Cell metabolism 1 Flashcards
TCA
tricarboxylic acid
How to liberate energy from combustion of glucose
Delta g for combustion of glucose : -2872 KJ/mol. To liberate that energy : apply heat. Same true for cellular oxidation of glucose. Process broken down into several steps. We overcome by use of enzymes or temp. Extraction of energy from Glucose around 40 % efficient.
complete oxidation of glucose results in how much ATP
38 ATP
Delta g for hydrolysis of phophoanhydride bond is
-31 KJ/mol (38 x-31 = 1178 kJ /mol) (40 % efficiency ). In car only 20 % efficient.
Two shuttles move electrons generated from NADH in glycolysis to mitochondria
Malate aspartate shuttle in kidney, liver and heart and and glycerol phosphate shuttle in the skeletal muscle and brain
Malate aspartate shuttle
Malate shuttled into mitoch and aspartate shuttled out, transferring e from Nadh in cytosole to mitochondria.
Reliant on two enzymes : aspartate transaminase and malate dehydrogenase.
Malate is oxidised to oxaloacetate by cytosolic and mitochondrial forms of enzyme malate dehydrogenase (MDH). Transfer of amino group of glutamate into oxaloacetate generates aspartate and a - ketoglutarate.
Substrates of kinases
serine, threonine and tyrosine. These all contain OH group which can be phosphorylated.
Phosphorylation of glucose means…
It can’t bind to glucose transporters
Lactate dehydrogenase (LDH)
Makes lactate (NADH can be regenerated). It catalyses the reduction of pyruvwte regenerating NAD + and allowing glycolysis to continue. Present in many body tissues. Elevated level - indicates stoke / heart attack.
Where does TCA take place
Mitochondrial matrix
Cancer may result in …
increased expression of glucose transporters eg GLUT1. Use this to our advantage - administer 18 F to patients.
Catabolic reaction
Larger molecule broken down into smaller one - eg - Energy contained in phosphoanhydride bond is a way for the cell to harness the energy liberated from breaking bonds within the food molecules. The energy released is harnessed as ATP.
Anabolic reactions
Building large molecules from smaller ones. e.g. membranes, DNA, and proteins.
Main stages of cellullar metabolism
- Glycolysis: the oxidation of glucose in the cytosol to produce ATP and NADH
- TCA (Krebs) Cycle: further oxidation of small molecules in the mitochondria to produce NADH, ATP and FADH2
- Oxidative Phosphorylation: generation of ATP in the mitochondria through the reduction of O2 and H2O (and the reoxidation of the products aformentioned)
6 reactions that make up cell metabolism
- Oxidation-reduction
- Ligation requiring ATP cleavage (forming covalent bonds)
- Isomerization (rearranging atoms to form isomers)
- Group transfer (transferal of functional grp from one molecule to other)
- Hydrolytic (cleaving with water)
- Addition or removal of functional groups (to and from double bonds)
2 stages of glycolysis
- Formation of a high energy compound using an investment of ATP
- Splitting of a high energy compound producing useful energy in the form of ATP
STage 1 glycolysis
See noteback for detailed notes
- Grp Transfer reaction (transf of phosphate group from ATP to glucose, irreversible) glucose —-hexokinase—->glucose-6-phosphate
- Isomerisation reaction : glucose-6-phosphate —phospoglucose isomerase——> fructose-6-phosphate
- Group transfer (phosphate group from ATP to fructose-6-phosphate, regulation of enzyme is key control step for entry into glycolysis) : fructose-6-phosphate —–phospofructokinase—-> fructose-1,6-biphosphate
- Formation of two high energy compounds : fructose-6-phosphate —–aldolase—–> glyceraldehyde 3-phosphate (G3P) + dihydroxyacetone phosphate (DHAP)
- Isomerisation, DHAP —–triose phosphate isomerase (TPI) ——–> G3P (G3P more useful high energy, only one that can be broken down to form ATP)(defiency in TPI is only fatal glycolytic enzymopathy)
Stage 2 glycolysis
These steps occur for both G3P molecules
- Redox/reduction using NAD+ as reducing agent( NADH generated can be used to generate ATP in O.P. later) : glyceraldehyde 3-phosphate —glyceraldehyde 3-phosphate dehydrogenase-——> 1,3-biphosphoglycerate
- Group transfer (phosphate group goes from 1,3-bisphosphoglycerate to ADP to form ATP) 1,3-bisphosphoglycerate—-phosphoglycerate kinase —–> 3-phosphoglycerate
- Phosphate group moved from the 3- to the 2- position via the removal and then addition of a phosphate group : 3-phosphoglycerate—-phosphoglycerate mutase—–> 2-phosphoglycerate
- Removal of functional groups (dehydration) to form a double bond : 2-phosphoglycerate —-enolase—-> phosphoenolpyruvate +H20
- Group transfer of phosphate from phosphoenolpyruvate to ADP to form ATP and pyruvate : phosphoenolpyruvate —-pyruvate kinase—-> pyruvate
- The net result of glycolysis is 2 ATP molecules and 2 NADH molecules (and 2 pyruvate)
Fates of pyruvate : alcoholic fermentation
Characteristic of yeast and can occur under anaerobic conditions
Fates of pyruvate : generation of lactate
- Anaerobic reaction
- Characteristic of mammalian muscles during intense activity when oxygen is a limiting factor.
- Allows NAD+ to be regenerated which allows glycolysis to continue* in conditions of O2 deprivation (when rate of NADH formation by glycolysis > rate of oxidation by respiratory chain)
- *NAD+ is required in Step 6 of glycolysis
Why is creatine phosphate imprtant
The amount of ATP produced in resting muscle is only enough to sustain contraction for around one second
Our muscles contain a reservoir of creatine phosphate to buffer demands for phosphate (for rapid formation of ATP)
Atp hydrolysis during exercise
ATP levels in muscles are rapidly hydrolysed
Creatine Phosphate buffers for a while
Which is why athletes use creatine as a dietary supplement
Acetyl coA generation
Occurs in the mitochondria thus the Acetyl CoA produced is committed to the TCA cycle
Catalysed by PDH complex
The thioester bond is a high energy linkage so it’s readily hydrolysed, enabling Acetyl CoA to donate the acetate (2C) to other molecules.
Beri Beri disease
A deficiency in thiamine, a derivative of vitamin B1, and a cofactor of the PDH complex which catalyses the formation of Acetyl CoA, is the cause of Beri-Beri.
Symptoms include damage to the PNS; weakness of the musculature; and decreased cardiac output.
The brain relies heavily on glucose metabolism which means that it is particularly vulnerable.
Tricarboxylic acid (krebs ) cycle
Krebs cycle enzymes, with one exception, are soluble proteins located in the mitochondrial matrix
The bulk of the ATP is generated when the reduced coenzymes are re-oxidised with the help of O2 in oxidative phosphorylation
Thus the TCA cycle only operated under aerobic conditions
Amino acids and TCA cycle
- General strategy of amino acid degradation is to remove the amino group (eventually excreted as urea) and the carbon skeleton is either funnelled into production of glucose or fed into TCA cycle
- Degradation of all 20 amino acids leads to the formation of only 7 molecules:
Pyruvate
Acetoacetyl CoA
Acetyl CoA
Alpha-ketoglutarate
Succinyl CoA
Fumarate
Oxaloacetate
Transamination reactions
Group transfer reactions in which an amine group is transferred from an amino acid to a keto acid to produce a new amino acid and keto acid
eg
Alanine undergoes transamination (through the action of alanine aminotransferase) to produce pyruvate and glutamate
Pyruvate can enter the TCA cycle
Glutamate is made into alpha-ketoglutarate generating NH4+ which ultimately becomes urea
Importance of shuttles
Glycolysis occurs in the cytosol, however the NADH produced is needed in the mitochondria to be used in oxidative phosphorylation and to regenerate NAD+
There is a finite amount of NAD+ so failure to regenerate would lead to glycolysis stopping
Thus it is transported via shuttles (glycerol phosphate shuttle
and malate-aspartate)
Glyerol phosphate shuttle
Electrons from NADH (rather than the whole molec) are carried aX the mitochondrial membrane
- Cytosolic glycerol 3-phosphate dehydrogenase transfers electrons from NADH to DHAP to generate glycerol 3-phosphate
- Membrane-bound glycerol 3-phosphate dehydrogenase transfers the electrons to FAD.
- The electrons are then passed to co-enzyme Q (part of the ETC)
Malate-aspartate shuttle (kidney, liver, heart)
- Contains transamination and redox reactions
- The electrons are transferred, not the cofactors
- AT and MDH have both cytosolic and mitochondrial membrane bound versions
Glycolysis and the TCA cycle provide starting point for many biosynthetic reactions
TCA cycle defects in cancer
Mutations in the genes marked in red are shown to decrease TCA activity and enhance aerobic glycolysis - which is where, even in sufficient O2, glucose will form lactate (The Warburg Effect)
Which aa can be phosphorylated
Serine, threonine and tyrosine all contain an OH group and can therefore be phosphorylated (in a group transfer reaction catalysed by a kinase)
LDH as diagnostic tool
- Elevated serum levels can be used to diagnose several disorders including stroke and myocardial infarction.
- Electrons need to be translocated from the cytosol to the mitochondrial matrix for oxphos; this is carried out by two shuttle systems; both which rely upon the action of cytosolic and membrane bound forms of the same enzyme.
- Malate-Aspartate shuttle in the kidney, liver and heart and the Glycerol-Phosphate shuttle in the skeletal muscle and brain.
- Cancer cells undergo selective reprogramming and use diff mechanisms to promote cell growth. One of these mechanisms is increased expression of the GLUT1 glucose transporter (found in endothelium and erythrocytes).
- You can administer radiolabelled 18F glucose to patients which allows the locating of tumours via PET imaging to monitor progress of chemotherapy.
