Exam 3 Flashcards
an inactive enzyme that must be partially digested to attain full enzymatic activity
zymogen
How does amylase digest carbohydrates?
cleaning the alpha-1,4 glycosidic bonds of starch
Which enzyme activates trypsin?
enteropeptidase
The transport of lipids in the lymph is carried out by
chylomicrons
______ pathways can be either anabolic or catabolic, depending on the energy conditions of the cell
amphibolic
ultimate acceptor of electrons in aerobic organisms
O2
the product of oxidation of carbon containing fuels in aerobic metabolism
CO2
this compound serves as an acyl carrier in metabolism
acetyl-CoA
the chemical currency of metabolism
ATP
the type of metabolism where useful energy is harvested
catabolism
serves as a reservoir of high potential phosphoric groups that can be readily transferred to ADP to regenerate ATP in vertebrate muscle
creatine phosphate
the metabolic pathways that require energy and are often biosynthetic processes are
anabolic
metabolic pathways are regulated by
- transcriptional regulation of the amount of enzyme
- allosteric control of enzyme activity
- the accessibility of substrates by compartmentalization
Explain how a metabolic pathway can contain an energetically unfavorable reaction yet still occur
the free-energy changes of the individual steps in a pathway are summed to determine the overall free-energy charge. Thus, a step that might not normally occur can be driven if it is coupled to a thermodynamically stable reaction.
What is an activated carrier? Provide two examples.
Activated carriers are molecules that are used as the carrier molecules of a particular molecule, atom, electron, or of protons. One example would be ATP, which is the activated carrier of phosphoryl groups. Flavin derivatives (FAD) and nicotinamide derivatives (NAD+) are examples of activate carriers of electrons.
stereoisomers that are mirror images of eachother
enantiomers
monosaccharides that differs at a single asymmetric carbon
epimers
the storage form of glucose in animals
glycogen
the enzymes that synthesize oligosaccharides
glycosyltransferases
the molecule to which most sugars are attached prior to transfer
UDP
a five-membered ring formed from a monosaccharide
furanose
formed when tow monosaccharides are linked together via a glycosidic bond
disaccharide
in N-linked glycoproteins, the carbohydrate portion is attached to an ____ residue in the protein
asparagine
The simplest carbohydrates are
dihydroxyacetone and D- and L-glyceraldehyde
an aldehyde and alcohol can react to form a
hemiacetal
the nutritional storage forms of glucose in plants
amylase and amylopectin
to which amino acid residues in glycoproteins are the sugars commonly linked
serine, threonine, and asparagine
Inhibitors against this viral enzyme have potential as anti-influenza agents
neuramidase
List three functions of carbohydrates and provide examples
- Fuel: monosaccharides like glucose, fructose and galactose have entry points into basic ATP-generating metabolic pathways.
- Structure: Cellulose with its beta linkages yields a straight chain capable of interacting with other cellulose molecules to form strong fibrils.
- Signaling/Recognition: Glycosylated proteins can serve as distinguishing marks within the cell and at the cell surface. The ABO blood groups are defines by characteristic carbohydrate patterns. The influenza virus protein hemagglutinin will recognize species-specific carbohydrate signatures at the cell surface for effective infection.
What is the difference between an enantiomer and a diastereoisomer?
An enantiomer is a stereoisomer that is a perfect (non superimposable or nonidentical) mirror image. A chiral molecule has one perfect mirror image, but for larger carbohydrates that have the same chemical formula and have multiple chiral centers, variations in asymmetric carbon structures mean that additional stereoisomers exist. The stereoisomers that are not mirror images of each other are called diastereoisomers.
What are the chemical and structural differences between cellular and glycogen.
Both are glucose homopolymers. Glycogen is a branched polymer and contains alpha-1,4 linkages with beta-1,6 branch points about every 10 residues. Cellulose is a linear polymer that contains that contains beta-1,4 linkages. Because of the beta linkages, cellulose can assemble into very long straight chains which can form interchain hydrogen bonds to produce fibrils
Describe the role of carbohydrates in determining human blood types.
All blood groups have a basal oligosaccharide signature designated Type O because there is no active glycosyltransferase for additional modification. In type A, N-acetylgalactosamine is added to the Type O oligosaccharide by a specific glysocyltransferase. In Type B, galactose is added to the Type O oligosaccharide by another transferase. The presence of unique oligosaccharide signatures identifies blood cells as ‘self’
What is the molecular basis of how erythropoietin protein relieves anemia?
Erythropoietin is a glycoprotein secreted by the kidney into the blood that stimulates the production of red blood cells. Glycosylation of erythropoietin enhances its stability in the blood, which leads to more stimulation of red blood cell production. For patients with anemias that are deficient in red blood cells, this extra stimulation of blood cell proliferation relieves anemia.
Explain the role of proteoglycans in cartilage.
Cartilage is composed, in part, of the proteoglycan aggrecan and the glycosaminoglycan hyaluronan. The glycosaminoglycan component of aggrecan cushions joints by releasing water on impact, and then rebinding water.
must be regenerated for glycolysis to proceed
NAD+
this molecule is an allosteric inhibitor of phosphofructokinase
ATP
an allosteric activator of glycolysis
AMP
how many moles of ATP does each mole of glucose produce in glycolysis
2
Why is glucose the most table hexose
the hydroxyl groups are all in the equatorial position
a potent allosteric activator of liver phosphofructokinase is _______, which is produced from fructose-6-phosphate by PFK2
fructose-2,6-biphosphate
increases the expression of most glycolytic enzymes and the glucose transporters GLUT1 and GLUT3
HIF-1
Which two 3-carbon molecules are generated by the cleavage of fructose-1,6-biphosphate?
glyceraldehyde-3-phosphate and dihydroxyacetone phosphate
What reaction is catalyzed by aldolase
reversible cleavage of F-1,6-BP to DHAP and GAP
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
What type of enzyme catalyzes the intramolecular shift of a chemical group?
mutase
Fructose can enter glycolysis at two distinct points, depending on the tissue. How is fructose metabolized in adipose tissue?
Fructose is converted to fructose-6-phosphate
Lactose intolerance is caused by a deficiency of
lactase
How are glycolytic enzymes regulated?
- transcriptional control
- reversible phosphorylation
- allosteric control
During exercise, glycolysis is stimulated by a
feed-forward stimulation of pyruvate kinase
Which two isomeraization reactions occur in glycolysis? Why are these steps necessary?
Glucose 6-Phosphate is isomerize to Fructose 6-Phosphate, converting an aldiss 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-biphosphate cleavage.
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 tautomerization of the enol to the more stable ketone.
What are the two fermentation pathways for pyruvate and how do they contribute to sustaining glycolysis?
Lactic acid fermentation and ethanol fermentation. They mole for mole regenerate the NAD+ reduced by glyceraldehyde-3-phosphate dehydrogenase
Why is it more sensible for phosphofructokinase, rather han hexokinase, to be an important control step?
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 glucose 6-phosphate is the first step in many different paths. Thus, glycolytic control would not be maintained by tight regulation of hexokinase
Describe the regulation of PFK in the liver.
PFK is still inhibited by ATP as in the muscle cells, but the ATP levels do not fluctuate as dramatically in the liver. PFT is inhibited by citrate, which indicates an abundance of precursors for the citric acid cycle. Thus, there is no need to further metabolize glucose for this purpose. Fructose-2,6-biphosphate (F2, 6BP) is an activator of PFK in a feedforward stimulation mechanism to ensure glycolysis is accelerated with glucose is abundant. As the concentration of Fructose-6-Phosphate (F-6P) increases as the result of high blood glucose, it is converted to F2, 6BP. F2, 6BP increases PFK’s affinity for F-6P and diminishes any inhibitory effect of ATP.
the major tissue in which gluconeogenisis takes place
liver
where does conversion of glucose-6-phosphate to glucose take place
endoplasmic reticulum
the reaction that uses GTP and not ATP as its high-phosphoryl-transfer potential donor
PEPCK (phosphoenolpyruvate carboxykinase) or oxaloacetate to PEP
an intermediate that is decarboxylated and phosphorylated to produce phosphoenolpyruvate
oxaloacetate
this essential cofactor is required for the carboxylation of pyruvate in humans
biotin
the enzyme that carboxylates pyruvate to oxaloacetate
pyruvate carboxylase
the cellular compartment where the first step is gluconeogenisis occurs
mitochondria
transport of oxaloacetate produced by PEPCK utilizes the mitochondrial and cytosolic enzyme _____
malate dehydrogenase
What do high levels of ATP and citrate do?
- indicate a high-energy-well-fed state
- promote gluconeogenesis
- inhibit glycolysis
the bifunctional enzyme
phosphofructokinase II
How does eating a meal influence the level of insulin released by the pancreas?
creates a high blood sugar which increases the level of insulin
acts as a glucose buffer for the rest of the body
liver
How does the liver behave (in regards to glucose) under low-energy conditions?
Does not utilize glucose, and it’s a producer of glucose
the primary raw materials for gluconeogenesis
lactate and amino acids
Explain how fructose 2,6-biphosphate (F-2, 6-BP) levels are regulated in the cell, and the relationship between F-2, 6-BP levels and the blood glucose levels.
The levels in the cell of F-2, 6BP are regulated by the bifunctional enzyme, phosphofructokinase2/ fructose 2,6-biphosphatase. When glucose levels in the blood are low, the phosphatase function of the enzyme is activated and the kinase function is inhibited, lowering F-2,6-BP levels. This inhibits the glycolytic pathway. Conversely, when blood glucose levels are high, the phosphatase function is inhibited and the kinase function is activated, raising cellular F-2, 6-BP levels. This activates the glycolytic pathway.
What is the role biotin has in pyruvate carboxylase catalytic mechanism?
Biotin is the activated carbon 1 carrier. The flexible arm of biotin allows the CO2 to move from one active site of the enzyme to the second set of active site amino acid, where pyruvate is carboxylated
How are gluconeogenesis and glycolysis coordinated by nucleotides?
ATP inhibits glycolysis at PFK, while ADP inhibits the flow of carbon from pyruvate to glucose at pyruvate carboxylase and phosphoenol pyruvate carboxykinase. When ADP levels are high, adenylate kinase converts ADP to AMP and ATP. The increase in AMP (only in a low-energy state) results in the inhibition of gluconeogenesis and activates glycolysis in the liver.
Which metabolic steps differ from glycolysis in gluconeogenesis?
There are 3 irreversible steps in glycolysis which require 4 different steps in gluconeogenesis:
- Pyruvate conversion to phosphoenolpyruvate via an oxaloacetate intermediate.
- F-1,6-BP hydrolysis
- The hydrolysis of glucose 6-phosphate
Describe the molecular basis of insulin resistance through PEPCK activity.
The presence of insulin in the blood indicates high glucose levels in the blood and should signal inhibition of gluconeogenesis. Insulin triggers the suppression of PEPCK gene transcription. In insulin resistance, PEPCK is not suppressed and gluconeogenesis remains active resulting in the liver delivering glucose into the bloodstream raising blood glucose levels.
What is a substrate cycle and how can it affect the relative flux of metabolic pathways?
Substrate cycles are pairs of irreversible reactions that produce and consume each others substrates/products. When substrate cycles are not equally paired with one another (one enzyme is faster than the enzyme in the other direction), they can magnify the effect of allosteric regulation of the enzymes.
How does a cell extract energy and reducing power from its environment
Catabolism/catabolic reactions
How does a cell synthesize the building blocks of its macromolecules and then the macromolecules themselves
Anabolism/anabolic reactions
What does the acidic environment of the stomach do?
denature proteins
digest proteins into amino acids and peptides
proteases and peptidases
digest dietary carbohydrates
alpha-amylase
digestive proteases and peptidases are first synthesized as these inactive precursor forms
proenzymes or zymogens
cleaves trypsinogen to form active trypsin
enteropeptidase
Why are inactive precursors important in digestive activity?
You don’t want active proteases in your cells degrading all your proteins
primary source of carbs
starch
initiates digestion by cleaving alpha-1,4 bonds but NOT alpha-1,6 bonds
alpha-amylase
secreted by the pancreas and convert the triacylglycerols into 2 fatty acids and monoacylglycerol
lipases
in the intestine, triacylglycerols are reformed from free fatty acids and monoacylglycerol and packaged into lipoprotein particles called
chylomicrons
why do chylomicrons enter the blood
so that the triaglycerols can be absorbed by tissues
activated carrier of phosphoryl groups
atp
what carries activated electrons derived from the oxidation of fuels
NAD+ and FAD
carry acyl groups
Coenzyme A and LIpoamide
what controls the amount of enzymes
transcription
what control the activity of enzymes
catalytic activity is regulated allosterically or by covalent modification. The energy status of the cells (relative amounts of ATP/ADP/AMP/Pi) is often an important regulator of enzyme activity.
what controls the accessibility of substrates
Opposing reactions, such as fatty acid synthesis and degradation may occur in different cellular components. Regulating the flux of substrates between compartments is used to regulate metabolism
have the same molecular formula but different structures
isomers
isomers that differ in the order of attachment of atoms
constitutional isomers
isomers in which atoms are connected in the same order but differ in the spatial arrangement
stereoisomers
non superimposable miroir images
enantiomers
isomers that are not mirror images
diastereoisomers
diastereoisomers that differ at one of several symmetric carbon atoms
epimers
diastereoisomers that differ at new asymmetric carbon atoms formed on ring closure
anomers
catalyse the formation of glycosidic bonds
glycosyltransferases
how are the monosaccharide substrates for glycosyltransferases activated
attachment to uridine diphosphate (UDP)
How are glycoproteins structured
carbs are attached to the nitrogen atom in the side chain of asparagine (N-linkage) or the oxygen atom of the side chain of serine or threonine (O-linkage)
what is cartilage composed of
the proteoglycan aggrecan and the glycosaminoglycan hyaluronan
what does the glycosaminoglycan component of aggrecan do
cushions joints by releasing water on impact and then rebinding water
one of the most abundant carbs in the world, also a glycosaminoglycan found in the exoskeleton of insects
chitin
conversion of one molecule of glucose to 2 molecules of pyruvate with the generation of 2 molecules of ATP
glycolysis
2 stages of glycolysis
1) Traps glucose in the cell and modifies it so that it can be cleaved into a pair of phosphorylated 3-carbon compounds
2) Oxidizes the 3-carbon compounds to pyruvate while generating 2 molecules of ATP
The 2 steps in the formation of glyceraldehyde 1,3-biphosphate
1) the highly exergonic oxidation of C1 in GAP to an acid
2) the highly endergonic formation glyceraldehyde 1,3 diphosphate from the acid
How are the 2 reactions in the formation of glyceraldehyde 1,3 diphosphate linked
by the formation of an energy-rich thioester in the active site of glyceraldehyde 3-phosphate dehydrogenase
ATP-generating pathways in which electrons are removed from one organic compound and pass to another organic compound (anaerobic)
Fermentation
The formation of ethanol from pyruvate in ethanol fermentation regenerates this
NAD+
oxidized by alcohol dehydrogenase, regenerating NAD+
NADH generated by glyceraldehyde 3-phosphate dehydrogenase in ethanol fermentation
can be oxidized by converting pyruvate to lactate in a reaction catalyzed by lactate dehydrogenase
NADH
the conversion of glucose into 2 molecules of lactate
lactic acid fermentation
In the liver, this activates PFK
F26BP
primary enzyme for glucose phosphorylation in the liver
glucokinase (isoenzyme)
when does glucokinase phosphorylate glucose
when blood glucose is high (has a low affinity for glucose)
why does glucose phosphorylation by glucokinase matter?a
This ensured that hexokinase in muscle and brain cells get first dibs on glucose when it is limiting. It also ensures no glucose is wasted when glucose is abundant.
inhibits pyruvate kinase in the muscle
ATP and alanine
synthesized by pyruvate by a single step
alanine
activates pyruvate kinase in the muscle
F16BP (a product upstream in glycolysis)
How is pyruvate kinase regulated int he liver?
Low blood glucose yields PK phosphorylation. High Blood glucose yields PK dephospho rylation.
secreted by the pancreas in response to an increase in blood glucose levels and stimulates glucose uptake by tissues
insulin
conversion of pyruvate into PEP begins with formation of
oxaloacetate
what vital does pyruvate carboxylase require as a cofactor
biotin
