Carbs: Study Guide Flashcards
Know the major elements required for life present in monosaccharides
*Carbon, Hydrogen, Oxygen
Are carbs considered “true polymers”
yes because they contain multiple monomers, and they are interlinked.
How do carbohydrates facilitate cellular interactions? How do they provide structural support?
*Forms a glycocalyx (bundle of carbs) to form a mesh-like bundle that surrounds the plasma membrane and prevents clotting
*They are on extracellular receptors and form noncovalent interactions with ligands.
A1C is formed via “non-enzymatic glycosylation” of hemoglobin. What does that mean?
*Non-enzymatic glycosylation is the attachment of glucose to proteins without the help of an enzyme.
Be able to identify different glycosidic bonds (i.e., O- vs. N-; α vs. β).
*O-bond is between carbon-1 and OH groups oxygen
*N-bond is between carbon-1 and NH2 groups nitrogen
*Alpha bond is OH on carbon-1 below the plane
*Beta bond is OH on carbon-1 above the plane
Be able to identify homopolymers and heteropolymers.
*Homopolymer is one sugar residue
*Heteropolymers would be multiple types of sugar residues
The terms “simple carbohydrate” and “complex carbohydrate” are often used on nutrition labels. What do those terms mean? Are all simple carbohydrates monosaccharides?
- Simple carbohydrates are referred to as quick energy, because they have fewer glycosidic bonds allowing them to enter glycolysis faster. Simple carbohydrates may contain monosaccharides or disaccharides.
-Complex carbohydrates are referred to as slow energy, because they have more glycosidic bonds and therefore enter glycolysis slower.
Glucose and sucrose were both subject to Fehling’s test. Know what happened to each molecule and why.
*Glucose’s color changed, meaning it is a reducing sugar. Sucrose’s color did not change, meaning that it is not a reducing sugar.
Why is glycogen storage limited in the human body?
*Because of the water uptake properties that glycogen has, we can only store so much
Besides glycogen, know one of the common larger polysaccharides – what it’s made of, what function/role it has, and what type of bonds it has.
*Chitin is a larger polysaccharide; it is a linear homopolymer linked by beta 1,4 glycosidic bonds. Chitin is a glucose derivative and serves as the structural component of an arthropod’s exoskeleton.
How do glycoproteins, glycolipids and glycoRNAs differ? How are they similar?
*All three have carbohydrates attached to their exterior surface
*Glycoproteins are both N and O linked, Glycolipids are N linked, and GlycoRNA’s are O linked.
Simple vs. Compound sugar
Simple sugars: also called monosaccharides, include glucose, fructose, and galactose (fewer glycosidic bonds,enters glycolysis faster)
Compound sugars: also called disaccharides or double sugars, are molecules made of two bonded monosaccharides; common examples are sucrose (glucose + fructose) (more glycosidic bonds, enters glycolysis slower)
Metabolically inert
They cannot reproduce or replicate themselves. Viruses themselves are metabolically inert and must rely on metabolic events in the cell to generate its component parts and to replicate new viral copies.
Rh factor
an inherited protein found on the surface of red blood cells.
Glycosyltransferase
enzymes that catalyze the formation of the glycosidic linkage to form a glycoside. These enzymes utilize ‘activated’ sugar phosphates as glycosyl donors, and catalyze glycosyl group transfer to a nucleophilic group, usually an alcohol.
Glycosylation
the attachment of carbohydrates to the backbone of a protein through an enzymatic reaction. A protein that is glycosylated is known as a glycoprotein
A1C is formed via “non-enzymatic glycosylation”. What does that mean?
Glucose can attach to hemoglobin without the assistance of an enzyme
How does glucose (Glc) differ from GlcN? What about from GlcNAc?
- Glc: glucose, hydroxyl located on carbon-2
- GlcN: glucosamine, amine group located on carbon-2
- GlcNAc: N-acetylglucosamine, N-acetyl group (N-C=O) on carbon-2
Glucose and sucrose were both subject to Fehling’s test. What happened? Would you expect maltose (has two glucose residues) to react to the reagent? Understand why.
-Sucrose was not able to participate in the redox (no color change =not a reducing sugar)
- glucose had a color change (aldehyde can participate in the reaction)
-fructose can’t participate bc it has a ketone
-maltose: reducing sugar, second sugar has the ability to partially open up and be free to be in aldehyde (therefore participating in a redox reaction)
Be able to identify different carbohydrates (i.e., disaccharide vs. oligosaccharide, homopolymer vs. heteropolymer, etc.) in images and/or descriptions.
Oligosaccharides: water-soluble; often attached to other biomolecules to facilitate cell recognition
Disaccharides: Sucrose, lactose, maltose (have the glucose residue)
homopoly: one type of sugar residue
Heterpoly: multiple types of sugar residues
Why do linear and high molecular weight polysaccharides have lower water solubility?
as molecular weight increases, the polymer would have a smaller solubility range
Know which conjugated molecules (i.e. glycoprotein, glycolipid, and glycoRNA) have O-glycosidic bonds vs. N-glycosidic bonds.
Glycoprotein: can have both N and O (glycoproteins, proteoglycans, mucins)
Glycolipids: sphingosine platform, cluster with cholesterol rafts (O bond)
glycoRNAs: monosaccharides attached to RNA via nucleobases (N linked)
Pancreas GLUT2 vs. Brain GLUT3
GLUT2: transporter for glucose in pancreas, stim. insulin release; HIGH Km, more glucose needed to reach 1/2 Vmax
GLUT3: low Km, saturated with glucose most of the time (due to high need of glucose in brain)
Redox reaction versus Oxidation number
increase in oxidation number through a reaction corresponds to an oxidation in the molecule, decrease means reduction
NAD+ and FAD comparison
FAD accepts 2 H+, NAD+ only accepts one
Understand how dietary carbohydrates are digested.
o A-Amylase in mouth
o Stomach: acidic environment denatures enzymes, generate uniform mixture by contractions
o SI: pancreatic a-1,4 amylase, A-dextrinase, Sucrase and maltase cleave disaccharides; Monosaccharides transported into intestinal cells by transporters
Do digestive enzymes cleave covalent bonds or noncovalent interactions?
cleave covalent bonds using water
Can humans metabolize oligosaccharides with α-glycosidic bonds? What about β-bonds?
Humans lack the enzymes to cleave the B-glycosidic bonds (lactase)
How do the two types of α-amylase differ? How are they similar?
Pancreatic Alpha-Amylase: cleaves remaining A- 1,4 glycosidic bonds
-generates oligosaccharides and disaccharides
-has 94% amino acid similarity to salvia alpha-amylase
Salvia Alpha-Amylase: cleaves random A-1.4 glycosidic bonds
-responsible for up to 30% of carb digestion
How do pancreatic α-amylase and α-dextrinase differ? How are they similar?
-Alpha-dextrinase cleaves A-1,6 glycosidic bonds
-pancreatic A-amylase cleaves A-1,4 glycosidic bonds
Does insulin stimulate the liver to synthesize or metabolize glycogen?
Insulin triggers insulin signal transduction pathway in muscle and fat cells
-high blood sugar, promotes insulin release to the pancreas, releases insulin (stimulates glucose uptake from blood), stimulates glycogen formation in the liver, lowers blood sugar
-low blood sugar, promotes glucagon release to the pancreas, releases glucagon to stimulate the breakdown of glycogen in the liver, which raises blood sugar
If dietary fiber is indigestible, why is it important that we regularly consume it? Be able to clarify the role of insoluble fiber vs. soluble fiber in our diet
Since it is indigestible, it enters the large intestine mostly intact; insoluble fiber helps improve the bioavailability of water-soluble vitamins within the LI (helps pass stool)
What can happen to dietary monosaccharides, following digestion?
-glucose stored as glycogen for later use
-can also be used as building blocks for new carbs
-if not needed, then metabolized (enter cellular respiration via glycolysis)
Know the four metabolic pathways involved in cellular respiration.
- glycolysis: glucose (input); pyruvate/NADH (output)
- pyruvate oxidation: Pyruvate (I); acetyl CoA/ NADH (O)
- Citric acid cycle: acetyl CoA (I); NADH/ FADH2 (O)
- Oxidative phosphorylation: NADH/FADH2 (O)
Be able to identify which molecule is oxidized and which is reduced in a redox reaction.
Oxidization: loss of electrons
Reduction: gain of electrons
During starvation, does the brain require glucose? How does it obtain it?
yes, the brain still requires glucose (the body will cannibalize muscle and fat)
-gluconeogenesis synthesizes glucose from non-carbohydrate sources
If you’re on a low-carb diet, how does the brain obtain glucose?
-brain still requires glucose
-non-carb precursors for gluconeogenesis obtained from dietary lipids and proteins
-consumed fats are also a source of acetyl CoA for ketone body synthesis
Phosphoryl-transfer potential
comparing the tendency of organic molecules to transfer a phosphoryl group to an acceptor molecule; form of energy transformation ie. formation of glucose 6 phosphate by dephosphorylation of ATP, endergonic rxn
Pyruvate translocase / MPC
the membrane protein that allows pyruvate to cross the inner mitochondrial membrane
Fructose 2,6-bisphosphate
regulation of glycolysis in LIVER, Increases phosphofructokinase’s (PFK) affinity for its substrate, producing fructose 1,6-biphosphate
Where does glycolysis occur within a prokaryotic cell? What about in a eukaryotic cell?
in cytoplasm for both
Know which (if any) of the 7 major metabolic intermediates can enter glycolysis.
glucose and fructose
Understand why the phosphorylation of glucose (reaction 1) is important.
-first reaction of glycolysis involves the phosphorylation of glucose by ATP
-through hexokinase (allosteric homodimer) and metal cofactor
-produces glucose 6-phosphate
-regulatory #1
Understand why the phosphorylation of fructose 6-phosphate (reaction 3) is important.
-third reaction of glycolysis involves the phosphorylation of fructose 6- phosphate by ATP
-through phosphofructokinase (allosteric homotetramer)
-reaction transfers a phosphoryl group creating fructose 1,6- bisphosphate
-this catalyzes the committed step in glycolysis
Understand why the isomerization of DHAP (reaction 5) is important.
Must occur to produce G3P to continue in cycle; have to keep G3P levels low to have the isomerization occur
How many molecules of pyruvate are generated from one glucose? Does glycolysis directly generate ATP?
2, no
Define the term “isozyme”. How do isozymes differ from isomers?
an enzyme with a different molecular structure that catalyzes the same reaction; isomers have same molecular structure but different reactions
Understand how each of the allosteric enzymes involved in glycolysis is regulated in the muscle versus in the liver. Know which environmental signals (like ATP, feedback inhibition, phosphorylation, etc.) suppress multiple allosteric enzymes
- Muscle:
o Feedback inhibition by glucose-6-phosphate
o Inactive phosphofructokinase results in inc in fructose-6-phosphate, isomerizes into glucose-6-phosphate
o ATP, changes in pH, and citrate inhibit function of phosphofructokinase
o Pyruvate kinase is inhibited by ATP, alanine, and phosphorylation of serine residues - Liver: responds to insulin and glucagon
o Fructose 2,6 bisphosphate is generated from excess fructose 6 phosphate, inc phosphofructokinase affinity for substrate
o Glucokinase (instead of hexokinase) lower glucose affinity, not inhibited by glucose 6 phosphate
Only phosphorylates glucose when blood glucose levels are high
Regulated with interactions with glucokinase reg protein; inactive when bound
o Phosphofructokinase inhibited by primarily citrate
o Pyruvate kinase
L-isozyme inhibited by phosphorylation by PKA (protein kinase a)
Gene expression regulated by transcription factor involved with lipogenesis and glycolysis
Does glucagon stimulate the liver to synthesize or metabolize glycogen?
stimulates liver to metabolize glycogen; low blood sugar–> high blood sugar
Where does pyruvate oxidation occur within a prokaryotic cell? What about in a eukaryotic cell?
cytoplasm for prokaryotes
mitochondria for eukaryotes
Know which (if any) of the 7 major metabolic intermediates can enter pyruvate oxidation.
pyruvate goes through pyruvate oxidation to produce acetyl CoA
How many molecules of acetyl CoA are generated from one pyruvate? Does pyruvate oxidation directly generate ATP?* ADD*
1 pyruvate = 1 acetyl CoA
Pyruvate oxidation products are acetly CoA and CO2; so No,ATP is generated in later steps
ATP also regulates pyruvate oxidation
Understand how pyruvate oxidation is regulated. Know which inhibitors also suppress allosteric glycolytic enzymes.
- Allosterically inhibited by phosphorylation of select serine residues
- PDH phosphatase stimulated by calcium and insulin
- Also regulated by NADH, acetyl CoA and ATP
- High ATP suppresses 2 allosteric enzymes
NAD+ is required for glycolysis, pyruvate oxidation, and the citric acid cycle. How is this coenzyme regenerated under aerobic conditions?
-the ratio of NAD+ to NADH (redox state) is important for hemostasis
-regenerated during oxidative phosphorylation (the last part of cellular respiration)
Synthetase vs. Kinase
Synthetase: An enzyme that catalyzes the synthesis of a compound
Kinase: an enzyme that catalyzes the transfer of phosphate groups from high-energy, phosphate-donating molecules to specific substrates.
NAD+ vs. NADP+
NAD+ and NADP+ are usually* reduced
IDH1 and IDH2 both reduce NADP+, while IDH3 reduces NAD+
NAD+ reduced to NADH
Succinate dehydrogenase vs. Complex II
also known as complex II or succinate: quinone oxidoreductase is an enzyme involved in both oxidative phosphorylation and tricarboxylic acid cycle
- Complex II is the succinate dehydrogenase enzyme of the TCA cycle oxidizing succinate to fumarate
Where does the CAC occur within a prokaryotic cell? What about in a eukaryotic cell?
-third part of cellular respiration: proceeds under aerobic conditions
-Prokaryotes: Cytosplasm
-Eukaryotes: Mitochondora
Know which (if any) of the 7 major metabolic intermediates can enter the CAC.
(7 = pyruvate, α-ketoglutarate, succinyl-CoA, fumarate, oxaloacetate, acetyl-CoA, or acetoacetate) ADD/FIX
Acetyl CoA, Oxaloacetate, α-ketoglutarate, Succinyl CoA, fumarate
Understand why the oxidative decarboxylation of isocitrate (reaction 3) is important.
- Catalyzes rate-limiting step* in citric acid cycle
- Reaction converts hydroxyl group to keto group at carbon-2 and removes carboxyl group at carbon-3
- Products: α-ketoglutarate (a five-carbon molecule) and CO2
*Slowest part of a metabolic pathway; impacts overall kinetics
How do the α-ketoglutarate dehydrogenase complex and the pyruvate dehydrogenase complex differ? How are they similar?
α-ketoglutarate dehydrogenase complex -
- Allosteric enzyme with 3 distinct enzymes; each has own active site
- Same structure and catalytic mechanism as pyruvate dehydrogenase complex
pyruvate dehydrogenase complex -
- Allosteric complex with 3 distinct enzymes; each has own active site
- Decarboxylation performed by E1, oxidation by E3, and transfer by E2
- Flexible lipoamide arm moves substrate from active site to active site
ATP is generated via substrate-level phosphorylation in glycolysis and the CAC. Why is the reaction in the CAC considered unusual?
Only example of substrate-level phosphorylation within mitochondria
Key differences from glycolysis: no kinase or donor molecule with phosphoryl-transfer potential
How many NADH and FADH2 are generated from one acetyl CoA? Does the CAC directly generate ATP?
Overall, 1 acetyl group from acetyl CoA generated:
1 molecule ATP
1 molecule FADH2
3 molecules NADH
Understand how each of the allosteric enzymes involved in the CAC is regulated. Do certain effectors regulate both enzymes
Regulation of isocitrate dehydrogenase
-Allosterically inhibited by ATP/ NADH
-IDH1(muscle) and IDH3 (liver)
Regulation of Alpha-keto
-different A-KG isozyme in the brain than in the muscle and liver
-Allosteric inhibition from ATP, NADH, Succinyl CoA
Know which inhibitors also suppress allosteric enzymes in glycolysis and pyruvate oxidation.
Pyruvate oxidation - Allosterically inhibited by phosphorylation of select serine residues
Glycolysis - inhibited by large amounts of ATP
Redox potential (E’0)
the electric potential of an electrochemical half-cell relative to a standard electrochemical half-cell under standard conditions.
Chemiosmosis
the process of moving ions (e.g. protons) to the other side of a biological membrane, and as a result, an electrochemical gradient is generated.
ATP synthase
mitochondrial enzyme localized in the inner membrane, where it catalyzes the synthesis of ATP from ADP and phosphate, driven by a flux of protons across a gradient generated by electron transfer from the proton chemically positive to the negative side.
Proton pump
a special kind of transporter that push hydrogen ions from areas of low concentration to areas with high concentration. I
Cytochrome C
protein that is localized in the compartment between the inner and outer mitochondrial membranes where it functions to transfer electrons between complex III and complex IV of the respiratory chain.
-supports ATP synthesis
Coenzyme Q
a redox-active lipid produced across all domains of life that functions in electron transport and oxidative phosphorylation
Where does oxidative phosphorylation occur within a prokaryotic cell? What about in a eukaryotic cell?
- fourth/ last part of cellular respiration (aerobic conditions)
Plasma membrane: prokaryotes
Inner mitochondrial membrane: eukaryotes
NADH and FADH2 are the primary inputs for the ETC. Which complex in the ETC accepts electrons from NADH? What about from FADH2? ADD
oxygen readily accepts electrons; serves as the final electron acceptor
-complex 1 intakes NADH
-complex 2 intakes FADH2
Is glycolysis the only metabolic pathway that generates NADH? Is the citric acid cycle the only metabolic pathway that generates FADH2?
CAC also produces NADH
Understand how electrons flow within the ETC – i.e., which complex receives electrons from which complex(es).
complex 1 (NADH) and 2 (FADH2) send e- to complex 3 using ubiquinone and coenzyme q respectively, received by cytochrome c in complex 3, e-sent to complex 4. all are proton pumps except complex 2
Does each ETC complex involve a single electron-carrier or multiple electron-carriers?
both, coenzyme q takes 2 protons, cytochrome c and upiquinone only accept one
Thinking about the electron carriers at complex III, why is the Q cycle necessary?
cytochrome c can only accept 1 proton but coenzyme q gives 2; a radical must be made for it to accept both
Which amino acid must be neutralized for ATP synthase’s c ring to turn? Review this AA’s characteristics. [Review Proteins 1]
glutamate
Fully combusting glucose ultimately yields 32 ATP. How many of those are generated in oxidative phosphorylation?
28
Which tissue type has the most mitochondria in the human body? Does this make sense, based on what you know about glucose as a primary fuel? [Review Carbohydrates 2]
the brain: 1-2 million per neuron; makes snese since glucose is primary fuel and mitochondria are site of pyruvate oxidation and CAC
Understand how oxidative phosphorylation is regulated, focusing on acceptor / respiratory control. Why is it important that NAD+ and FAD are regenerated?
- When ADP conc is low, NADH and FADH2 are produced by earlier parts of cell respiration but not oxidized back to NAD+ and FAD
- Electrons cross the inner mitochondrial membrane via malate-aspartate shuttle, NADH cannot cross
- ETC regulation:
o Protein chaperones for complex assembly and e- carrier insertion, post translational modifications of tryptophan - ATP synthase regulation:
o IF1 prevents reverse rotation during o2 deprivation
o Post translational mod of phosphorylation of b-subunit
Reciprocal regulation
a pathway can have anabolic or catabolic effects on metabolism; ie. maintaining blood glucose levels by insulin or glucagon
Photosynthesis vs. Photorespiration
photorespiration is undergone when o2 is present and CO2 is not
Phosphoglucomutase
A key enzyme in glycolysis and gluconeogenesis, regulating both glycogen and trehalose metabolism in insects.
Phosphoryl group transferred to carbon-1 by phosphoglucomutase (in carbohydrate synthesis in humans)
In what organ does gluconeogenesis occur in humans? Why must reactions 3-7 occur in the cytoplasm?
Only occurs in the liver within humans
- The enzymes for gluconeogenesis are located in the cytoplasm, except for pyruvate carboxylase (in the mitochondria) and glucose 6-phosphatase (membrane bound in the endoplasmic reticulum).
Know which (if any) of the 7 major metabolic intermediates can enter gluconeogenesis.
No, there are precursors that can be turned into intermediates
- Gluconeogenesis relies on non-carbohydrate precursors
- Glucogenic carbon skeletons from amino acid metabolism
- Glycerol from triacylglycerol metabolism
- Lactate from lactic acid fermentation
- Propionyl CoA from fatty acid metabolism
Explain why gluconeogenesis is not a reversal of glycolysis.
- Bypasses the (almost) irreversible reactions in glycolysis
- Does so via four enzymes unique to the pathway
- Energy unavailable to force -ΔG glycolytic reactions to go backwards
To raise the body’s blood-glucose level, would gluconeogenesis end at reaction 9 or 10?
Reaction 10 because Glucose is then transported to cytoplasm and released into blood
Understand how gluconeogenesis is regulated.
Gluconeogenesis and glycolysis are reciprocally regulated within liver
One pathway active, when other pathway inactive
Is the Calvin Cycle catabolic or anabolic? What is the source of the carbons for glucose?
Anabolic reactions that occur within chloroplasts in absence of light (dark reactions aka calvin cycle)
Source for carbons for glucose is via CO2 (traps carbon in the cell)
Why is Rubisco labeled an inefficient enzyme?
- Most abundant enzyme in plants but inefficient (low kcat)
- Also requires both bound CO2 and substrate CO2
Know why it is important that chloroplast-containing plant cells can perform both gluconeogenesis and the Calvin Cycle
in Calvin cycle stage 2, it uses the same phosphate from the liver in the plant cell. 3-phosphoglycerate metabolized via gluconeogenesis (reactions 5-8)
(crossover with the same phosphate in each of the two reactions)
What is the primary input / starting material for lactic acid fermentation? What coenzyme is regenerated via this anaerobic pathway?
- Pyruvate interconverted to form lactate, which can enter gluconeogenesis
- Anaerobic process in which organic compounds act as both electron donors and acceptors
- In the process, NAD+ is regenerated for glycolysis
(pyruvate and NAD+)
Explain what happens in the Cori Cycle in the muscle versus in the liver.
Cori cycle -A series of cooperative reactions between skeletal muscles and liver
- Shifts burden of metabolizing lactate to liver, while providing muscles with glucose for glycolysis
- Muscle:
o Anaerobic glycolysis: glucosepyruvate
o Pyruvate lactate in lactic acid fermentation - Liver:
o Lactatepyruvate
o Pyruvateglucose (gluconeogenesis)
In what organs does glycogenesis occur in humans?
Muscle and Liver
Know which enzymes involved in glycogenesis form α-1,4- vs. α-1,6-glycosidic bonds
glycogenin builds an oligosaccharide to generate a short oligosaccharide with a-1,4 glycosidic bonds
- Glycogen synthase extends oligosaccharide using UDP-glucose
- Generates larger linear polysaccharide with α-1,4-glycosidic bonds
- Branching enzyme breaks an existing α-1,4-glycosidic bond and transfers the fragment to a more interior site
- Generates larger branched polysaccharide with α-1,6-glycosidic bonds
Understand how glycogenesis is regulated.
- Glycogen metabolism and synthesis are reciprocally regulated
- Involves glycogen phosphorylase and glycogen synthase
- Via ATP, Ca2+ (muscle only), glucose (Liver only) and phosphorylation
Where are the products of the β-oxidation pathway and of glycerol metabolism entering cellular respiration?
β-oxidation pathway - metabolizes fatty acids
Glycerol metabolism - metabolizes glycerol in a triacylclycerol
NADH (from both) Enter in oxidative phosphorylation through complex 1
FADH2 (from β-oxidation pathway) enter in oxidative phosphorylation through complex 2
Thinking about the headline “Sugar is making you fat” explain how glucose can be converted into a fatty acid, identifying the metabolic pathways involved.
- Insulin is released by pancreas when blood sugar is high, causes liver to take in glucose and can convert to fatty acids in de novo lipogenesis
Describe the differences with dietary fiber
-insoluble: contains beta glycosidic bonds, non-fermentable (metabolically inert)
-soluble fiber: contains alpha glycosidic bonds, fermentable
