Tutorial - Week 10 - Carbohydrate Metabolism Flashcards
Define Pathways:
consecutive reactions catalysed by enzymes
The products of one reaction become the reactants of
the next one
Define Catabolic pathways:
pathways that degrade nutrients/biomolecules into simpler end products
to extract chemical energy and convert it into a form useful to the cell (e.g. ATP, NADH, NADPH, FADH2)
- These processes are often oxidative (i.e. substrates are oxidised, lose electrons),
electrons are transferred to electron carriers (e.g. NADH is obtained) - These processes are energy releasing and drive synthesis of ATP (i.e. energy
stored in chemical bonds of nutrients is transferred/captured as ATP
Define Anabolic pathways:
pathways that start with small precursor molecules and convert them
into more complex molecules (e.g. glycogen, proteins, lipids) and store energy in the chemical
bonds of these molecules. These require the use of energy.
- these processes are often reductive (i.e. substrates are reduced), they use electrons from
electron carriers, which become oxidised (e.g. NAD(P)H is converted to NAD(P) + ) - These processes are energy requiring, they use ATP (i.e. ATP is converted to ADP or AMP
Define Metabolism:
the overall network of enzyme catalysed pathways
(anabolic & catabolic)
It is convenient to divide metabolic processes into “pathways”, but such
separation does not exist in the living cell. Pathways occur simultaneously
in living organisms
- Catabolic pathways: degradation of complex macromolecules into
smaller molecules, releasing energy - Anabolic pathways: synthesis of complex macromolecules, using
energy
ATP is…?
NADH/NADPH/FADH are…?
the universal currency of energy in biological systems
electron carriers, they provide reducing power
How does ATP store energy?
What can ADP be hydrolysed into?
ATP, adenosine triphosphate: chemical energy stored in bonds
* breakage of a phopshoanhydre bond generates ADP + Phosphate ion (Pi), very exergonic reaction that is normally coupled to endergonic reactions in cells
- ADP can also be hydrolysed to AMP + Pi to obtain energy
What is NADH
nicotinamide adenine dinucleotide: reducing power, electron carrier
- Donate electrons in processes that generate ATP (e.g. in respiratory chain)
What is NADPH
nicotinamide adenine dinucleotide phosphate: reducing power, electron carrier
- Drive reductive steps in biosynthetic (anabolic) pathways
Metabolic pathways maintain the levels of…?
key metabolites, such as ATP and NADH in cells and glucose in the blood.
The rate of metabolic pathways is regulated by ____________, which can be regulated in different ways:
enzyme activity
See image for different ways
Regulation of the amount or the activity of an enzyme:
Define course control - describe both answers
- long term control of enzymes, it can take hours, or days
- it normally relates to changing the amount of enzyme
available
See image for explanations
Regulation of the amount or the activity of an enzyme:
Define fine control - describe all answers
- short term control, it can take milliseconds up to min
- it normally relates to a direct effect on the catalytic
activity of existing enzyme
See image for explanations
Define Control of the amount of certain enzymes
- Extracellular signals (e.g. hormones such as insulin) (1)
- Transcription of genes (2)
- mRNA processing or degradation (3)
- mRNA translation & posttranslational modification (4)
- Enzyme stability (turnover: synthesis vs. degradation) (5)
Define control by spacial segregation
- Multienzyme complexes that require assembly of subunits
- Compartmentation: enzymes and substrates locate in
different compartments/organelles (6)
Define Control of catalytic activity of existing enzyme molecules
- Changes in substrate concentration (7)
- Allosteric regulations (e.g. non-competitive uncompetitive inhibition (8)
Define Control by the energy status and availability of reducing power
- NAD(P)+/NAD(P)H ratios (8)
- ATP/ADP ratios (energy change) (8)
Define Control via allosteric regulation
- feedback inhibition
Define Control via covalent modification
- phosphorylation/dephosphorylation (reversible) (9)
- proteolytic activation: zymogens (irreversible)
This catabolic process is often subdivided into three pathways:
What are they?
Anaerobic processes:
When do they occur? In the presence or absence of?
Where do they occur in the cell?
Inefficient…?
Where in the body does this occur?
Anaerobic processes (AKA as fermentation or anaerobic respiration):
- occur in the absence of oxygen: glycolysis
- occur in the cytoplasm
- Inefficient energy yield: 2ATP/glucose molecule
Occurs in red blood cells (no mitochondria)
and anaerobic microorganisms (e.g. yeast)
Aerobic processes:
When does it occur (in the presence of what)/what is this process called?
Where do they occur in the cell?
Energy yield efficiency?
Where in the body do they occur?
Aerobic processes (AKA as aerobic respiration, cellular respiration) :
- occur in the presence of oxygen: oxidative phosphorylation
- occur in the mitochondria
- efficient energy yield: 32ATP/glucose molecule
Occurs in cells/tissues with mitochondria
What does glycolysis mean?
Lysis of glucose
Glycolysis:
Does it occur in most cells?
1 molecule of glucose is degraded in…?
Some of the energy released from glucose is conserved in the form of…?
Yes
a series of enzyme-
catalysed reactions to obtain 2 molecules of pyruvate (2 x C3
ATP and NADH
What are the three stages glycolysis is subdivided into?
Glycolysis uses ________ and generates ________; the balance is 2 ATP molecules
2 ATP
4 ATP
_________ of _________ is degraded in a series of _______________
reactions to yield ___ molecules of _________
1 molecule
glucose
10 enzyme-catalysed
2
pyruvate
In the sequential reactions of glycolysis, three types of chemical transformations are particularly relevant:
What are they?
(1) degradation of the carbon skeleton of glucose to yield pyruvate;
(2) phosphorylation of ADP to ATP;
(3) transfer of electrons to NAD+, forming NADH.
There are three regulatory enzymes, which catalyse irreversible reactions:
What are they?
- Hexokinase (reaction 1): inhibited by the products
- Phosphofructokinase-1 (PFK-1; reaction 3)
- Pyruvate kinase (reaction 10, last step): inhibited by ATP & acetyl-CoA
is the key enzyme in the control of glycolysis
(it catalyses the phosphorylation of fructose-6-phosphate to fructose-1,6-biphosphate)
Phosphofructokinase-1
__________ allosterically inhibits PFK-1 (it reduces its affinity for the substrate)
__________ allosterically activates PFK-1 (it increases its affinity for the substrate)
High [ATP]
High [AMP]
High [ATP]/[AMP] ___________, decreases glycolysis
________________ PFK-1 activity, and increases glycolysis
decreases PFK-1 activity
Low [ATP]/[AMP] increases
T/F: Pyruvate can be an intermediary in anabolic pathways
How?
Pyruvate can be an intermediary in…?
How?
other catabolic pathways, to obtain more energy:
___________ is the metabolite that links two central catabolic pathways: ____________ and the ____________
Pyruvate
glycolysis and the TCA cycle
________________ (“new formation of sugar”), converts pyruvate and related three-
and four-carbon compounds to glucose.
Glucogenesis
Why is glucogenesis important?
Important when glucose levels are low:
- The brain alone requires about 120 g of glucose each day — more than
half of all the glucose stored as glycogen in muscle and liver. - Between meals and during longer fasts, or after vigorous exercise,
glycogen is depleted. - When the glycogen is depleted, organisms need a method for
synthesizing glucose from noncarbohydrate
n gluconeogenesis: the three ____________ steps are bypassed by a separate
set of enzymes, they catalyze reactions that are sufficiently exergonic to be
effectively irreversible in the direction of glucose synthesis
irreversible
What is the result of the glucogenesis reaction?
1 Glucose is formed
2 pyruvate, 4ATP, 2GTP and 2NADH are consumed
What is meant by the Reciprocal regulation of glycolysis and glucogenesis?
One pathway is relatively inactive, while the other is highly active
Molecules that induce reciprocal regulation by allosteric control on the enzymes that differ in
the two pathways:
What are they?
What is the most tightly regulated enzyme in glycolysis?
Phosphofructokinase-1:
What is the most tightly regulated enzyme in gluconeogenesis
Fructose 1,6 biphosphatase-1
What is the major allosteric regulator factor of the
two pathways (it activates glycolysis, and inhibits gluconeogenesis)
Fructose 2,6-biphosphate (F26BP)
What is this reaction catalysed by?
What is the reaction responsible for?
Reaction catalysed by Pyruvate dehydrogenase complex (PDH)
(it occurs in the mitochondrial matrix)
Pyruvate + CoA + NAD + —> Acetyl-CoA + CO2 + NADH + H
Responsible for catalysing the synthesis of acetyl-CoA
(i.e. activated acetate)
Activity of pyruvate dehydrogenase is tightly regulated:
What is it activated and inhibited by?
What do all these molecules get turned into?
What are the 8 steps, roughly, of the TCA cycle?
What is the TCA cycle also known as?
It is a ________ pathway
Where does it occur?
Central pathway in…?
1 ________ enters the cycle, 2 _______ leave the cycle
It includes 8 _____________
It is a source of…?
It converts chemical energy from…?
Describe TCA cycle regulation
What is Isocitrate dehydrogenase?
is the key regulatory enzyme in the TCA cycle and
responds to levels of ATP, ADP and NADH in the cell:
- if ATP and NADH are high, TCA cycle is inhibited
- If ADP levels are high, TCA cycle is activated
There are two ways to obtain ATP in cells: What are they? Describe them
See image
- Oxidative phosphorylation is the culmination of…?
- All the oxidative steps in the degradation of _________________ converge to
this pathway to synthesise ATP - It occurs in…?
catabolism in aerobic organisms
sugars, fats and proteins
the inner mitochondrial membrane
Oxidative phosphorilation:
- electron flow?
- The free energy, G, from _____________ is…?
- The flow of protons back down provides…?
- Electrons flow from electron carriers (NADH and FADH2 generated in TCA cycle) through a chain of membrane-bound carriers to a final electron acceptor (O2 )
- The free energy, G, from exergonic electron flow is coupled with
transport of protons (H + ) across a proton-pump in the membrane
(electron transport chain) - The flow of protons back down provides free energy for synthesis
of ATP, catalysed by a membrane protein complex: ATP synthase
Cellular respiration occurs in the mitochondria and refers to?
(LEARN) What are the four parts to carbohydrate catabolism? Explain each step
Complete oxidation of _____ molecule of glucose to CO2 using ________________
yields ____________ molecules
1
oxidative phosphorylation
30 or 32 ATP
The catabolic pathways that allow for carbohydrate catabolism to occur are regulated by cellular energy status (2 points)
- High [ADP] increases the pathways
- High [ATP] decreases them
Key concept:
Catabolism vs anabolism & basic strategies of metabolism (3 points)
- Catabolic pathways degrade nutrients/biomolecules into simpler end products to extract chemical energy
and convert it into a form useful to the cell (e.g. ATP, NADH) - Anabolic pathways start with small precursor molecules and convert them into more complex molecules (e.g. glycogen, proteins, lipids) and store energy in the chemical bonds of these molecules.
- Nucleotides are used to store energy, enzymes control the rate of the reactions,
Key concept:
Regulation of metabolic regulation: coarse control and fine control strategies: (2 points)
- Coarse control: long term control of enzymes, it can take hours, or days; it normally relates to
changing the amount of enzyme available - Fine control: short term it can take milliseconds up to min; it normally relates to a direct effect on
the catalytic activity of existing enzyme
Key concept:
Overview of glycolysis pathway (1 point)
1 molecule of glucose is degraded in a series of 10 enzyme-catalysed reactions to yield 2 molecules of pyruvate, 2
molecules of ATP, 2 molecules of NADH. It occurs in the cytoplasm
Key concept:
Overview of gluconeogenesis pathway (2 points)
- Synthesis of glucose molecules; it converts pyruvate and related three- and four-carbon compounds
into glucose: 1 Glucose is formed; 2 pyruvate, 4 ATP, 2 GTP and 2 NADH are consumed. - Reciprocal regulation of glycolysis and glucogenesis
Key concept:
Overview of TCA cycle (3 points)
- It is a central pathway in the catabolism of sugars, fats and proteins; it uses compounds derived from
the breakdown of carbohydrates, lipids and proteins), which are converted to CO2 - 1 Acetyl-CoA enters the cycle, 2 CO2 leave the cycle
- 3 NADH, 1 FADH 2 , 1 ATP (or 1GTP)
Key concept:
Overview of oxidative phosphorylation (4 points)
- All the oxidative steps in the degradation of sugars, fats and proteins converge to this
pathway to synthesise ATP - ATP is generated using electron flow from electron carriers (NADH and FADH2
generated in TCA cycle) to O2 - The protons gradient provides free energy for synthesis of ATP
- Complete oxidation of 1 molecule of glucose to CO2 yields 30 or 32 ATP molecules
