2.1-2.2 Cell Metabolism Flashcards
What does glycolysis produce?
2 x Pyruvate
2 x NADH
2 x net ATP
What reaction is irreversible and commits the cell to glycolysis and why?
Conversion of glucose to glucose-6-phosphate by hexokinase
Traps glucose in the cell as phosphate carries a negative charge
What is the complete reaction steps of glycolysis?
Glucose
G6P by hexokinase (GT)
F6P by phosphoglucose isomerase (iso)
F-1,6-BP by phosphofructokinase (GT)
GAL-3-P and DHAP by aldolase (hydrolytic)
GAL-3-P by triose phosphate isomerase (iso)
2 x 1,3-bisphosphoglycerate by GAL-3-P dehydrogenase (redox, GR)
2 x 3-phosphoglycerate by phosphoglycerate kinase (GT)
2 x 2-phosphoglycerate by phosphoglycerate mutase (iso)
2 x phosphoenolpyruvate by enolase (dehydration)
2 x pyruvate by pyruvate kinase (GT)
Which step of glycolysis does phosphofructokinase control?
The conversion of fructose-6-phosphate into fructose-1,6-bisphosphate
What is the rate limiting step of glycolysis and why?
Conversion of F6P to F-1,6-BP
Phosphofructokinase is an allosteric enzyme regulated by negative feedback
When ATP levels rise, its affinity for F6P is reduced
What does hexokinase catalyse?
The conversion of glucose into glucose - 6 - phosphate
What is the only glycolytic enzymopathy which is fatal?
Deficiency in TPI
Which reactions produce ATP?
1,3-bisphosphoglycerate to 3-phosphoglycerate by phosphoglycerate kinase
Phosphenolpyruvate to pyruvate by pyruvate kinase
What do kinases do?
Transfer phosphate groups to molecules
Which step in glycolysis needs NAD+?
Glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate by GAL-3-P dehydrogenase
Which steps of glycolysis require ATP?
Glucose → glucose - 6 - phosphate
Fructose - 6 - phosphate → Fructose - 1,6 - bisphosphate
Does glycolysis need oxygen to occur?
No - it is anaerobic
Where does glycolysis occur?
In the cytoplasm
Why are high energy phosphate groups added to some of the substrates involved in glycolysis?
Adding the phosphate groups makes them easier to split as they are more reactive
What are the three fates of pyruvate?
- Lactate generation
- Acetyl CoA production
- Alcohol fermentation
What enzyme is needed to generate Acetyl CoA from pyruvate?
Pyruvate dehydrogenase complex
What does pyruvate dehydrogenase needed as a cofactor?
Thiamine
What condition does a deficiency in thiamine result in and what are the 3 symptoms of this condition?
Beri-Beri
1. Damage to peripheral nervous system
2. Weakness of musculature
3. Decreased cardiac output
What happens in alcohol fermentation?
How is pyruvate converted into lactate?
Using lactate dehydrogenase complex
What does the fermentation of alcohol and lactate production both regenerate?
NAD+ which is needed for glycolysis to continue occurring anaerobically
How can lactate dehydrogenase (LDH) be used as a diagnostic tool?
LDH is released into the circulation as a result of necrosis
Used to diagnose tissue damage
E.g. stroke, MI, liver disease, muscle injury, muscular dystrophy, pulmonary infarction
What happens to the Acetyl CoA that is produced?
It enters into the Krebs Cycle
Which high-energy bond joins the acetyl group onto CoA and what is it useful for?
Thioester bond
It is readily hydrolysed, enabling acetyl CoA to donate the acetate to other molecules
What are the stages of the Krebs Cycle?
Acetyl CoA combines with oxaloacetate to make Citrate
Then Alpha - keto glutarate
Succinyl CoA
Succinate
Fumarate
Malate
What does the Krebs cycle produce?
2 x CO2
1 x GTP
3 x NADH
1 x FADH
What reaction in the Krebs cycle produces FADH?
The conversion of succinate to fumerate
Where does the krebs cycle occur?
The mitochondrial matrix
What is a transamination reaction?
Removing an amine group from one amino acid and transferring it to a ketoacid, producing another ketoacid and amino acid
Give an example of transamination
alanine + alpha-ketoglutarate → pyruvate (ka) + glutamate (aa)
Elevated levels of which enzyme are a diagnostic tool for hepatic disorders such as hepatitis C?
Alanine aminotransferase
Which 3 amino acids can be phosphorylated and why?
Serine, threonine and tyrosine as they have an OH group
What are the 7 molecules that could arise from degradation of all 20 amino acids?
Pyruvate
Succinyl CoA
Acetoacetyl CoA
Acetyl CoA
Oxaloacetate
Alpha-ketoglutarate
Fumarate
Why is it essential that NAD+ is regenerated?
Needed for dehydrogenation of glyceraldehyde-3-phosphate to produce ATP
Allows glycolysis to occur anaerobically
What is the Warburg effect?
Mutations occur in genes of fumarase, succinate dehydrogenase and isocitrate dehydrogenase, which decreases Kreb’s Cycle activity which enhances anaerobic glycolysis
What are glucogenic amino acids?
Catabolism produces pyruvate or other glucose precursors
What are ketogenic amino acids?
Catabolism produces ketone body precursors like Acetyl CoA
During fasting, which type of metabolism dominates?
Fat metabolism, resulting in the production of ketone bodies
What happens when the levels of Acetyl CoA is not equal to Oxaloacetate?
The excess Acetyl CoA is used to make ketone bodies
What are the names of the three ketone bodies?
Acetone, acetoacetate and 3-B-hydroxybutyrate
Why can ketone bodies be utilised by the brain but fatty acids cannot?
Fatty acids can’t cross the blood brain barrier
What is the purpose of the shuttles?
To regenerate NAD+ from NADH and carry its electrons across the matrix as the inner mitochondrial membrane is impermeable to NADH
Where in the body is the Glycerol-Phosphate Shuttle used?
Brain and skeletal muscle
Explain how the Glycerol-Phosphate Shuttle works
Cytosolic glycerol-3-phosphate dehydrogenase transfers electrons from NADH to dihydroxyacetone phosphate (DHAP), producing glycerol-3-phosphate
Mitochondrial glycerol-3-phosphate dehydrogenase regenerates DHAP and transfers the electrons to FAD, which passes them on to coenzyme Q
Where in the body is the Malate-Aspartate Shuttle used?
Liver, kidney and the heart
Give the transamination reaction which occurs in the malate-aspartate shuttle
Oxaloacetate (ka) + glutamate (aa) →
alpha-ketoglutarate (ka) + aspartate (aa)
Outline the processes in the Malate-Aspartate Shuttle
- Malate dehydrogenase reduces oxaloacetate to malate and oxidises NADH to NAD
- Malate is shuttled into mitochondria and mitochondrial MDH forms NADH and oxaloacetate
- Oxaloacetate (ketoacid) and glutamate (amino acid) undergo transamination by aspartate transaminase to form aspartate (amino acid) and alpha-ketoglutarate (ketoacid)
- Aspartate is shuttled out the mitochondrion and cytosolic AT carries out reverse transamination to produce glutamate (amino acid) and oxaloacetate (ketoacid)
What kind of transporters are used to shuttle the molecules between the cytosol and mitochondrion?
Antiporters (transport in 2 directions)
When malate enters the mitochondrion, alpha-ketoglutarate leaves
When aspartate leaves the mitochondrion, glutamate enters
What is a ligation requiring ATP cleavage reaction?
Formation of covalent bonds eg carbon-carbon bonds
What is an isomerisation reaction?
The rearrangement of atoms to form isomers
What is a group transfer reaction?
The transfer of a function group from one molecule to another
What is a hydrolytic reaction?
The cleavage of bonds by the addition of water
What are the two main concepts of glycolysis?
The formation of a high energy compound, and the splitting of this compound
Why is the generation of NADH useful?
NADH can be later used to generate yet more ATP within the mitochondria in oxphos
Why is creatine phosphate used as a buffer?
In muscle, the amount of ATP needed during exercise is only enough to sustain contraction for around one second
Describe how creatine phosphate is used to generate ATP?
Creatine phosphate -> creatine + ATP
How is Acetyl CoA able to donate the acetate group (2C) to other molecules?
The thioester linkage which it contains is a high energy linkage so is readily hydrolysed
What are the symptoms of Beri Beri?
Damage to the peripheral nervous system, weakness of the musculature and decreased cardiac output
What enzyme is used in the transamination reaction of alanine?
Alanine (C3) undergoes transamination by the action of the enzyme alanine aminotransferase.
What does the transamination of alanine produce?
Pyruvate and glutamate
What type of reaction is occuring in the Malate-Aspartate Shuttle?
Transamination reaction
Mutations in which TCA genes have been shown to decrease TCA activity and enhance aerobic glycolysis?
Isocitrate dehydrogenase
Succinate dehydrogenase
Fumerase
How many ATP does the oxidation of one Acetyl CoA produce?
12
How many ATP molecules are formed by the reoxidation of each NADH?
3
How many ATP molecules are formed by the reoxidation of each FADH2?
2
What is the name of the process of fatty acid metabolism?
Beta oxidation
Where does beta oxidation occur?
In the mitochondria
How are fatty acids converted into an Acyl CoA Species?
Using Acyl CoA synthetase
What is the first step of beta oxidation of fatty acids?
Acyl CoA synthetase combines fatty acid with coenzyme A
This produces an acyl CoA species
2 high energy phosphoanhydride bonds in ATP are hydrolysed, producing AMP and 2 inorganic phosphates
Where does the generation of Acyl CoA occur?
In the outer mitochondrial membrane
What is the name of the shuttle used to transport Acyl CoA species into the miatrix?
Carnitine shuttle
Describe how the carnitine shuttle works
Carnitine acyltransferase I transfers the acyl group from acyl CoA to carnitine, forming acyl carnitine
Translocase shuttles acyl carnitine across the membrane
Carnitine acyltransferase II converts acyl carnitine back to carnitine and forms acyl CoA inside the matrix
What is used as a supplement when you have primary carnitine deficiency?
Carnitor / levocarnitine
What type of deficiency is primary carnitine deficiency?
Autosomal recessive disorder
What are some symptoms of primary carnitine deficiency?
Encephalopathies (brain disease), cardiomyopathies, muscle weakness, and hypoglycaemia
What causes cardiomyopathy and muscle weakness in primary carnitine deficiency?
Fatty acids are not entering mitochondria and they provide ATP for muscle and heart
Describe the mutations involved in primary carnitine deficiency
Mutations in a gene known as SLC22A5 which encodes a carnitine transporter
This results in reduced ability of cells to take up carnitine, needed for the β-oxidation of fatty acids
What are the four reactions in Beta-oxidation?
Oxidation, hydration, oxidation and thiolysis
What does one cycle of beta oxidation result in?
1 Acetyl CoA
1 Acyl CoA species 2 carbons shorter than the original
1 FADH2
1 NADH
What does beta oxidation of palmitic acid (16 C) generate?
Palmitoyl CoA + 7 FAD + 7 NAD+ + 7 H2O + 7 CoA → 8 acetyl CoA + 7 FADH2 + 7 NADH
How many beta-oxidation cycles occur for palmitic acid (16C)?
7
What happens to the acetyl CoA generated through beta-oxidation?
Enters the TCA cycle only if B-oxidation and carbohydrate metabolism are balanced, as oxaloacetate is needed for entry
What does Acetyl CoA form during fasting?
Ketone bodies
D-3-hydroxybutyrate, acetone and acetoacetate
Why does Acetyl CoA form ketone bodies during fasting?
During fasting, fat breakdown predominates, thus forming ketone bodies that can be used by brain/muscle where they are converted back into acetyl CoA
What is fatty acid biosynthesis known as?
Lipogenesis
What are the two enzymes involved in fatty acid biosynthesis?
Acetyl CoA carboxylase
Fatty acid synthase (contains 7 enzymes)
What are the three enzymes which are involved in lipogenesis (contained in fatty acid synthase)?
Ketoreductase, dehydratase and enol reductase
During lipogenesis, what is the growing fatty acid group linked to?
An Acyl carrier protein
What are the differences in carriers between synthesis and degradation of fatty acids?
Lipogenesis uses Acyl Carrier Protein
B-oxidation uses CoA
What are the differences in reducing powers between synthesis and degradation of fatty acids?
Lipogenesis uses NADPH
B-oxidation uses FAD/NAD+
What are the differences in location between synthesis and degradation of fatty acids?
Lipogenesis occurs in the cytoplasm
B-oxidation occurs in the mitochondrial matrix
What is the overall reaction of lipogenesis to form palmitate (C16)?
Acetyl CoA (C2) + 7 Malonyl CoA (C3) + 14 NADPH + 14 H+
—-> Palmitate (C16) + 7 CO2+ 6 H2O + 8 CoA-SH + 14 NADP+
What does the desaturation of fatty acids required?
The action of fatty acyl-CoA desaturases
What is the enzyme that creates oleic acid from stearate and palmitoleic acid from palmitate?
Delta-9 desaturase
In adults where is de novo Faty acid biosynthesis restricted to?
The liver, adipose tissue and lactating breasts
What is meant by Acyl-CoA-dehydrogenase specificity?
Each acyl-CoA-dehydrogenase can bind a fatty acid chain of varying lengths
How much ATP does the reoxidation of 1 NADH generate?
3
How much ATP does the reoxidation of 1 FADH2 generate?
2
What is the net gain in ATP following the β-oxidation of one palmitate molecule?
129
Reoxi of 7 NADH = 21
Reoxi of 7 FADH2 = 14
8 acetyl CoA enter TCA cycle = 8 x 12 = 96
Initial hydrolysis of ATP to AMP in B-oxidation = -2
What is MCADD?
Medium chain Acyl CoA dehydrogenase deficiency
What type of diet should patients with MCADD sick to?
A high carbohydrate diet
What must patients with MCADD need if their illness manifests as apetite loss or severe vomiting?
IV glucose to make sure their body is not dependant on fatty acids for energy
How can MCADD be treated?
Lifestyle adaptations
How are fats stored in the adipocytes?
As triglycerides
What are triglycerides broken down into?
glycerol and fatty acids using lipase
How many ATP does the generation of the Fatty Acyl CoA need?
2
How many NADH and FADH2 does beta oxidation produce?
1 of each
What is TPI needed for?
Converstion of DHAP into G-3-P
What is the only fatal enzyme glycoltic enzymopathy?
TPI deficiency
What are the 5 main classes of lipids?
Free fatty acids
Triglycerides
Phospholipids
Glycolipids
Steroids
What are fatty acids?
Hydrocarbon chains with a terminal carboxylic acid group
What makes triglycerides ideal for storage?
The 3 fatty acids are attached to a glycerol via ester linkages
This neutralises the COOH group to keep cell pH in normal range
Thus the FAs are reduced and anhydrous
Why is does it make sense that ox phos happens in mitochondria?
Acetyl CoA produced from both major food molecules (sugars and fats) in mitochondria so most of ox phos happens there too to make majority of cellular ATP
What 3 primary sources are fats derived from?
Diet
De novo biosynthesis in liver
Storage depots in adipose
Where are bile salts made, stored and where do they pass to?
Generated in liver
Stored in gall bladder
Pass from bile duct into intestine during digestion
What do bile salts do?
Emulsify fats
What feature do bile salts have that help them emulsify fats?
Hydrophobic face that contacts the triglyceride
Outer hydrophilic face allowing the entire micelle to be solubilised
So that enzymes like pancreatic lipase can digest the TGs into FAs
What vitamins do bile salts help absorb?
Fat soluble
A, D, E, K
What is steatorrhea?
Fatty stool
Lack of bile salts means fat passes through gut undigested and unabsorbed
What do chylomicrons do and what are their source?
From intestines
For dietary transport
What do VLDLs do and what are their source?
From liver
For endogenous fat transport
What do IDLs do and what are their source?
From VLDLs
Precursor for LDL
What do LDLs do and what are their source?
From IDLs
For cholesterol transport
What do HDLs do and what are their source?
From liver
For reverse cholesterol transport
Why are lipoproteins important?
Carry fatty acids (hydrophobic, can’t dissolve in plasma) in the circulation
How are chylomicrons formed?
Digested products are absorbed by enterocytes lining the small intestine brush border
Triglycerides are reformed by enzymes, then packaged into chylomicrons
What do chylomicrons do once formed in enterocytes?
Travel from lacteals to thoracic duct to left sublavian vein
Enter blood and pick up apoproteins from HDL
Interact with lipoprotein lipase on capillary endothelial cells
What do the lipoprotein lipases do to chylomicrons?
Break down the chylomicrons
TGs are thus digested into FFAs and glycerol
FFAs absorbed by cell and undergo B-oxidation
Glycerol returned to liver for gluconeogenesis
How does VLDL become IDL?
Transfers apoproteins to HDL
How can HDL become LDL?
Transfer its cholesterol esters to IDL
