Carbohydrate Metabolism

Question 1

Anaerobic Carbohydrate Metabolism

Answer 1

1. Glucose Transport
2. Glycolysis
3. Other Monosaccharides
4. Pyruvate Dehydrogenase
5. Glycogenesis and glycogenolysis
6. Gluconeogenesis
7. Pentose Phosphate Pathway

Question 2

Glucose Transport

Answer 2

4 Types:
Glut 1
Glut 2
——– Specific to
1. pancreatic cells: sense high glucose
concentration and respond with insulin
release
2. hepatocytes: Store glucose as it travels
from intestine to the liver through hepatic portal
vein if glucose concentration is >1/2km
Glut 3
Glut 4
——– Specific to
1. adipose tissue: with increased insulin levels,
convert excessive glucose to first, DHAP, and
second to glycerol phosphate, which can store
fatty acids as triacylglycerol
2. muscle tissue: with increased insulin levels, uptake
excessive glucose and convert it to glycogen

****Review 291*********

Question 3

Glycolysis

Answer 3

An anaerobic energy yielding pathway that converts glucose to 2 pyruvates in addition to producing 2 electron carriers [NADH] and 2 ATPs

Certain cells without oxygen and mitochondria like erythrocytes can produce energy only through glycolysis****

Question 4

Glycolysis Steps

Answer 4

1. Transport of Glucose inside the cell using Glucokinase
2. Phosphorylation of glucose to G-6-P by Hexokinase to prevent its transport using GLUT
3. G-6-P———Isomerase————-F-6-P
4. F-6-P———PFK-2——————–F 2,6 BP
   [F 2,6 BP activates PFK1]
5. F-6-P——–PFK-1———————F 1,6 BP
6. F-1,6 BP—-Aldolase—————-Glyceraldehyde 3-P
7. Glyceraldehyde 3-P—–Glyceraldehyde-3-P- Dehydrogenase———NAD>NADH &—-1,3 BPGlycerate
8. 1,3 BPG——————-Phosphoglycerate Kinase—————————-ADP>ATP—————3-phosphoglycerate
9. 3-phosphateglycerate—-mutase–2-phosphoglycerate
10. 2-phosphoglycerate–Enolase–phosphoenolpyruvate
11. PEP———-Pyruvate Kinase—–ADP>ATP———————————————————-Pyruvate

Question 5

Fermentation

Answer 5

The process of NAD+ replenishment and lactate production through reduction of pyruvate with NADH oxidation.

**Occurs in absence of oxygen & mitochondria**

pg. 294

Question 6

High Energy-Yielding Intermediates of Glycolysis

Answer 6

1,3 BPG [Bisphosphoglycerate]
&
PEP [Phosphoenolpyruvate]

Both of them produce ATP anaerobically through substrate-level phosphorylation***

Question 7

DHAP

Answer 7

AKA [Dihydroxyacetone-phosphate]
Glycolysis intermediate formed from F 1,6 BP by rxn of aldolase that gets isomerized into Glycerol-3-P by glycerol-3-p-dehydrogenase before being converted into glycerol backbone which can be used for triacylglycerol storage by adipose and hepatic cells

Question 8

Gluconeogenesis

Answer 8

Production of glucose from other biomolecules through liver

**The reverse process o glycolysis**

Question 9

Irreversible Enzymes of Glycolysis

Answer 9

1. Glucokinase/Hexokinase
2. PFK-1
3. Pyruvate Kinase

These enzymes are replaced by others in gluconeogenesis*******

Question 10

Glycolysis and Hemoglobin O2 Dissociation Curve in Erythrocytes

Answer 10

Erythrocytes, due to their lack of mitochondria, only employ anaerobic glycolysis as a mean for energy production.

**They use their BPG mutase to convert 1,3 BPG to 2,3 BPG which upon binding to the beta chain of HbA they carry, decreases their affinity for oxygen and forces them to release more oxygen in the tissues.

****O2 Saturation in the lungs remains at 100% unless the concentration of 2,3 BPG increases too much, leading to dramatic rightward shift of HbA O2 dissociation curve, indicating excessive O2 release in the tissues.

*******2,3BPG does not bind well to HbF; therefore fetuses obtain their oxygen from their mother through transplacental transmission.

Question 11

Types of Hemoglobins

Answer 11

HbA———-Adult Hemoglobin

HbF———-Fetal Hemoglobin

Question 12

Monosaccharides Metabolised by Cells

Answer 12

1. Glucose
2. Galactose
3. Fructose

Question 13

Source of Galactose in Diet

Answer 13

Lactose in Milk that divides into 1. Galactose & 2. Glucose by Lactase

Question 14

Lactase

Answer 14

Brush-border enzyme of duodenum that breaks down lactose into galactose and glucose

Question 15

Galactose Metabolism

Answer 15

1. Galactose, broken down from lactose by lactase, reaches the liver through hepatic portal vein
2. Galactokinase phosphorylates it to G-1-P to trap it inside the cell
3. G-1-P Uridyltransferase & an epimerase convert G-I-P to glucose-I-phosphate

Question 16

Epimers

Answer 16

Diastereomers that differ only at one chiral carbon

Question 17

Epimerases

Answer 17

Enzymes that catalyze conversion of one sugar epimer to another

Question 18

Diet Source of Fructose

Answer 18

1. Fruits,
2. Honey
3. Sucrose

Question 19

Fructose Metabolism

Answer 19

1. Sucrase divides sucrose into fructose and glucose
2. Fructose travels up to the liver from the intestine through the hepatic portal vein
3. Fuctokinase phosphorylates fructose to F-I-P
4. Aldolase cleaves F-I-P into DHAP & glyceraldehyde
5. Proximal renal tubules metabolize the smaller segments
   pg. 299

Question 20

Pyruvate Dehydrogenase Complex

Answer 20

Complex of enzymes that convert pyruvate to acetyl-CoA to start either 1. citric acid cycle if ATP is needed, or 2. Fatty acid synthesis if ATP is sufficiently present

• ***Require multiple cofactors and coenzymes such as
  1. lipoic acid
  2. NAD
  3. FAD
  4. CoA
  5. Thiamine pyrophosphate

Question 21

Possible Fates of Pyruvate at the end of Glycolysis

Answer 21

1. Conversion into Acetyl-CoA by PDH
2. Conversion into lactate by dehydrogenase
3. Conversion into oxaloacetate by pyruvate carboxylase

Question 22

Glycogen

Answer 22

Identity:?
Source of glucose storage in muscles and liver where glucose becomes mobilized either to maintain an optimal level of blood sugar or to enable muscle contraction

Chemical Nature
In the cells, glycogen gets stored in the cytoplasm in granule forms where linear or branched chains of glycogen radiate outward from a central, core protein

Question 23

Starch

Answer 23

Excessive glucose stored in plant cells

Question 24

Glycogenesis

Answer 24

Synthesis of glycogen granules that starts with
1. conversion of glucose-6-phosphate to G-I-P
follows with:
2. interaction of G-I-P with UTP to yield Pp and UDP-glucose
& ends with
3. activation of glycogen synthase with insulin & G-6-P to produce the alpha 1,4 glycosidic bonds of the linear glucose chains that will bind to glycogenin

Question 25

Glycogenin

Answer 25

Core protein in glycogen granules to which glycogen chains attach

Question 26

Glycogen Synthase

Answer 26

Enzyme in glycogenesis that produces the alpha 1,4 glycosidic bonds found in the linear glucose chains of glycogen granules after it gets activated by insulin and G-6-P ****Glucagon and epinephrine inhibit the action of glocogen synthase****** ******Rate limiting step of glycogenesis******

Question 27

Function of Branching Enzymes in Glycogenesis

Answer 27

1. Glycogen Synthase synthesizes an alpha 1,4-linked polyglucose chain 2. Branching enzyme hydrolyzes an alpha 1,4-bond 3. Branching enzyme attaches the oligoglucose unit as a branch with an alpha-1,6-bond 4. Glycogen synthase extends the branches

Question 28

Glycogenolysis

Answer 28

Process of breaking down glycogen with a phosphorylase 1. Glycogen Phosphorylase, activated by glucagon in the liver and by AMP & epinephrine in the muscles, breaks down alpha-1,4-links of glucose chains of glycogen granules and releases G-I-P * *******************does not break alpha-1,6-bonds********* 2. De-branching enzyme hydrolyses the alpha-1,4-bond closest to the branch point 3. De-branching enzyme transfers the oligoglucose to the end of another chain 4. De-branching enzyme hydrolyzes the alpha-1,6-bond, releasing the single glucose from the former branch 5. Mutase converts G-I-P to G-6-P 6. G-6-phosphatase converts G-6-P to Glucose

Question 29

Gluconeogenesis

Answer 29

I. Reversal process of glycolysis that takes place in hepatocytes to produce glucose for other cells when adequate supply of ATP exists in hepatocytes II. Obtains its energy from beta-oxidation of fatty acids released from triacylglycerols held in hepatic adipose tissues I. Fatty acids convert to ATP III. Convenient during lengthy hours of starvation Uses the following substrates: 1. glycerol-3-phosphate [triacylglycerol] from adipose tissues 2. Lactate [anaerobic glycolysis] 3. Glucogenic amino acid

Question 30

Amino Acid Subcategories

Answer 30

1. Glucogenic Amino Acid All except leucine and lysine can be converted to intermediates that can feed into gluconeogenesis 2. Ketogenic Amino Acid Can be converted into ketone bodies that can be used as a source of energy during prolonged periods of starvation

Question 31

Enzymes Converting Intermediates into Glucose through Gluconeogenesis

Answer 31

1. Lactate to pyruvate by Lactate dehydrogenase 2. Glycerol-3-P to DHAP by glycerol-3-phosphate dehydrogenase 3. Alanine to pyruvate by alanine aminotransferase

Question 32

Enzymes Responsible for Irreversible Steps of Glycolysis

Answer 32

1. Glucokinase 2. PFK1 3. Pyruvate Kinase

Question 33

Gluconeogenesis's Enzymes that Catalyze the Irreversible Steps of Glycolysis for Glucose Synthesis

Answer 33

1. Pyruvate Carboxylase I. Converts pyruvate to OAA in the Mitochondria II. OAA gets reduced to Malate that can leave the mitochondria via malate- aspartate shuttle III. In the cytoplasm, malate gets oxidized to OAA 2. PEPCK [Phosphoenolpyruvate Carboxykinase] I. converts OAA to PEP in the cytoplasm using GTP II. Circumvents the irreversible catalysis of pyruvate kinase 3. Fructose-1,6-Bisphosphatase I. Circumvents PFK1 II. Converts F-1,6-BP to F-6-P in the cytoplasm III. Activated by ATP IV. Inhibited by AMP & F-2,6-bisphosphate V. Rate-limiting step of gluconeogenesis 4. Glucose-6-Phosphatase I. Circumvents Glucokinase & Hexokinase II. Convert G-6-P to Glucose in the Lumen of endoplasmic reticulum [ER] ****Free glucose then travels back to the cytoplasm to be transported by GLUT************** ********3.IV. In glycolysis, F-2,6-BP activates PFK1; therefore it logically inhibits F-1,6-Bphosphatase****

Question 34

Phosphatase

Answer 34

Enzyme that dephosphorylates | Opposes the action of kinases

Question 35

Pentose Phosphate Pathway

Answer 35

--- AKA Hexose Monophosphate Shunt [HMP] ---Location ----Takes place in cytoplasm of cells ---Purpose I. Produces NADPH II. Produces ribose 5-phosphate **necessary for nucleotide synthesis ---Important rate-limiting Enzyme I. G6PD [Glucose-6-phosphate dehydrogenase]`

Question 36

PPP Steps

Answer 36

pg. 312

Question 37

Difference B/w NAD+/NADH & NADP+/NADPH

Answer 37

-NAD+ is a potent oxidizing agent and electron acceptor that once reduced to NADH can produce ATP after being fed into the electron transport chain --NADPH is a reducing agent and electron donor that contributes to I. biosynthesis of fatty acids and cholesterol II. protection against free radical oxidative damage by maintaining supply of reduced glutathione [serves as an antioxidant] III. bacterial activity by producing bleach in certain white blood cells

Question 38

Glutathione

Answer 38

reducing agent that reduces free radical formation to prevent damage to cell DNA and cell membrane pg. 313