Lecture 40

Question 1

role of energy production

Answer 1

in muscle cells, ATP required for muscle contraction

pg 1060

Question 2

adenosine triphosphate (ATP)

Answer 2

• has 2 high-energy phosphate bonds (β and γ) which can be cleaved by H₂O
• this reaction of hydrolysis of ATP to form ADP and a free phosphate can be coupled with unfavorable reactions to drive them forwards

pg 1061

Question 3

energy production: substrates

Answer 3

• amino acids, glucose, and fatty acids transported into the cells via proteins
• amino acids used in the cell for synthesis of proteins, but they can also produce energy in extreme conditions; form contractile machinery of muscle cells
• fatty acids use a fatty acid transport protein

pg 1062

Question 4

glucose transporters: GLUT 4

Answer 4

• found in adipose tissue, skeletal muscle, heart muscle
• insulin-sensitive transporter -> when insulin is present, the number of GLUT 4 transporters increases
• high affinity system
• GLUT 4 stored in intracellular vesicles until insulin is present

pg 1063

Question 5

glucose transport in muscle cells

Answer 5

• when insulin binds insulin receptor, it triggers a downstream signaling pathway which tells the intracellular vesicle storing GLUT 4 to translocate to the plasma membrane
• GLUT 4 embeds in the plasma membrane and allows glucose uptake
• clinical correlation: insulin resistance and T2D

pg 1064

Question 6

energy production: GLUT 4

Answer 6

• glucose transported into the cell by GLUT4 when insulin in present (high in absorptive/well-fed state)
• once in cell, glucose activated to glucose-6-phosphate
• G-6-P undergoes glycolysis to form pyruvate, ATP, and NADH
• ATP formed from substrate level phosphorylation (transfer of phosphate from one substance to another)
• NADH is electron carrier
• 2 ATP are formed from 1 glucose

pg 1065

Question 7

glycolysis overview

Answer 7

• central metabolic pathway
• consist of 10 steps: 3 irreversible, 7 reversible (regulated by availability of substrate/product
• steps are divided in 2 phases: energy investing (2 ATP lost) and energy harvesting (4 ATP gained)
• net energy yield for 1 glucose is: 2 ATP, 2 NADH
• ATP is needed initially to destabilize glucose-6-phosphate energetically
• 3 irreversible steps: 1 (hexokinase/glucokinase), 3 (phosphofructokinase-2), 10 (pyruvate kinase)

pg 1066

Question 8

nicotinamide adenine dinucleotide (NAD)

Answer 8

• NAD+ has an oxidized nicotinamide and NADH has a reduced nicotinamide
• pellagra: a deficiency of niacin -> niacin involved in NAD+ synthesis

pg 1067-1068

Question 9

activation of glucose

Answer 9

Hexokinase - 1st Reversible Step

• 3 isoforms in muscle -> hexokinase I, II, III
• found in most tissue including muscle
• inhibited by the end product
• high affinity for glucose (low K_m) (how glucose stays in the cell)
• low maximal velocity (V_max)
• tissue specific regulation - in muscle -> inhibitors: glucose 6-P (product)
• converts D-glucose to glucose-6-phosphate and requires ATP
• analog is glucokinase in liver cells -> glucokinase has a low affinity (high Km) and high Vmax

pg 1070

Question 10

glycolysis: PFK-1 step

Answer 10

Phosphofructokinase-1 (PFK-1) - 3rd step

• irreversible step
• rate-limiting and committed step
• most important control point
• in muscle, regulated allosterically by: inhibitors: ATP (high energy), citrate, H+; activators: F-6-P (substrate level), fructose 2,6-bisphosphate (in liver in response to high insulin)
• converts fructose-6-phosphate to fructose 1,6-bisphosphate and needs ATP to do so

pg 1072

Question 11

glycolysis: pyruvate kinase

Answer 11

• last step of glycolysis
• irreversible step
• tissue specific isoforms: M1 (in skeltal muscle), M2 (in kidney, adipose tissue, lungs), L (in liver, regulated by glucagon), R (in RBCs) -> allows for more specific and precise regulation
• regulated step (tissue-dependent)
• M-type PK regulation is allosteric (NOT regulated by glucagon): inhibitors: ATP (high energy), acetyl-CoA, Ala; activators: AMP (low energy), F-1,6-bisP (feedforward -> upstream intermediate)
• converts 2 molecules of phosphoenol-pyruvate to 2 molecules of pyruvate and releases 2 ATP in the process

pg 1074

Question 12

summary of glycolysis regulation

Answer 12

• short-term regulation: allosteric, covalent modifications (phosphorylation/dephosphorylation)
• long-term regulation (in the liver): protein expression

pg 1075

Question 13

energy production: pyruvate

Answer 13

• pyruvate enters the mitochondria where it is converted to acetyl CoA
• acetyl CoA goes into the TCA cycle to release NADH and FADH2 (electron carriers)
• NADH and FADH2 go to the electron transport chain (ETC)
• after the ETC, ADP + Pi and O2 are used for oxidative phosphorylation and the release of ATP
• O₂ is the final electron acceptor (captures the electron to allow production of ATP)

pg 1076

Question 14

fate of pyruvate: aerobic glycolysis

Answer 14

• glycolysis itself does not need oxygen, but its subsequent steps do
• in cells with mitochondria and sufficient oxgen available:
• pyruvate convered into acetyl CoA in the mitochondrial matrix
• acetyl CoA will undergo further breakdown in the TCA cycle
• NAD+ is regenerated through the electron transport chain
• in order for glycolysis to proceed, NAD+ has to be regenerated
• net energy yield: 36-38 ATP

pg 1077

Question 15

NADH transport into mitochondria

Answer 15

• NADH is a large molecule so it donates electrons to smaller molecules that can pass through the membrane
• these molecules donate the electrons back to NAD+ or FAD in the mitochondria
• uses 2 shuttles: the glycerol-3-phosphate shuttle (yields 1.5 ATP) OR the malate-aspartate shuttle (yields 2.5 ATP)

pg 1078-1079

Question 16

energy production: lack of oxygen

Answer 16

if oxygen is not present, NAD+ regenerated by converting pyruvate to lactate in the cytosol

pg 1080

Question 17

fate of pyruvate: anaerobic glycolysis

Answer 17

if there is a lack of mitochondria or decreased oxygen supply:

• pyruvate will be reduced to lactate by lactate dehydrogenase
• this step will regenerate NAD+ and allow glycolysis to proceed
• net energy yield: 2 ATP

pg 1081

Question 18

anaerobic glycolysis

Answer 18

lactate production in skeletal muscle (normal):

• exercising skeletal muscle
• lactate build up results in lowered pH (acid) and cramping
• can be released into the plasma, taken up by other tissues (liver), and metabolized back to pyruvate (the Cori cycle -> converts lactate from circulation back to glucose via gluconeogenesis)

reaction:

• pyruvate to lactate requires lactate dehydrogenase enzyme in a reversible reaction; NAD+ is released and can go back to accept electrons

pg 1082

Question 19

anaerobic glycolysis in muscle and other cells

Answer 19

lactate production in muscle and other cell types (abnormal):

• hypoxia (lack of oxygen in tissues) -> cell damage, dead and necrosis
• lactic acidosis -> lowers the pH of the blood (normal lactate, hyperlactemia, lactic acidosis

pg 1083

Question 20

myocardial infarction

Answer 20

• cardiomyocytes are highly oxidative
• can NOT tolerate lactate accumulation
• result in cell and tissue damage

pg 1083

Question 21

lactic acidosis

Answer 21

lactate levels increase to 4-5 mmol/L (normal is less than 2)

pg 1084