Metabolism Flashcards
What are the three steps in protein breakdown and what enzymes are required?
1) Transamination
- transaminases
ALT and AST
2) Formation of ammonia by oxidative deamination
Glutamate dehydrogenase and sometimes Glutamine Synthase for free ammonia
3) Formation of Urea (requires the arginase enzyme
What occurs in the transamination reaction?
ALT alanine+ α-ketogluterate→pyruvate+glutamate
AST aspartate+ α-ketogluterate oxoloacetate +glutamate
Amino acid will react with alpha ketoglutarate to form alpha keto acid and glutamate. There are AST (aspartate transaminases) and ALT (alanine transaminases). Pyruvate and oxaloacetate can be converted to make glucose. High levels of AST and ALT will indicate liver damage.
What is the role of transaminases in the reaction and what do they require to function?
Transaminases transfer an amino group from the amino acid to the alpha keto acid and require a pirodoxal phosphate required from vitamin B6
What amino acids do not undego transamination?
Threonine and lysine
What indicates liver damage?
High levels of AST and ALT
How is ammonia produced?
By oxidative deamination using glutamate dehydrogenase. Occurs in the matrix of the mitochondria
Free ammonia can combine in tissues with glutamate and ATP forming glutamine and ADP. This reaction requires glutamine synthase and is a way of transporting more than one amine residue.
Glutamate+N〖H4〗^++ATP □(→┬(Glutamine Synthase ) ) Glutamine+ADP
What molecule is the main acceptor of nitrogen what does it do?
Glutamate, will donate nitrogen for biosynthesis of amino acids, nucleotides and sugars
What happens in the urea cycle?
CO2 + NH4+ -> carbomyl phosphate
Carbomyl phosphate + Ornithine -> citrulline
Citrulline + Aspartate -> to produce arginino succinate
Arginino succinate will split into arginine + fumarate
Arginine is converted to urea using enzyme arginase
What happens to fumarate?
Fumarate will be converted to malate and then oxaloacetate which will further react with alpha amino acids to produce aspartate and alpha ketoacid
What is the glucose alanine cycle?
Pyruvate produced in the muscle will react with glutamate to produce alanine and alpha keto glutarate which will then be transported to the liver and converted back into Glutamate and pyruvate and converted to urea and pyruvate is used in to make glucose by gluconeogenesis
How are amino acids broken down in the muscle?
In muscle proteins are broken down as an energy source for contracting muscle. The muscle has transaminases, but it does not have enzymes to produce urea therefore transported to the liver using the glucose-alanine cycle. Nitrogen will be transferred via glutamate and pyruvate to alanine. Alanine will then be transported to the liver where it is converted back to glutamate and pyruvate. Glutamate will then produce ammonia and form urea and pyruvate will be converted into glucose.
What are the types of amino acids?
Ketogenic aa = involved in the production of ketone bodies
Glucogenic amino acids = feed into the points of the TCA cycle
Are proteins stored?
No proteins can either be structural or functions there are not specific protein stores. Amino acids are supplied by the diet and then excess protein will be broken down and secreted
What is a normal nitrogen balance?
The loss and intake or protein is balanced
What is a positive nitrogen balance?
The intake of nitrogen exceeds the amount lost resulting in an increase in the amino acid pool. Occurs in children (during periods of growth) and in excersice (enhanced by anabolic steroids)
What is a negative nitrogen balance?
The amount of nitrogen excreted from the body is greater than the nitrogen consumed
Associated with burns, serious tissue injuries fevers and hyperthyroidism
Also a response due to catabolic hormones and lack of anabolic hormones
What are the functions of lipids?
Membranes
Uptake of lipid soluble vitamins
Energy store
Precursor of steroid hormones
Why is fat an important energy store?
1g of fat produces 37kj of energy
whereas protein only produces 17kj and carbohydrate 16kj
How is acetyl CoA transported?
Acetyl CoA cannot be transported out of the mitochondria therefore it combines with oxaloacetate to produce citrate which is then transported out of the mitochondria to the liver cytoso
What is the citrate malate antiport and what does it produce?
Citrate will be converted back into acetyl CoA and oxaloacetate which is then converted into malate and then pyruvate to be transported back into the mitochondria. This requires NADH and will produce NADPH which will be used in the elongation step of FA synthesis
What is the first step of FA synthesis
Activation by citrate will activate the Acetyl CoA carboxylase enzyme which will convert acetyl CoA and ATP into Malonyl CoA, ADP and Pi
What does Acetyl CoA carboxylase require?
Activation by citrate (positive feed forward)
Vitamin biotin
Activated by insulin (blood glucose levels are low therefore energy is needed from other sources
EXPRESSION IS INCREASED BY high carbohydrate and low fat
What decreases the activity of Acetyl-coA carboxylase?
Palmitate (product inhibition
Inhibited by glucagon (carboxylase is inhibited by phosphorylation glucagon stimulates phosphorylation therefore inhibits the enzyme
EXPRESSION OF ACETYL COA CARBOXYLASE IS DECREASED by low carbohydrate and high fat
What is the elongation step in FA synthesis? `
Malonyl CoA (C3) binds to an ACP covalently. This forms malonyl ACP. Acetyl CoA (C2) also binds and is activated by ACP. The two react forming Acetoacyl (C4) ACP which is then reduced x2 using NADPH produced in the citrate malate antiport or the pentose phosphate pathway, aswell as dehydrated to produce butyryl ACP (C4) this then reacts with another malonyl ACP (C3) to produce a C6 molecule.
- the process will always involved the sequential addition of 2C units
- Enzyme required is fatty acid synthase
What are the features of fatty acid synthase?
Fatty acid synthase exists as a dimer = makes fatty acid synthesis efficient as continual movement of reactant to product
Forms a multi functional complex
Acetyl CoA is passed from one active site to the next
What are the features of cholesterol?
Rigid hydrophobic molecule, insoluble in water
Precursor of steroid hormones, sterols and bile salts
Transported in the circulation as cholesteryl esters
Cannot be oxidised to O2 or H2O = no energy
Membrane component = fluidity
Cholesterol is mainly synthesised in the ER
What are the steps of fatty acid degradation and where do they happen?
Mobilisation = in the adipocyte Activation = in the hepatocyte cytosol Degradation = in the liver mitochondria
What happens in mobilisation? (FA degradation)
cAMP mediated response will activate cAMP which will activate pKA which will phosphorylate triacylglycerol lipase.
Triacylglycerol lipase will break down triglycerides into its components of glycerol, fatty acids and cholesterol.
Glycerol converted to Glyceraldehyde-3-phosphate and is used in glycolysis (partially) or gluconeogenesis (mainly)
Fatty acids will be transported to the liver and activated by acyl CoA synthase
What happens in the activation step of FA degradation?
Long chain fatty acids will be converted to acyl CoA-ATP OMM using Acyl CoA synthase.
Acyl CoA produced will then be transported to the mitochondria bound to the alcohol carnitine forming Acyl carnitine. In the mitochondria Acyl will bind back with a CoA molecule
What happens in the degradation step of FA degradation?
Acyl CoA will be degraded by a series of steps forming Acetyl CoA, NADH and FADH2.
Small amount of acetyl CoA will be used in TCA cycle in the presence of glycolysis
Complete oxidation of palmitate = yields 106 ATP
Odd chain length yields propionyl CoA in last round
Odd numbered double bonds removed by = isomerase
Even numbered double bonds removed by = reductase + isomerase
Acetyl CoA will be used in ketogenesis in the production of ketone bodies which are used when the body is in starvation, or taken up by tissues cardiac and renal cortex depending on the flow of carbohydrates in glycolysis.
What is ketogenesis
Production of ketone bodies from acetyl CoA
Acetyl CoA converted to HMG-CoA
HMG-CoA converted to acetoacetate
Acetoacetate reduced to 3-beta-hydroxybutyrates or non enzymatically acetone
Taken up by many tissues mainly cardiac muscle and renal cortex dependant on the flow of carbohydrates in glycolysis it will be used in the cells for OXPHOS and generation of ATP.
In starvation 70% comes from ketone bodies
What is the hormonal regulation of the fat metabolism involving insulin?
Insulin
inc. glycolysis in the liver
inc. FA synthesis in liver (promotes the enzyme Acetyl CoA carboxylase)
inc. TG in adipose tissue
What is the hormonal regulation of the fat metabolism involving glucagon?
Increases TG mobilisation = increases breakdown of FA by Triacylglycerol lipase
(also stimulates phosphorylation of acetyl CoA carboxylase which will inhibit the enzyme)
What is the comparison between synthesis and degradation of fatty acids?
Reciprocally regulated Synthesis in the cytosol Intermediates linked to acyl carrier protein, acetyl CoA is passed from one active site to the next Sequential addition of 2C units Reductant NADPH Fatty acid synthase enzyme complex
Degradation in mitochondria
Intermediates linked to Co enzyme A
Sequential removal of 2C units
Oxidants FAD and NAD (become FADH2 and NADH)
Carried out by individual enzymes
Triacylglycerol lipase, Acyl CoA synthase
Requires carnitine for transport to mitochondria cytosol
What is catabolism?
The breakdown of complex molecules to release energy or carry out mechanical work
What is anabolism?
The synthesis of new molecules from less complex molecules
Why should we study metabolism?
Understand the metabolic basis of disease e.g diabetes, artherosclerosis and gall stones
Can use the changes in metabolites to aid diagnosis
How much ATP is used during excersice?
0.5kg per minute
What is the mass of ATP in the body?
100g must be broken down and regenerated to meet energy demands
What are the enzymes used in glycolysis?
Hexokinase (all tissues)/ Glucokinase (liver)
Phosphofructokinase converts fructose-6-phosphate to fructose-1,6-biphosphosphate
Pyruvate kinase
converts phosphenol pyruvate to pyruvate
How is hexokinase regulated?
Hexokinase is inhibited by its own product G6P
How is PFK regulated in the muscles?
Citrate = start of the TCA cycle indicates that the precursors are abundant = negative feedback ATP = high levels of ATP means that less glucose needs to be converted to pyruvate H+ = build up of lactate will cause decrease in H+ ions and inhibit PFK activity
AMP = positive regulator of PFK and indicator of energy status in the muscle. Two molecules of ADP will combine to form ATP and AMP. This indicates that the cell is starced for energy and needs to promote the affinity for fructose-6-phosphate
How is PFK regulated in the liver?
More complex as there are more functions
Citrate
ATP
Not inhbited by H+ as the liver does not produce lactate and therefore unaffected by a low pH
Why do glucokinase and hexokinase have different Km values?
Glucokinase has a high Km value which means it has a low affinity for glucose and is only active at high concentrations of glucose providing extra capacity for the glucose by phosphorylating and storing glucose as glycogen.
