Cell metabolism 1

Question 1

TCA

Answer 1

tricarboxylic acid

Question 2

How to liberate energy from combustion of glucose

Answer 2

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.

Question 3

complete oxidation of glucose results in how much ATP

Answer 3

38 ATP

Question 4

Delta g for hydrolysis of phophoanhydride bond is

Answer 4

-31 KJ/mol (38 x-31 = 1178 kJ /mol) (40 % efficiency ). In car only 20 % efficient.

Question 5

Two shuttles move electrons generated from NADH in glycolysis to mitochondria

Answer 5

Malate aspartate shuttle in kidney, liver and heart and and glycerol phosphate shuttle in the skeletal muscle and brain

Question 6

Malate aspartate shuttle

Answer 6

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.

Question 7

Substrates of kinases

Answer 7

serine, threonine and tyrosine. These all contain OH group which can be phosphorylated.

Question 8

Phosphorylation of glucose means…

Answer 8

It can’t bind to glucose transporters

Question 9

Lactate dehydrogenase (LDH)

Answer 9

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.

Question 10

Where does TCA take place

Answer 10

Mitochondrial matrix

Question 11

Cancer may result in …

Answer 11

increased expression of glucose transporters eg GLUT1. Use this to our advantage - administer 18 F to patients.

Question 12

Catabolic reaction

Answer 12

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.

Question 13

Anabolic reactions

Answer 13

Building large molecules from smaller ones. e.g. membranes, DNA, and proteins.

Question 14

Main stages of cellullar metabolism

Answer 14

1. Glycolysis: the oxidation of glucose in the cytosol to produce ATP and NADH
2. TCA (Krebs) Cycle: further oxidation of small molecules in the mitochondria to produce NADH, ATP and FADH2
3. Oxidative Phosphorylation: generation of ATP in the mitochondria through the reduction of O2 and H2O (and the reoxidation of the products aformentioned)

Question 15

6 reactions that make up cell metabolism

Answer 15

• 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)

Question 16

2 stages of glycolysis

Answer 16

• 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

Question 17

STage 1 glycolysis

Answer 17

​See noteback for detailed notes

1. Grp Transfer reaction (transf of phosphate group from ATP to glucose, irreversible) glucose —-hexokinase—->glucose-6-phosphate
2. Isomerisation reaction : glucose-6-phosphate —phospoglucose isomerase——> fructose-6-phosphate
3. 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
4. Formation of two high energy compounds : fructose-6-phosphate —–aldolase—–> glyceraldehyde 3-phosphate (G3P) + dihydroxyacetone phosphate (DHAP)
5. 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)

Question 18

Stage 2 glycolysis

Answer 18

These steps occur for both G3P molecules

1. 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
2. Group transfer (phosphate group goes from 1,3-bisphosphoglycerate to ADP to form ATP) 1,3-bisphosphoglycerate—-phosphoglycerate kinase —–> 3-phosphoglycerate
3. 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
4. Removal of functional groups (dehydration) to form a double bond : 2-phosphoglycerate —-enolase—-> phosphoenolpyruvate +H20
5. Group transfer of phosphate from phosphoenolpyruvate to ADP to form ATP and pyruvate : phosphoenolpyruvate —-pyruvate kinase—-> pyruvate
6. The net result of glycolysis is 2 ATP molecules and 2 NADH molecules (and 2 pyruvate)

Question 19

Fates of pyruvate : alcoholic fermentation

Answer 19

Characteristic of yeast and can occur under anaerobic conditions

Question 20

Fates of pyruvate : generation of lactate

Answer 20

• 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

Question 21

Why is creatine phosphate imprtant

Answer 21

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)

Question 22

Atp hydrolysis during exercise

Answer 22

ATP levels in muscles are rapidly hydrolysed

Creatine Phosphate buffers for a while

Which is why athletes use creatine as a dietary supplement

Question 23

Acetyl coA generation

Answer 23

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.

Question 24

Beri Beri disease

Answer 24

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.

Question 25

Tricarboxylic acid (krebs ) cycle

Answer 25

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

Question 26

Amino acids and TCA cycle

Answer 26

* 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

Question 27

Transamination reactions

Answer 27

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

Question 28

Importance of shuttles

Answer 28

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)

Question 29

Glyerol phosphate shuttle

Answer 29

Electrons from NADH (rather than the whole molec) are carried aX the mitochondrial membrane 1. Cytosolic glycerol 3-phosphate dehydrogenase transfers electrons from NADH to DHAP to generate glycerol 3-phosphate 2. Membrane-bound glycerol 3-phosphate dehydrogenase transfers the electrons to FAD. 3. The electrons are then passed to co-enzyme Q (part of the ETC)

Question 30

Malate-aspartate shuttle (kidney, liver, heart)

Answer 30

* Contains transamination and redox reactions * The electrons are transferred, not the cofactors * AT and MDH have both cytosolic and mitochondrial membrane bound versions

Question 31

Glycolysis and the TCA cycle provide starting point for many biosynthetic reactions

Answer 31

Question 32

TCA cycle defects in cancer

Answer 32

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)

Question 33

Which aa can be phosphorylated

Answer 33

Serine, threonine and tyrosine all contain an OH group and can therefore be phosphorylated (in a group transfer reaction catalysed by a kinase)

Question 34

LDH as diagnostic tool

Answer 34

* 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.