Carbs and metabolism

Question 1

What is a glycosidic linkage?

Answer 1

the bond created between two monosaccharide sugars to to form a disaccharide.

Question 2

What is glycolysis? What is the reactants and products? Where does it take place?

Answer 2

Glycolysis is the first of 4 main steps in the the break down of Glucose (6C)

Glycolysis involves the splitting of glucose into two molecules of pyruvate (3C each)

glycolysis occurs in the cytoplasm (mitochondria) and can occur in the absence of oxygen

Question 3

What are all of the products of glycolysis?

Answer 3

2 molecules of pyruvate
2 molecules of ATP
2 molecules of NADH

Question 4

does glycolysis require ATP?

Answer 4

yes! glycolysis requires ATP to get it going but the net result of glycolysis is the production of + 2 ATP!!

Question 5

explain the general reaction flow of glycolysis from glucose to 2 pyruvate molecules (include enzymes)

(4 steps, 3 enzymes)

Answer 5

glucose –> glucose-6-phosphate (G6P) catalyzed by hexokinase
G6P –> F6P
F6P —> F-1,6-bp catalyzed by phosphofructokinase (PFK)

Series of reactions that lead to 2 molecules of PEP

2PEP —> 2 pyruvate via pyruvate kinase
this is the step that generates 2 ATP molecules

Question 6

Explain the actions of Hexokinase, phosphofructokinase, and pyruvate kinase.

Answer 6

hexokinase converts glucose to G6P
PFK converts F6P to F-1,6-bP
pyruvate kinase converts 2 PEP to 2 pyruvate (3C)

Question 7

What is the “committed step” of glycolysis? this is the step in which the glycolysis pathway is set and non-reversible.

Answer 7

The reaction that PFK catalyzes to convert F6P to F-1,6-bP is very thermodynamically favourable. Therefore once this conversion takes place, it is essentially irreversible.

PFK is biochemical valve in some sense.

note PFK is allosterically regulated by ATP in which which conc. of ATP lowers its activity.

Question 8

NADH and FADH2 are the high energy electron carriers within the body. They are responsible for becoming oxidized and creating a proton gradient that makes ATP (electron transport).

These molecules must be converted back to NAD+ and FAD in order to continuously make ATP. This is achieved in aerobic conditions when the electron transport chain oxidizes them.

What happens in anaerobic conditions?

Answer 8

The NAD+ in glycolysis if fully used up in anaerobic conditions. With no NAD+ left, no NADH would be formed and therefore no ATP synthesis may occur.

However, with a lack of oxygen our bodies are able to reproduce NAD+ through fermentation

Question 9

Explain how fermentation works? What is its products?

Answer 9

Rather then the electron transport chain oxidizing NADH back to NAD+, fermentation utilizes the pyruvate made in glycolysis.

The pyruvate molecules are reduced by NADH (which in turn is oxidized). The re-made NAD+ goes back to glycolysis to produce more ATP

in yeast the pyruvate reduction makes ethanol
in humans it produces lactate.

Question 10

what is the pyruvate dehydrogenase complex? PDC

Answer 10

in aerobic conditions when fermentation is not necessary, the pyruvate product of glycolysis interacts with the pyruvate dehydrogenase complex (PDC).

PDC takes pyruvate and performs a decarboxylation in which pyruvate turns into an acetyl group and NADH and CO2 are produced.

The acetyl group formed is then attached to a molecule called coenzyme A.

The PDC is located in the mitochondrial matrix

Question 11

what are the products of the PDC decarboxylation reaction?

Answer 11

1 NADH
1 CO2
and the molecule Acetyl-CoA (after the acetyl group joins coenzyme A)

Question 12

what is a prosthetic group?

Answer 12

a prosthetic group is a cofactor that is tightyl bound or covalently bound to its enzyme. The PDC contains a TCC prosthetic group which aids in decarboxylation

Question 13

TCC is a prosthetic group in PDC. What important vitamin is in it?

Answer 13

Vitamin B1 also known as thiamine

Question 14

Acetyl Co-A leaves PDC and makes its way to the Krebs cycle. What is the first step of the cycle?

Answer 14

Acetyl-CoA reacts with a molecule called oxaloacetate (OAA) which together forms citrate

acetyl-CoA (2 carbons - only consider acetyl not enzyme)
Oxaloacetate (4 carbons)
when these combine they form the 6 carbon citrate molecule.

Question 15

true or false, Oxaloacetate is used up by the krebs cycle and is required in our diet for replenishment.

Answer 15

false, OAA is recycled with each cycle.

Question 16

Once citrate has been formed (by acetyl-CoA + OAA) what occurs next in the krebs cycle?

what is the carbon count of our converted citrate after these steps?

Answer 16

A series of decarboxylation reactions occur in which 2 molecules of carbon dioxide and 2 molecules of NADH are produced. (These numbers are per pyruvate molecule)

citrate is a 6 carbon molecule. After the 2 decarboxylation rxns its a 4 carbon molecule called Succinyl-CoA

Question 17

how many carbons of coenzyme A are donated in the kreb cycle?

Answer 17

none! only acetyl carbons and oxaloacetate carbons shift around

Question 18

once succinyl-CoA has been produced, what are the final steps of the Krebs cycle?

Answer 18

The final steps are the reproduction of oxaloacetate. In this process, 1 GTP, 1 NADH, and 1 FADH2 (per pyruvate) is made.

Question 19

In terms of per glucose molecule, explain the products of ATP / NADH / FADH2 / GTP / CO2 after each process

1. glycolysis
2. PDC
3. Krebs cycle

Answer 19

1. After glycolysis: 2 ATP and 2 NADH
2. After PDC: 2 CO2 and 2 NADH
3. After Krebs: 6 NADH, 2FADH2, 2GTP, and 4 CO2

only 10 molecules of NADH so far and 2 ATP molecules (2 GTP gets converted to ATP so technically 4 ATP)

Question 20

Mitochondrial organization, what is the inner and outer membranes, porins, and cristae

Answer 20

Outer membrane - permeable membrane which contains porin proteins which create pores

Inner membrane - quite impermeable and folds deeply into the matrix where PDC and other respiration molecules are found. These folds are called Cristae

the electron transport chain and the ATP synthase are found embedded in the inner membrane of the mitochondria

Question 21

What is the key difference between glycolytic NADH and Krebs cycle NADH?

Answer 21

NADH produced from glycolysis must be transported into the mitochondrial matrix whereas all other NADH is produced in the right location already.

Question 22

What is the major difference between oxidative phosphorylation in eukaryotes and prokaryotes?

Answer 22

both perform oxidative phosphorylation (electron chain) except prokaryotes use their cytoplasmic membrane and eukaryotes use their inner mitochondrial membrane.

Question 23

Oxidative phosphorylation:
There are 5 membrane proteins in the electron transport chain. There are 3 large proteins and 2 smaller electron carriers. They are organized like so…

L - s - L - s - L

what are the names and roles of each carrier?

Answer 23

the purpose of the chain is to use the high energy of the electrons to pump H+ from the matrix into the inter membrane space.
the large carriers (contain heme groups) are responsible for pumping H+ out each time they are reduced.

The first molecule in the chain is NADH dehydrogenase which oxidizes NADH to NAD+. The next molecule is called ubiquinone or coenzyme Q.

ubiquinone / coenzyme Q passes electrons to cytochrome C reductase which passes electrons to cytochrome C which passes electrons to Cytochrome C oxidase.

Question 24

What are NADH dehydrogenase / coenzyme Q reductaze and cytochrome C reductase and Cytochrome C oxidase responsible for?

Answer 24

when these receive electrons they pump out protons into the inter-membrane space.

Question 25

How do protons generate ATP?

Answer 25

when they pass back through ATP synthase this reaction is catalyzed by ADP combining with Pi t`o form ATP.

Question 26

per one molecule of NADH, how many protons are pumped and how many molecules of ATP are made?

Answer 26

between the three proton pumps of the electron chain 10 protons are pumped out per NADH. 4 protons are required for the synthesis of 1 ATP therefore 2.5 ATP molecules are made per NADH

Question 27

What is the main difference between FADH2, cytoplasmic NADH, and matrix NADH?

Answer 27

FADH2 and glycolytic NADH skip over NADH dehydrogenase and deliver directly to the second transporter of the chain = ubiquinone / coenzyme Q

as a result these only pump out 6 protons and produce 1.5 ATP per molecule.

note: NADH from glycolysis gets to the matrix via the glycerol phosphate shuttle.

Question 28

how much ATP is produced in total per glucose in prokaryotes and eukaryotes?

Answer 28

30 = eukaryotes and 32 = prokaryotes

Question 29

What is gluconeogenesis?

Answer 29

gluconeogenesis is the reverse process of glycolysis. Molecules such as lactate, pyruvate, amino acids, are converted into glucose.

since glycolysis is so thermodynamically favoured, other enzymes opposite to hexokinase, phosphofructokinase, and pyruvate kinase are used. Additionally 6 equivalents of ATP are used.

Question 30

The glycolysis pathway converts glucose into pyruvate.

rxn 1. glucose –> G6P by hexokinase
rxn 3. F6P —> F-1,6-bp by phosphofructokinase
rxn 9. 2PEP—>2 pyruvates by pyruvate kinase

What are the enzymes used in gluconeogenesis to reverse these reactions?

(4 reactions with enzymes)

Answer 30

A molecule of pyruvate is either made from other waste or found (in the liver)

1. Pyruvate –> oxaloacetate by pyruvate carboxylase (which carboxylates it)
2. Oxaloacetate –> PEP by PEP carboxykinase ( which decarboxylates it and adds a P group)
3. series of rxns
4. F-1,6-bP —> F6P by fructose-1,6-biphosphatase
5. G6P –> glucose by glucose-6-phosphatase

Question 31

What is reciprocal control?

Answer 31

when one molecule regulates enzyme activity of two opposing enzymes.

Question 32

What is one important molecule that exhibits reciprocal control between glycolysis and gluconeogenesis?

Answer 32

F-2,6-bP found in the liver

Question 33

What two enzymes does F-2,6-bP act on?

Answer 33

PFK in the glycolysis pathway and Fructose-1,6-biphosphotase in the gluconeogenesis pathway.

Question 34

Insulin stimulates the formation of F-2,6-bP and glucagon inhibits the formtation (promotes the breakdown) of F-2,6-bP. With this info, what effect does F-2,6-bP have on PFK and F16-biphosphatase?

Answer 34

Insulin seeks to reduce blood sugar. Insulin will bind to a hepatocyte stimulate the formation of F-2,6-bP. This molecule will then activate PFK to promote glycolysis and inhibit F-1,6-biphosphatase to prevent gluconeogenesis.

the opposite is true for glucagon.

Note: insulins job is to reduce blood sugar and gluconeogenesis results in sugar being released into the blood.

Question 35

how would high levels of ATP affect PFK?

Answer 35

it would act to inhibit the activity of PFK which breaks down glucose to form ATP. Additionally, the high levels of ATP will act on fructose-1,6-biphosphatase to promote gluconeogenesis.

Question 36

Glycogen is the polymer of glucose expect to form glycogen glucose is first converted to G6P by _____. After this conversion 3 more enzyme involving steps occur to create glycogen. Explain each step.

Answer 36

Glucose is first converted to G6P by hexokinase (first step of glycolysis. This could isomerize to F6P and then have PFK convert it to F-1,6-bP)

In glycogenesis however,

2. G6P is converted to G1P by phosphoglucomutase
3. G1P is converted to UDP-glucose by UDP-glucose pyrophosphorylase
4. UDP-glucose is converted to glucose by glycogen synthase

Question 37

What is glycogenolysis? What enzymes are involved?

Answer 37

to convert glycogen to glucose.

Glycogen phosphorylase converts one terminal part of glycogen to G1P which is then converted to G6P by phosphoglucomutase.

This G6P can be re-enter the glycolysis pathway or be acted on by glucose-6-phosphatase to become glucose.

Question 38

What are the effects of insulin and glucagon on glycogen metabolism?

Answer 38

insulin will promote the synthesis of glycogen (therefore it stimulates hexokinase). This makes sense since insulin stimulate glycolysis as-well. Therefore hexokinase converts glucose to G6P which can either enter glycolysis or enter glycogenesis.

Glucagon promotes glycogenolysis which, with the use of glycogen phosphorylase and then phosphoglucomutase and then glucose-6-phosphatase gets glycogen into glucose. (which can diffuse into blood)

Question 39

Why do hepatocytes have glucose-6-phosphatase but skeletal muscle cells do not?

Answer 39

The role of the liver is to store and release glucose to maintain homeostatic blood sugar. Therefore, the liver cells contain glucose-6-phosphatase to ensure G6P can become glucose and diffuse into the blood when glucagon acts on it.

Note: G6P is charged and cannot move unless de-phosphorylated

Skeletal muscle cells do not contain G-6-phosphatase since rather then maintaining glucose levels, the muscles need to G6P from glycogenolysis to enter the glycolytic pathway for energy.

Question 40

What is the pentose phosphate pathway? What is its main two products

Answer 40

starting with glucose we can either enter glycolysis or glycogenesis. There is however another pathway that glucose can be shunted into call the PPP. This pathway produces NADPH and ribose-5-phosphate.

Question 41

What are NADPH and ribose-5-P ?

Answer 41

NADPH is an important reducing agent which helps cells get rid of reactive oxygen species which are harmful

Ribose-5-phosphate is an important molecule for nucleotide synthesis

Question 42

What is the on enzyme used to convert G6P to the PPP pathway?

Answer 42

glucose-6-phosphate dehydrogenase (G6PDH)

Question 43

true or false, aside from NADPH and ribose-5-P, glycolytic intermediates are also produced in the PPP which can be shunted back to glycolysis.

Answer 43

True

Question 44

people who lack G6P dehydrogenase are susceptible to what?

Answer 44

disease involving reactive oxygen species

Question 45

what is glycogens glycosidic linkage? how is its regular structure different from its branched structure?

Answer 45

glycogen is a polymer of alpha-glucose linkages

backbone is alpha-1-4 linkages

branching is alpha-1-6 linkages

Question 46

what linkage is in starch?

Answer 46

polymer of alpha glucose. this is the carb energy store in plants and is typically unbranched

Question 47

what linkage is in cellulose?

Answer 47

this differs from starch and glycogen and is actually a beta glucose polymer

we cannot digest cellulose