Biochemistry Ch. 16 Glycolysis and Gluconeogenesis
Glucokinase
A SECOND FORM OF HEXOKINASE aka Hexokinase D or type IV Adult liver and kidney senses glucose levels
fructose-2,6-bisphosphate
A potent allosteric activator of liver phosphofructokinase is _____________________, which is produced from fructose-6-phosphate by PFK2.
Adenylate Kinase
ADP + ADP → ATP + AMP Salvages ATP from two ADP molecules. This is the primary reason AMP "represents" low energy charge.
What is substrate level phosphorylation?
ATP synthesis when the phosphate donor is a substrate with high phosphoryl transfer potential
Aldolase A Reaction Type
Aldol cleavage
Phosphofructose Kinase-1 (PFK-1) Allosterically regulated
Allosterically regulated by "energy charge" Stimulated by F 2,6 BP and AMP Inhibited by ATP, citrate, H+
Hexokinase Induced fit
Binding of glucose causes conformational change (cleft closing) in Hexokinase Active site around glucose becomes more nonpolar Favors donation of gamma phosphate Excludes water from active site Prevents hydrolysis of gamma phosphate of ATP by water Substrate-induced cleft closing is a general feature of kinases
What is a common mechanistic feature of kinases?
Binding of substrate induces cleft closing
Hexokinase substrate specificity
Broad substrate specificity phosphorylates several hexoses
Enolase Reaction type
Dehydration
Hexokinase Cofactor
Divalent cation (usually Mg2+) complexed with ATP.
3-Bisphosphoglycerate Kinase Free Energy Change
Endergonic
Triose Phosphate Isomerase Free energy change
Endergonic
Aldolase A Free Energy Change
Exergonic
Phosphofructose Kinase-1 (PFK-1) Free Energy Change
Exergonic
Phosphohexose Isomerase Free Energy Change
Exergonic
Pyruvate Kinase Energy Transfer
Exorgonic
Fructose is converted to fructose-6-phosphate
Fructose can enter glycolysis at two distinct points, depending on the tissue. How is fructose metabolized in adipose tissue?
Describe the biochemical explanation for galactosemia.
Galactose is metabolized by conversion to Gal-1-P by galactokinase. The enzyme, galactose-1-phosphate uridyl transferase transfers a uridyl group from UDP-Glc to Gal-1-P to produce UDP Gal. Then UDP-Gal is epimerized to UDP-Glc. UDP-galactose is a necessary intermediate in the metabolism of galactose. Individuals deficient in galactose-1-phosphate uridyl transferase activity cannot metabolize galactose, which leads to elevated levels of galactose in the blood and urine. This genetic disease is referred to as galactosemia
Both hexokinase and glucokinase phosphorylate glucose. The function of glucokinase is to phosphorylate glucose in liver cells as a means to regulate blood-sugar levels. Would you expect its Km to be higher or lower than hexokinase?
Glucokinase must be responsive to elevated blood-glucose concentrations, thus it should have a higher Km. This lower binding affinity allows this enzyme to become more active at high glucose concentrations, which saturate hexokinase. Hexokinase serves to phosphorylate glucose (and other hexoses) in the cytosol, and has higher affinity for glucose, or a lower Km value.
What two isomerization reactions occur in glycolysis? Why are these steps necessary?
Glucose-6-phosphate is isomerized to fructose-6-phosphate, converting an aldose to a ketose, which then allows phosphorylation at the number 1 carbon. Later in the pathway, dihydroxyacetone-phosphate is converted to glyceraldehyde-3-phosphate, utilizing both of the molecules formed from fructose-1,6-bisphosphate cleavage
2
Glycolysis produces a net of ________ moles of ATP per one mole of glucose.
Regulatory Enzymes of Glycolysis
Hexokinase Phosphofructose Kinase 1 Pyruvate Kinase
Enzymes of Glycolysis
Hexokinase Phosphohexose Isomerase (Phosphoglucose Isomerase) Phosphofructose Kinase-1 (PFK-1) Aldolase A Triose Phosphate Isomerase Phosphoglyceraldehyde dehydrogenase 3-Bisphosphoglycerate Kinase Phosphoglyceromutase Enolase Pyruvate Kinase (PK)
Glucokinase Km
High Km = low affinity for glucose Km> 5-10 mM
6
How many high-energy phosphate bonds are expended in gluconeogenesis?
thiamine or B1
In alcoholic fermentation, the decarboxylation of pyruvate requires a coenzyme that contains the vitamin ______________.
HIF-1, or hypoxia-inducible transcription factor
In the absence of oxygen, _____________________ increases the expression of most glycolytic enzymes and the glucose transporters GLUT1 and GLUT3.
Hexokinase Allosterically regulated...
Inhibited by G6-P
Triose Phosphate Isomerase Reaction type
Isomerization
Phosphohexose Isomerase Reaction type
Isomerization Conversion of aldose C1 into ketose on C2.
Which of the following are reasons that glucose is a common metabolic fuel used by living organisms?
It has a stable ring structure and is unlikely to glycosylate proteins. It has been found as one of the monosaccharides formed under prebiotic conditions.
lactase
Lactose intolerance is caused by a deficiency of
Hexokinase Km
Low Km = high affinity for glucose Km< 0.1 mM permits efficient metabolism of glucose
Why do muscles need to generate ATP under aerobic and anaerobic conditions?
Muscles initially function aerobically. However, when bursts of energy are required, the oxygen supply does not meet the demand. In order to generate sufficient ATP for energy demands during periods of extreme activity, the muscle depends on activity-dependent anaerobiosis.
Phosphoglyceraldehyde Dehydrogenase Coenzyme
NAD+
Phosphohexose Isomerase Allosterically regulated
No
3-Bisphosphoglycerate Kinase Coenzyemes, cofactors, prosthetic groups
None
Aldolase A Coenzymes, cofactors, and prosthetic groups
None
Enolase Coenzymes, cofactors, prosthetic groups
None
Phosphofructose Kinase-1 (PFK-1) Coenzymes, cofactors, and prosthetic groups
None
Phosphoglyceraldehyde Dehydrogenase Cofactors, prosthetic groups
None
Phosphohexose Isomerase Coenzymes, cofactors, and prosthetic groups
None
Pyruvate Kinase Coenzymes, cofactors, prosthetic groups
None
Triose Phosphate Isomerase Coenzymes, cofactors, prosthetic groups
None
α-amylase
Pancreatic ______________ is the digestive enzyme primarily responsible for the hydrolysis of dietary plant starch.
Why is it more sensible for phosphofructokinase to be an important control step, rather than hexokinase?
Phosphofructokinase catalyzes the first committed step in the glycolytic pathway. At this point, the molecule is committed to entering the glycolytic path. In contrast, production of G6P is the first step in many different paths. Thus, glycolytic control would not be maintained by tight regulation of hexokinase.
How does citrate influence glycolysis?
Phosphofructokinase is inhibited by citrate, which is an intermediate in the citric acid cycle. Thus, if citrate levels are high, the enzyme is inhibited, and fewer glucose molecules are metabolized. In this sense, citrate serves as a cell indicator. High levels of citrate in the cytoplasm means that biosynthetic precursors are abundant, and so there is no need to degrade additional glucose for this purpose
Phosphoglyceromutase Reaction type
Phosphoryl shift from C-3 to C-2
Phosphofructose Kinase-1 (PFK-1) Reaction type
Phosphoryl transfer
Hexokinase Reaction Type
Phosphoryl transfer "traps" glucose in cell No transporters exist to transport phosphorylated glucose (or other phosphorylated carbohydrates) Activates "destabilizes" glucose
Pyruvate Kinase Reaction Type
Phosphoryl transfer ATP produced substrate level phosphorylation
3-Bisphosphoglycerate Kinase Reaction type
Phosphoryl transfer from 1,3 BP to ADP ATP produced substrate level phosphorylation
Phosphoglyceraldehyde Dehydrogenase Reaction type
Phosphorylation coupled to oxidation of aldehyde to carboxylic acid
How is glycolysis maintained under anaerobic conditions?
Pyruvate can be reduced to either lactate or ethanol, and this reaction is accompanied by the oxidation of NADH to regenerate NAD+.
Enolase Reversible or not?
Reversible
Phosphoglyceraldehyde Dehydrogenase reversible or not
Reversible
Pyruvate Kinase Reversible or not?
Reversible
Triose Phosphate Isomerase Reversible or not?
Reversible
3-Bisphosphoglycerate Kinase Reversible or not
Reversible this is unusual for a kinase
Pyruvate Kinase Allosterically regulated
Simulated by F-1,6-BP (Feed forward) Inhibited by ATP and Alanine
Phosphofructose Kinase-1 (PFK-1) Hormonal Regulation
Stimulated insulin Inhibited by glucagon
Phosphofructose Kinase-2 (PFK-2) Function
Synthesis of Fructose 2,6 phosphate activates glycolysis inactivates gluconeogenesis
At equilibrium, there is far more DHAP than GAP. Yet the conversion of DHAP by triose phosphate isomerase proceeds readily. Why?
The GAP formed is immediately removed by subsequent reactions, resulting in conversion of DHAP into GAP by the enzyme.
Cori
The ________________ cycle refers to the metabolic reactions by which glucose is converted into lactate in skeletal muscle, and then lactate converted back into glucose in the liver.
enediol
The catalytic mechanism of the isomerization of a ketose into an aldose proceeds through an _________________ intermediate.
Rossmann fold
The common structural domain of NAD+ binding dehydrogenases is often called a ___________________ after the scientist who first recognized it.
How is the conversion of phosphoenolpyruvate to pyruvate accompanied by ATP formation?
The enol phosphate possesses very high potential for phosphoryl transfer, which is due to the driving force of the tautamerization of the enol to the more stable ketone.
How are gluconeogenesis and glycolysis regulated reciprocally?
The enzymes involved in two substrate cycles are control points. Figure 16.28 in the text shows the glycolytic path activation by F-2,6-BP, AMP, and F-1,6-BP; whereas ATP, alanine, citrate, and protons inhibit glycolysis. Gluconeogenesis is activated by citrate and acetyl CoA and inhibited by F-2,6-BP, AMP, and ADP.
carboxylation
The first step in gluconeogenesis is the ___________________ of pyruvate to form oxaloacetate.
phosphofructokinase
The key enzyme that regulates the pace of glycolysis is ____________________.
lactate and alanine
The primary raw materials for gluconeogenesis are
What two functions are attributed to substrate cycles?
The substrate cycles regulate glycolytic path flux by amplifying metabolic signals and they generate body heat produced by the hydrolysis of ATP.
What is the function of a thioester intermediate such as the one formed from GAP?
The thioester allows the two-step reaction to be coupled so the second reaction, the energetically unfavorable phosphorylation, can proceed.
Which metabolic steps differ from glycolysis in gluconeogenesis?
There are three irreversible steps in glycolysis, which require four different steps in gluconeogenesis: pyruvate conversion to phosphoenolpyruvate via an oxaloacetate intermediate, fructose 1,6-bisphosphate hydrolysis, and the hydrolysis of glucose 6-phosphate.
obligate anaerobes
These organisms cannot survive in the presence of oxygen
What astounding discovery was made by the Buchners?
They were the first to dispute Pasteur's assertion that fermentation required intact cells when they demonstrated that fermentation can occur in yeast extracts.
galactosemia
This condition is a result of a genetic deficiency of a single "transferase" enzyme.
biotin
This essential nutrient is required for the carboxylation of pyruvate in humans.
UDP-glucose
This intermediate is necessary for the conversion of galactose to glucose.
AMP
This is an allosteric activator of glycolysis.
Embden Meyerhof pathway
This is another name for glycolysis.
gluconeogenesis
This is the process by which noncarbohydrate precursor molecules are converted into glucose.
ATP
This molecule is an allosteric inhibitor of phosphofructokinase.
NAD+
This substance must be regenerated for glycolysis to proceed
GLUT5
This transporter is responsible for fructose uptake in the intestine.
Describe the two isoforms of lactate dehydrogenase.
Two forms exist, called M and H, which predominate in skeletal and heart muscle, respectively. The two forms are products of different genes, but are similar in structure, and can form tetramers in various H:M ratios. The two forms differ in their sensitivity to pyruvate. H4 functions primarily to oxidize lactate to pyruvate, which serves as a fuel for aerobic metabolism. In contrast, M4 produces lactate so that glycolysis can continue under anaerobic conditions.
2,3-bisphosphoglycerate
What is the additional metabolite that is required for the conversion of 3-phosphoglycerate to 2-phosphoglycerate?
glyceraldehyde-3-phosphate and dihydroxyacetone phosphate
What two 3-carbon molecules are generated by the cleavage of fructose-1,6-bisphosphate?
Phosphoglyceromutase Free energy change
endergonic
What are the primary metabolic fates of pyruvate?
ethanol lactate acetyl CoA
Phosphoglyceromutase Coenzymes, cofactors, prosthetic groups
none
What is the function of glyceraldehyde 3-phosphate dehydrogenase?
oxidation by NAD+ and formation of acyl-phosphate
Aldolase A reversible or irreversible?
reversible
Phosphoglyceromutase Reversible or not?
reversible
What reaction is catalyzed by aldolase?
reversible cleavage of F-1,6-BP to DHAP and GAP
Hexokinase Hormonal regulation
stimulated by insulin prohibited by glucagon
What is the purpose of phosphorylating glucose in cytosol?
to trap glucose in the cell to destabilize glucose and facilitate the next series of metabolic steps
How are the glycolytic enzymes regulated?
transcriptional control reversible phosphorylation allosteric control
Phosphofructose Kinase-2 (PFK-2) Hormonal regulation
well fed - stimulated by high insulin, low glucogon after carb rich meal starvation - inhibited by low insulin, high glucogon inhibits glycolysis stimulates gluconeogenesis
