used are phosphoglycerate dehydrogenase, aminotransferase and phosphoserine phosphatase
Which biosynthesis of amino acid reactions used transamination?
Alanine, aspartate and glutamate are all synthesised by simple transamination reactions, using an aminoacidtransferase and a coupled reaction with an amino acid going to a-keto acid
We have a taste receptor that recognises glutamate - therefore we add MSG to foods to improve flavour
Give an overview of glutmaine synthesis?
Glutamine synthesis is a mechanism for taking up ammonium ions
It can be viewed as a storage form of ammonium
It is a donor of amino groups e.g. Purines and pyrimidines as well as amino sugars
The amino sugars glucosamine and galactosamine have important roles in connective tissue, synovial fluid in joints
Glutamine synthetase is a central control point in nitrogen metabolism
What is the difference with bacterial enzymes in glutamine synthesis?
Bacterial enzymes are more complex as this is the point where fixed nitrogen is assimilated into biologically
The ammonia is combined with a-ketoglutarate
E.coli is allosterically controlled by 9 different modulators
Modulators include 6 different end-products of reaction pathways from glutamine
Less adenylated glutamine synthetase is more active
Describe tryptophan synthase?
It is a bifunctional enzyme with two subunits and two active sites an α-subunit and a β-subunit
The indole is never released from the enzyme – instead it is channelled directly from the a-subunit to the b-subunit
Give an overview of nitrogen?
All of the nitrogen in biological molecules is derived from atmospheric N2
N≡N bond is very stable: 945 kJmol-1
Sources of Nitrogen - fertiliser, lightening and nitrogen fixing bacteria that colonise the root nodules of legume plants
What is the nitrogen cycle?
Nitrogen fixation: atmospheric N2 -> ammonia (nitrogenase)
Ammonification: ammonia nitrogen containing biomolecules (saprobiotic bacteria)
Nitrification: ammonium -> nitrite/nitrate (oxidised using nitrate reductase)
Denitrification: nitrates -> atmospheric N2 (de-nitrifying bacteria)
Describe the enzyme in nitogen fixing?
Nitrogenase - the nitrogen fixing bacteria (diazotrophs)
It is a complex of 2 proteins – an Fe-S containing protein (a homodimer) and a tetrameric protein containing both Fe and Mo
Fe and Mo are unusual redox cofactors
Each dimer has 2 bound redox centers: the P-cluster and the FeMo-cofactor
Is nitrogen fixing - energy requirements?
This is energetically very costly needing 20-30 ATP per N2 fixed
ATP usage is increased by futile cycling in the first step
Nitrogenase reduces water to hydrogen
Hydrogen reacts with the diamine to form nitrogen
This process would have needed 16 ATPs without this stage
How are electrons used in nitrogen fixing?
ATP hydrolysis is coupled to electron transfer in nitrogenase
Electrons are generated oxidatively/phosophosynthetically depending on the organism
Electrons are tranferred ti ferredoxin - that transfers electrons to the nitrogenase
What is used to prevent degredation of nitrogenase?
Leghemoglobin has a high affinity for O2 within the cyanonacteria, which keeps the pO2 low enough to protect the nitrogenase but maintain O2 for the aerobic bacterium
