Carbohydrate and Lipid Metabolism Flashcards
What is oxidative stress in diabetes?
Due to an increase in glucose levels
Toxic oxidising molecules which can damage proteins and biochemical processes, causes cardiovascular and nerve tissue damage
Name three main Reactive Oxygen Species (ROS):
Hydrogen Peroxide H2O2
Superoxide O2*
Hydroxyl radicals HO*
Describe Glutathione (GSH) and how does it work?
Tripeptide with an essential cysteine a.a thiol
Glutathione maintains an intracellular reducing environment and helps neutralise ROS
Thiol reacts with oxidative species, reducing it to water and the glutathione converts to disulphide
What is the cellular conc of GSH?
10mM (high conc)
How is glutathione made back from disulphide?
Glutathione reductase uses a reducing co-factor of NADPH-> NADP+ and reduces it into 2 thiol molecules
Need a high conc of NADPH too
Describe hyperglycaemia- induced superoxide production:
Increase amount of glucose, glycolysis of Krebs cycle
Increase in NADH/FADH2 (in Krebs cycle) increase in proton gradient across inner mitochondrial membrane
This inhibits electron transfer from complex-III ion electron transport chain
So electrons don’t go all the way down chain to react with O2 to produce H2O
Instead some of electrons half way through will leak out into solution early to react with water to produce superoxide (single e- on O2 molecule)
How does superoxide cause hyperglycaemic damage?
Superoxide overproduction inhibits GAPDH (for glycolysis)
This diverts upstream glycolysis metabolites causing four pathways of hyperglycaemic damage
Name the four pathways of hyperglycaemic damage:
Polyol pathway
Hexosamine pathway
Protein kinase C pathway
AGE pathway
Describe the Polyol pathway:
Aldose reductase converts aldehydes into alcohol (sorbitol)
Glucose has an aldehyde
Aldose reductase has a high Km so low affinity for glucose
As conc of glucose increases, more aldose reductase activated (using more NADPH ->NADP+)
If more glucose then more aldose reductase so more NADPH used so decrease in cellular conc of NADPH
So less Glutathione disulphide (GSSG) into GSH, so cells can’t neutralise reactive oxygen species
ROS can lead to oxidation of alcohols to aldehydes so even more aldose reductase needed, on going cycle
Describe the Protein Kinase C pathway:
Excess DHAP converted to Diacyl glycerol (DAG)
DAG activated PKC
PKC activity then affects function of various proteins:
- blood flow abnormalities
- capillary and vascular occlusion
- pro- inflammatory gene expression
* increased NADPH oxidase activity
NADPH + 2O2 -> NADP+ + H+ + 2O2* (superoxide)
What is Methylglyoxal (MG) and how is it made?
A toxic byproduct of glycolysis
As DHAP and GAP build up, they chemically degrade to produce methylglyoxal
MG is only a low level side reaction under normal conditions, if conc of DHAP and GAP increase, so does conc of MG
What does AGE stand for and what occurs during it?
Advanced Glycation End- products
MG is a very reactive carbonyl
The aldehyde can easily react with the amine groups on lysine side chains to form imines (AGE products)
These proteins cause damage of proteins by several mechanisms
How do AGE products cause damage?
- AGE precursors bind to AGE receptors on macrophages and induce oxidative stress and produces reactive oxygen species
- Other AGE receptor binding interactions can also activate transcription factor NF-kB causing pathological changes in gene expression
Describe how MG can cause a decrease in GSH:
Increase levels of oxidative stress decrease levels of GSH so less effective at detoxifying MG which can cause more oxidative stress
MG also detoxified by polyol pathyway, which lowers NADPH levels so less GSSG-> GSH
What are the functions of insulin?
Stimulates glucose -> cells
Stimulates conversion of excess glucose into glycogen
Promotes Triacyl Glycerol (TAG) biosynthesis
Inhibits synthesis of TAG
What type of insulin do type 1 diabetics need?
We are in constant need of insulin even when not eating so:
Basal insulin (24 hour insulin)
Fast acting insulin (before each meal)
What causes ketoacidosis?
Significantly lowered (or zero) basal insulin
What happens in ketoacidosis?
Increase in TAG hydrolysis which leads to:
Increase in fatty acid levels which are catabolised to give:
Increased Acetyl -CoA levels
Excess Acetyl CoA is converted into ketone bodies, acidic side chains which can lower blood pH
What is a diagnostic indicator of ketoacidosis?
Smell of acetone on breath
What is a major cause of extreme hyperglycaemia?
An osmotic diuresis that leads to a loss of water and electrolytes in urine so the blood is more concentrated with glucose
How would you monitor long term blood glucose control and why?
Over 2-3 months measure levels of Hb glycation (Hb1Ac)
A RBC lives for around 120 days
Glucose naturally binds to Hb which creates glycated haemoglobin (Hb1Ac)
Why is it possible to measure long term blood glucose control?
0.02% of glucose exists in a ring open form
The aldehyde on the ring open form of glucose forms an imine with the amino acid group at the end of the Hb polypeptide chain
The amine produce -> amadori product
The higher the conc of glucose the increase in the quantity of the ring open form so the faster the reaction takes place
Can measure this, useful for clinical diagnosis
What is the value for the Hb1Ac value for uncontrolled diabetes?
Higher than 8.6mmol/L
What is metabolism?
Catabolism + anabolsim
The sum total of all the biochemical reactions within a living organism
What is catabolism?
Where large biochemical molecules are broken down into smaller ones (releases energy)
What is anabolism?
Where small biochemical molecules are joined together to form larger ones (requires energy)
What is NAD+?
Nicotinamide Adenine Dinucleotide
Redox co-factor- can catalyse oxidation or reduction processes
It oxidises
What is NADH?
Same as NAD+, just reduces and doesn’t have an extra proton
What is NADPH?
Same as NADP but has another phosphate group
What is FAD?
Flavin Adenine Dinucleotide
Redox co-factor
Has a flavin ring
What is SCoA and how is it important?
An acyl-carrier
Acetyl Coenzyme A
Has an important thioester bond
How are carbohydrates metabolised?
Blood glucose-> oxidised into ATP
Blood glucose-> excess storage, anabolic into glycogen, stored in muscles, produced in liver
Describe the formation of glycogen from glucose:
Glycogenesis -2ATP into glycogen
Glycogenolysis, hydrolyses glycosidic bonds into glucose
Describe glycolysis in carbohydrate metabolism:
(enzymes)
Glucose (hexokinase)-> glucose-6-phosphate using ATP
Into Fructose-6- phosphate into Fructose1,6bisphosphate using ATP
Into (Aldolase) 2x Glyceraldehyde 3-phosphate into 1,3 biphosphate glycerate using 2NAD+ into 2NADH
Eventually into two molecules of pyruvate from 1 molecule of glucose
4 molecules of ATP
Describe the citric acid cycle (krebs) in carbohydrate metabolism:
Pyruvate into CoA using NAD+ into NADH and releasing CO2
CoA into citrate (6C) into 5C sugar using NAD+
5C into succinate (4C) using GDP-> GTP and NAD+
Succinate into oxaloacetate (4C) back round to CoA using NAD+ and FAD-> FADH2
What are the outcomes of the Krebs cycle?
1x FADH2
3x NADH
1x GTP
2x CO2
Where does the electron transport chain occur?
Mitochondria
What are the steps in the electron transport chain (ETC)?
Proteins are found in the intermembrane
NADH= 1st and 2nd steps FADH2= 3rd step
Once electrons are added in parts to different proteins, small amounts of energy allow them to pass protons (H+) from the inner mitochondrial membrane to the inter membrane space
In stages seven and eight those electrons and protons are combined to produce H2O
Describe oxidative phosphorylation in carbohydrate metabolism for ATP:
The buildup of H+ in the intermembrane space generates an electrochemical gradient across the inner mitochondrial membrane
The flow of protons is allowed by ATP synthase, as H+ passes through, it gathers the energy from them to generator a linkage of ADP+Pi
How many molecules of ATP are produced in carbohydrate metabolism?
10 per CoA catabolised
What are triacylglycerols?
Energy storage (main metabolised)
What are two types of lipids?
Triacylglycerols
Cholesterol
What are the products of hydrolysis of lipids?
Glycerol
3 fatty acid chains combined with CoA, can vary in length and double bonds
How does the hydrolysis of ester bonds generate ATP?
B-oxidation of fatty acid to enter the inter membrane, removes two carbons at a time and yields:
1x FADH2
1x NADH
2x CoA leads to Krebs cycle
With addition of ATP and NAD+, glycerol can form dihydroxyacetone phosphate which causes glycolysis
Why is lipid metabolism better than carbohydrate metabolism?
From a single 18 chain fatty acid, from B oxidation, you can generate 122 ATP molecules
Compared to 30 ATP with glucose
What type of molecules does skeletal muscle use as an energy source and when?
Glucose in an active state
Fatty acids in an inactive state
What type of molecules does cardiac muscle use as an energy source and when?
Use fatty acids first but may use ketone bodies, glucose and lactate
What type of molecules does the liver use as an energy source and when?
Fatty acids
What type of molecules does the brain function use as an energy source and why?
Maintained by glucose and ketone bodies since fatty acids can’t cross the BBB
In diabetes, decrease in sugar so limited energy source for brain
What is lipogenesis?
ACoA can be used to make lipids
What is ketogenesis?
Metabolic pathway by which ketone bodies are synthesized from “excess” acetyl CoA which cannot be used in the citric acid cycle because of low oxaloacetate levels
Describe the ketogenesis pathway:
2x Acetyl CoA into acetoacetyl CoA
3-hydroxy-3-methylglutaryl CoA into the ketone bodies e.g acetone
3-hydroxy-3-methylglutaryl also made into cholesterol
Name 3 ketones and how are they bad?
B-hydroxy butyrate, acetoacetate, and acetone
Carboxylic acids which can affect blood pH
How are amino acids metabolised?
Ammonia is a toxic byproduct and converted into non-toxic urea
Describe the degradation of amino acids:
Carbon metabolism- triacylglycerols via fatty acid biosynthesis, glucose via glucogeonesis, ATP via citric acid cycle, ketone bodies via ketogenesis
Nitrogen metabolism- Elimination via urea, biosynthesis of non-essential a.a, biosynthesis of non protein nitrogen containing products e.g nucleic acids
What is trans-amination?
Enzymes which catalyse the interchange of the amino group of an alpha amino acid with the keto group of an alpha keto acid
e.g alanine with a.a-> pyruvate with keto acid
e.g aspartate with a.a-> oxaloacetate with keto acid
What is a hetrogeneous polysaccharide?
Multiple different sugars
How are monosaccharides classified as aldoses or ketoses?
Depending on whether the carbonyl group is an aldehyde or ketone
How do you number the carbon chain of monosaccharides?
The numbering starts at the end of the chain which is closer to the carbonyl group
What are axial substituents?
Substituents pointing directly above or below the ring
What are equatorial substituents?
Substituents pointing out of the ring
Give an example of a hydrolysable lipids?
Triacylglycerols
Give an example of a non-hydrolysable lipids?
Steroid hormones e.g cholesterol
What is the function of cholesterol?
Unsaturated fatty acids import a liquid like character membrane
Presence of rigid cholesterol molecules regulate fluidity
How are excess amino acids broken down into urea?
Are transferred to glutamate and converted into glutamate via glutamate dehydrogenase
Why are primary alcohols more reactive than secondary alcohols?
They are less hindered
Why are axial alcohols less reactive than equatorial OH’s?
More steric hindrance on axial, as closer to ring
What can primary alcohols be oxidised to?
Aldehydes and carboxylic acids
What can secondary alcohols be oxidised to?
Ketones
Give 5 examples of functional groups prone to hydrolysis:
Esters
Amides
Lactones and lactams^ cyclic versions
Thioesters
Imines
What are imines and how are they formed?
Nitrogen equivalent of an aldehyde/ ketone
The addition of 1º amine to a carbonyl group, followed by elimination of water
Under which conditions does imine synthesis occur?
Mildly acidic, 4-6
If too acidic then the amine will be fully protonated so no nucleophilic attack
At higher pH ranges there will be no elimination
At physiological pH this reaction occurs but at a very slow rate
