Biochem Exam 5

Where is α-amylase stated to be present?

In saliva and pancreatic secretions

α-amylase is described as which type of enzyme?

Endoglycosidase

Which product results from α-amylase cleavage of amylopectin or glycogen

All of the above

In starch and glycogen degradation, what does the debranching enzyme cleave?

Limit dextrins

Which is one of the two activities of the debranching enzyme described on the slide?

It transfers trisaccharide groups

Which is one of the two activities of the debranching enzyme is described:

Cleaves the remaining single glucose units from the main chain

As α-amylase nears branch point its going to reduce activity and end up with:

Highly branched limit dextrins

The debranching enzyme’s glucanotransferase activity transfers what?

A trisaccharide unit

In order for alpha amylase to easily digest, we have a debranching enzyme called ____ activity

glucanotransferase

The digestive breakdown of starch and dietary glycogen is:

Unregulated

Which form of glycogen is described as tightly regulated?

Synthesis and degradation of storage glycogen

Tissue glycogen is described on the slide as:

An important energy reservoir

___ cleaves a sugar unit from which part of the glycogen chain and uses inorganic phosphate to phosphorylate the glucose. (cleaves and phosphorylates at the same time)

Glycogen phosphorylase

What does glycogen phosphorylase use to phosphorylate the released glucose?

Inorganic phosphate

True or False: Glycogen phosphorylase requires the use of ATP and inorganic phosphate to phosphorylate glucose

False

After glycogen phosphorylase activity, what enzyme degrades the resulting limit dextrins?

Debranching enzyme

Which statement best describes the enzymes present in glycogen granules?

Catabolic and anabolic enzymes are present

True or False: In the phosphorolysis reaction, the glycosidic bonds are split by phosphates and NOT H2O

True

If glycogen is degraded in muscles, the reaction enters:

Enters glycolysis

If glycogen is degraded in liver then:

Glucose-6-phosphate is hydrolyzed to glucose

If glycogen is degraded in the liver, glucose-6-phosphate is hydrolyzed to glucose for:

Transport to other tissues

Where is the active site located within each glycogen phosphorylase subunit?

At the center of the subunit

The allosteric effector active site of glycogen phosphorylase lies near which region?

The subunit-subunit interface

Which residue serves as the regulatory phosphorylation site on each subunit of glycogen phosphorylase?

Ser14

Regulatory control of glycogen phosphorylase is influenced by which structural site?

Glycogen-binding site

Which molecules act as allosteric inhibitors of glycogen phosphorylase?

ATP and glucose-6-phosphate

Which molecule serves as an allosteric activator of glycogen phosphorylase?

AMP

What happens to glycogen breakdown when ATP and glucose-6-phosphate levels are abundant?

Breakdown is inhibited

True or False: When cellular energy reserves are LOW (meaning high AMP and low ATP-G6P), glycogen metabolism is stimulated.

True

Which condition would most strongly activate glycogen phosphorylase?

High AMP and low ATP

What general rule describes the relationship between cellular energy status and glycogen breakdown?

Both

Phosphorylation of which Ser14 converts phosphorylase b into phosphorylase a causing:

A large conformational change that decreases allosteric sensitivity

Which form of glycogen phosphorylase is less sensitive to allosteric regulation?

Phosphorylase a

Phosphorylation of glycogen phosphorylase shifts the enzyme’s equilibrium toward which state?

R state

Once phosphorylated, glycogen phosphorylase becomes more active because it no longer requires:

AMP as an allosteric activator

Phosphorylation converts glycogen phosphorylase into a form that is:

Active without allosteric activation

Phosphorylase kinase, which activates glycogen phosphorylase, is itself activated by which mechanism?

Phosphorylation

Which hormones stimulate the signaling cascade that activates phosphorylase kinase?

Epinephrine and glucagon

Epinephrine’s role in regulating glycogen breakdown is to:

Prepare the body for large energy demands

Which hormone inhibits glucagon secretion?

Insulin

Which abnormal pattern occurs in diabetes regarding glycogen breakdown?

Glycogen is degraded even when glucose levels are high

cAMP is classified as what type of molecule in hormone signaling?

Second messenger

Which enzyme converts ATP into cAMP?

Adenylyl cyclase

What happens when cAMP binds to the regulatory subunits of cAMP-dependent protein kinase?

The catalytic subunits dissociate and become active

The catalytic subunits of cAMP-dependent protein kinase phosphorylate which target in the glycogen breakdown pathway?

Phosphorylase kinase

Which molecule activates glucose needed for glycogen synthesis? (not what activates glycogen synthesis, there’s a difference)

UDP

Glycogen synthesis differs from glycogen degradation because synthesis requires:

Activation of glucose as a sugar nucleotide

What provides the driving force that makes the overall reaction of glycogen synthesis favorable?

Hydrolysis of pyrophosphate (PPᵢ)

The large glycogen particle is built around which single core protein?

Glycogenin

The first glucose in a glycogen molecule is attached to which amino acid on glycogenin?

Tyrosine

Glycogen synthase transfers glucose from which activated donor molecule?

UDP-glucose

Where does glycogen synthase add new glucose units?

At the nonreducing end

Formation of the glycosidic bond involves which reaction intermediate?

Oxonium ion

Which statement describes the function of glycogen synthase of glycogen synthesis?

It transfers glucosyl units from UDP-glucose to the C-4 hydroxyl of a nonreducing end of a glycogen strand

Glycogen metabolism is regulated through reciprocal control of which two enzymes?

Glycogen synthase and glycogen phosphorylase

Which molecules allosterically inhibit glycogen phosphorylase?

ATP, glucose-6-phosphate, and caffeine

Glycogen synthase is stimulated by which metabolite?

Glucose-6-phosphate

Which regulatory mechanism regulates both glycogen synthase and glycogen phosphorylase?

Covalent modification by phosphorylation

True or false: Glycogen synthase is phosphorylated at multiple sites by protein kinases, including cAMP-dependent protein kinase.

True

True or False: Dephosphorylation is carried out by PPi for BOTH glycogen synthase and glycogen phosphorylation

True

Which form of glycogen synthase has high activity and does not require glucose-6-phosphate?

The dephosphorylated form

True or False: Phosphorylation has opposite effects on glycogen phosphorylase (the catabolic enzyme) and glycogen synthase (the anabolic enzyme).

True

Insulin triggers which metabolic response when blood glucose rises?

Glycogen synthesis

Insulin helps lower blood glucose by:

Stimulating glycogen synthesis and inhibiting glycogen breakdown

Insulin stimulates which of the following processes?

All of the above

Insulin’s effects include all of the following except:

Inhibition of glycolysis

What is the main role of phosphocreatine in muscle?

All of the above

During intense exercise such as a 100 m sprint, the immediate sources of energy are:

All of the above

Why can phosphocreatine only provide energy for a few seconds?

It is rapidly depleted during intense activity

What is the first energy source used during the FIRST second of intense exercise?

Free ATP already present in the muscle

During excersice, anaerobic glycolysis cannot continue for long because:

It leads to acidosis

In longer term exercise, which energy source becomes more dominant?

Glycogen breakdown and aerobic metabolism

Symptoms of this disease: limited ability to perform strenuous exercise, but gentle exercise is possible.

McArdle disease

Symptoms of this disease: Only the outermost branches of glycogen can be degraded.

Cori disease

Symptoms of this disease: Glucose-6-phosphate accumulates in the liver because it cannot be converted to glucose, stimulating glycolysis, causing lactate acidosis.

von Gierke’s disease

Cori disease is caused by deficiency of which enzyme?

Debranching enzyme

von Gierke’s disease is caused by a deficiency in:

Glucose-6-phosphatase

McArdle disease is caused by a deficiency in:

Muscle glycogen phosphorylase

What is another name for the pentose phosphate pathway?

All of the above

What do cells require for reductive biosynthesis in PPP?

NADPH

The pentose phosphate pathway occurs primarily in the cytosol of which tissues?

Liver and adipose cells

NADPH produced by the PPP is used in the cytosol primarily for:

Fatty acid synthesis

The oxidative phase of the pentose phosphate pathway converts glucose-6-phosphate into which product?

Ribulose-5-phosphate

Which of the following best represents the overall reaction of the oxidative phase?

this Glucose-6-phosphate + 2 NADP+ → ribulose-5-phosphate + 2 NADPH + CO₂

Which of the following are true for Ribulose-5-phosphate

All of the above

Which enzyme catalyzes the rate-controlling step of the pentose phosphate pathway?

Glucose-6-phosphate dehydrogenase

Which of the following best describes the function of glucose-6-phosphate dehydrogenase in the pentose phosphate pathway?

Oxidizes glucose-6-phosphate and reduces NADP⁺ to NADPH

Glucose-6-phosphate dehydrogenase is allosterically stimulated by:

NADP+

True or False: in mammals, PPP occurs exclusively in the cytoplasm.

True

The pentose phosphate pathway is most active in which tissues?

Liver, mammary gland, and adrenal cortex

The primary product formed in Step 1: G-6-P dehydrogenase after the oxidation of glucose-6-phosphate is:

6-phospho-D-gluconolactone

What is the normal ratio of NADPH:NADP⁺ in liver cytosol?

100:1

True or False: Cytosol is a highly reducing environment

True

How do NADPH-utilizing pathways, such as fatty acid synthesis, affect the activity of glucose-6-phosphate dehydrogenase?

They increase NADP⁺ and stimulate the enzyme

This step of the PPP pathway is unstable and spontaneously hydrolyzed, accelerating the opening of the ring.

Glucolactonase

What enzyme step catalyzes the oxidative decarboxylation of 6-phosphogluconate?

Phosphogluconate dehydrogenase

What molecule is produced during phospho-gluconate of the pentose phosphate pathway?

Ribulose-5-phosphate

The non-oxidative phase allows carbon to be redirected toward:

glycolysis or gluconeogensis

This step of the PPP converts ribulose-5-phosphate into ribose-5-phosphate using an enediol intermediate. Exchange of groups between carbons.

Phosphopentose isomerase

This step of the PPP interconverts ribulose-5-phosphate and xylulose-5-phosphate using an enediol intermediate and occurs at C-3. Exchange of groups on a single carbon.

Phosphopentose epimerase