Carbohydrate metabolism 1 Flashcards
Where in the cell does glycolysis take place?
The cytoplasm
What is the “big-picture” goal of glycolysis?
to synthesize thousands of ATP molecules used for various cellular metabolism.
For each molecule of glucose consumed, two pyruvate are formed. What other biomolecules are formed by glycolysis?
When glucose is broken down one molecule of NAD+ in the broken down form of NADH is formed , ATP is formed and 2 pyruvate molecules
In the first (preparatory) stage of glycolysis:
glucose →
+ 2 ADP + 2 PI + 2 NAD+ -> 2 PYRUVATE + 2 ATP + 2NADH + 2 H+ + 2 H20
At the end of which stage of glycolysis does the production of ATP finally pay off the ATP debt of the preparatory phase?
At the end of the second stage is where the ATP debt is finally paid off.
Which enzyme catalyzes the second step of glycolysis?
Phosphoglucose isomerase
ADP
Which steps in glycolysis consume ATP?
step 1 , step 3
Which steps in glycolysis produce ATP?
step 9 and step 2
Which enzyme catalyzes the 6th step of glycolysis?
enzyme glyceraldehyde-3-phosphate dehydrogenase
What are the three metabolic pathways which pyruvate can take?
actic acid fermentation if there is oxygen absent, the Krebs cycle if there is oxygen present, and ethanol fermentation if there is oxygen absent.
Glycolysis
Responsible for the capture of some of the bond energy of carbohydrates
Responsible for the storage of that energy in the molecular form of adenosine triphosphate (ATP).
Glycolysis actually releases and stores very little (2.2%) of the potential energy of glucose, but the pathway also serves as a source of biosynthetic building blocks.
It also modifies the carbohydrates in such a way that other pathways are able to release as much as 40% of the potential energy.
ATP: The Cellular Energy Currency
Adenosine triphosphate, or ATP for short, is the energy currency of life.
ATP is a high-energy molecule found in every cell. Its job is to store and supply the cell with needed energy.
Catabolism is the set of metabolic pathways that break down complex macromolecules into simpler ones and, in the process, harvest part of their potential energy for use by the cell.
One of those energy-requiring functions is anabolism, or biosynthesis.
Overview of Catabolic Processes
Carbohydrates, fats and proteins can be degraded to release energy, but carbohydrates are the most readily used energy source.
When we eat a meal, we are eating quantities of these nutrients that will provide the energy required for life processes.
The primary function of all catabolic pathways is to harvest the chemical energy of fuel molecules and to store that energy by the production of ATP.
This continuous production of ATP is what provides the stored potential energy that is used to power most cellular functions.
Three major products of Glycolysis
- Chemical Energy in the form of ATP
Four ATP molecules are formed by the process of substrate-level phosphorylation group from one of the substrates is transferred to ADP to form ATP. - Chemical Energy in the form of reduced NAD+, NADH.
Nicotinamide adenine dinucleotide (NAD+) is a coenzyme derived from the vitamin niacin. - Two Pyruvate Molecules
- At the end of glycolysis, the six-carbon glucose molecule has been converted into two three-carbon pyruvate molecules.
The fate of the pyruvate also depends on whether the reactions are occurring in the presence or absence of oxygen.
Under aerobic conditions, it is used to produce acetyl CoA destined for the citric acid cycle and complete oxidation.
Under anaerobic conditions it is used as an electron acceptor in fermentation reactions.
Genetic Disorders of Glycolysis
Muscle Myopathy – involves damage to the muscle as a result of the inability to extract energy from food molecules.
Mildest forms – can cause exercise intolerance
More severe forms – muscle breakdown
Hemolytic anaemia – anaemia that results from the lysis of red blood cells. Red blood cells are completely dependent on glycolysis for their ATP. A defect in one of the enzymes of glycolysis results in insufficient ATP and resultant cell death.
Glycolysis : steps 1 and 2
s1: glucose + ATP > hexokinase > glucose-6-phosphate
s2: glucose-6-phosphate + ADP > phosphoglucose isomerase > Fructose-6-phosphate
Glycolysis : step 3
fructose-6-phosphate + ATP > phosphofructokinase > fructose-1,6-biphosphate
Glycolysis : step 4 and 5
fructose-1,6-biphosphate > aldolase > D-glyceraldehyde-3-phosphate + dihydroxyacetone phosphate > triosephosphate isomerase > D-glyceraldehyde-3-phosphate
Glycolysis : step 6
glyceraldehyde-3-phosphate + NAD+ + HPO4 2- > glyceraldehyde-3-phosphate dehydrogenase > glycerate-1,3-biphosphate
glycolysis : steps 7 and 8
1,3-biphosphoglycerate > phosphoglycerate kinase > 3-phosphoglycerate + atp > phosphoglycerate mutase > 2-phosphoglycerate
glycolysis steps 9 and 10
2-phosphoglycerate > enolase > phosphoenolpyruvate > pyruvate kinase > pyruvate + ATP
Regulation of Glycolysis
Energy-harvesting pathways, such as glycolysis, are responsive to the energy needs of the cell.
Reactions of the pathway speed up when there is a demand for ATP.
They slow down when there is abundant ATP to meet the energy requirements of the cell.
One of the major mechanisms for the control of the rate of glycolysis is the use of allosteric enzymes.
Allosteric enzymes are enzymes that change their conformational ensemble upon binding of an effector, which results in an apparent change in binding affinity at a different ligand binding site.
In addition to the active site, which binds to the substrate, allosteric enzymes have an effector binding site, which binds a chemical signal that alters the rate at which the enzyme catalyses the reaction.
Effector binding may increase (positive allosterism) or decrease the rate of reaction (negative allosterism).
Glycolysis: Three possible fates for pyruvate
In yeast (fermentation)
Glucose -> pyruvate -> ethanol
In muscle (anaerobic respiration)
Glucose -> pyruvate -> lactate
In muscle (aerobic respiration)
Glucose -> pyruvate -> acetyl CoA
Metabolism of pyruvate to ethanol
In the anaerobic state, yeast and other microorganisms ferment glucose to ethanol and CO2 and oxidise NADH to NAD+
Two step process
Pyruvate is decarboxylated to acetaldehyde by pyruvate decarboxylase
Acetaldehyde is reduced to ethanol by NADH and alcohol dehydrogenase
Metabolism of pyruvate to lactate
Anaerobic conditions:
Unlike yeast, mammals lack pyruvate decarboxylase and cannot produce ethanol from pyruvate
In mammals, lactate is formed from pyruvate in skeletal muscles during strenuous exercise
transported to liver where converted to pyruvate by hepatic LDH for gluconeogenesis
When supply of O2 to tissues is inadequate, all tissues produce lactate (anaerobic glycolysis)
Accumulation of lactic acid in the blood – lactate acidosis
