BIOCHEM501: Exam 4

hormones

small molecules/proteins that connect all organs in body, released into the bloodstream, and act through specific receptors to alter cellular activities

signal, reception, transduction, response

Principles of signal transduction

glucagon

released from pancreas when blood glucose is low (fasting state)

signaling results in glycogen breakdown in liver cells

binds on liver/fat cells

epinephrine

released from adrenal glands during activity (excercise/stress)

signals to breakdown glycogen in muscle/liver and breakdown triacylglycerol in fat cells

binds on liver, fat, and muscle cells

insulin

released from pancreas when blood glucose is high (fed state)

binds fat, liver, and muscle cells

leptin

released from fat cells after a meal

binds receptors in the brain to signal to stop eating (suppress appetite)

fed state

after eating a meal

during feeding, body decreases glucose by transporting it to cells

pathways include glycogenesis, glycolysis, fatty acid synthesis, cholesterol synthesis, pentose phosphate pathway

fasting state

body releases/produces stored forms of fuel to be used

pathways include glycogenolysis, gluconeogenesis, lipolysis, and ketogenesis

carbohydrates

aldehydes/ketones with at least 2 hydroxyl groups, or substances that yield such compounds on hydrolysis

empirical formula = (CH2O)n

monosaccharides

simple sugars, consist of a polyhydroxy aldehyde or ketone unit
example: D-glucose

disaccharides

two monosaccharide units joined together by glycosidic bond

example: sucrose (D-glucose and D-fructose)

oligosaccharides

short chains of monosaccharide units, or residues, joined by glycosidic bonds

polysaccharides

sugar polymers with 10+ monosaccharide units
examples: cellulose (linear), glycogen (branched)

anomer

isomers that differ at a new asymmetric C atom formed on ring closure

alpha form anomer

hydroxyl at anomeric C is on opposite face of ring relative to exocyclic C

beta form anomer

hydroxyl at anomeric C is on same face of ring relative to exocyclic C

glycosidic bond

covalent linkage of 2 monosaccharides

O glycosidic bond

formed when the hydroxyl group of one sugar molecule reacts with the anomeric C of another

glycogen

polysaccharide, animal starch

storage form of glucose in animal cells

mostly lined by alpha 1,4 glycosidic bonds that form compact hollow cylinders

branches formed by alpha 1,6 glycosidic bonds

amylose

glucose storage in plants

cellulose

structural component of plants that is insoluble

homopolymer glucose units with beta 1,4 glycosidic bonds

straight chain capable of reacting with other cellulose molecules to form fibrils that exclude H2O

reducing sugar

sugars that react with oxidizing agents

open chain form is reactive

end of chain with free anomeric C involved

glucose drops below normal

• pancreas releases glucagon

• liver binds glucagon to release glucose

• adipose tissue binds glucagon for fatty acid release

What physiological changes occur during the fasting state?

• glucagon = hormone

• glycogen = polymer of glucose

What is the difference between glucagon and glycogen?

glycogen

breakdown of this polymer in liver replenishes blood glucose levels

breakdown of this polymer in muscle provides energy for muscle contraction

• NOT released in response to decrease of blood glucose levels

glycogenolysis

breakdown of cellular glycogen to glucose 6 phosphate

1. Release of glucose-1-phosphate from glycogen by breaking alpha 1,4 bond

• phosphorylysis reaction

2. Debranching enzyme transfers branches onto main chains AND releases residue at alpha 1,6 branch as free glucose

3. Rapid release of glucose 1-phosphate from glycogen favors isomerization to release glucose 6-phophate

• reversible reaction, highly dependent on concentration

What are the steps of glycogenolysis?

phosphorylysis

chemical reaction that involves the breaking of a bond between 2 parts of molecule with the addition of a phosphate group

analogous to hydrolysis reaction

• muscle - enters glycolysis to supply ATP to cells

• liver - phosphate removed so free glucose can be transported out to replenish blood glucose

What happens to the glucose-6-P from glycogen breakdown?

• Protein Kinase A catalyzes glycogen breakdown AND gluconeogenesis

• Allows for very quick activation because protein already synthesized

Why is the glucagon signaling pathway so complex?

1. Glucagon binds the receptor

2. GTP stimulates adenylate cyclase to convert ATP to cAMP

3. cAMP binds regulatory subunits of PKA

4. PKA → phosphorylase kinase → glycogen phosphorylase → glucose molecule

Steps of glucagon signaling

G protein coupled receptor (GPCR)

contains the 7 transmembrane receptor that binds glucagon and heterotrimeric G protein

heterotrimeric G protein

conserved family of signaling protein with 3 subunits (alpha, beta, and gamma)

alpha subunit is binding site for GDP/GTP

kinase

enzyme that modifies a substrate by phosphoryl group transfer from a nucleosidetriphosphate

typically phosphorylated on hydroxyl groups (i.e. serine, threonine, tyrosine)

often post translationally modified

phosphorylase

enzyme that catalyzes phosphorylysis rxn (which utilizes an inorganic phosphate)

1. Receptor interaction is reversible

2. Ga has an inherent GTPase activity that cleaves the bound GTP to GDP

3. cAMP phosphodiesterase converts cAMP to AMP, which stops the activation of PKA

How is the glucagon signaling pathway shut down?

lipolysis

glucagon signaling activates PKA

PKA activates lipase via phosphorylation

active lipase catalyzes the hydrolysis of triacylglycerol to free fatty acids

fatty acids transported to other tissues provide energy via beta oxidation and aerobic respiration

3 net; first step skipped

How much ATP does glycolysis generate in the muscle?

in reverse, steps 1, 3, and 10 are HIGHLY endergonic

What barrier prevents glycolysis from running in reverse to synthesize glucose?

gluconeogenesis

pathway that converts pyruvate and related 3/4 C compounds to glucose

occurs in all animals, plants, fungi, and microorganisms

mainly in the liver

can start with reactants such as lactate, glucogenic amino acids, or glycerol

lactate, glucogenic amino acids, glycerol

What are three possible starting reactants for gluconeogenesis?

Cori cycle

1. lactate produced in muscle during aerobic respiration

2. liver cells remove lactate from blood and convert into glucose

3. liver synthesized glucose can be returned to muscles to be used again/converted to glycogen

leucine; lysine

Every amino acid, except for ___ and ____ can be glucogenic.

ketogenic amino acids

enter the citric acid cycle as acetyl CoA and exit as ketone bodies

there is no evidence that acetyl CoA can be converted to glucose

glycerol → DHAP

DHAP enters glycolysis at step 4

How does the process of glycolysis change when glycerol is the starting material for gluconeogenesis?

bypass reactions

in gluconeogenesis, these steps replace irreversible glycolytic reactions 

bypass 1a

pyruvate → oxaloacetate via pyruvate carboxylase

carboxylation reaction

occurs inside the mitochondria

oxaloacetate must be converted to malate to exit the mitochondria

generates cytoplasmic NADH (required for the continuation of gluconeogenesis)

bypass 1b

oxaloacetate → phosphoenolpyruvate via PEP carboxylase

occurs outside the mitochondria, in the cytoplasm

GTP is used, CO2 is released

bypass 2/3

hydrolysis reactions (with the removal of a phosphate → dephosphorylation)

facilitated by a phosphatase enzyme

reciprocal regulation

glycolysis predominates when glucose is abundant

gluconeogenesis is active when glucose is scarce

fructose 2,6 biphosphate

main determinant of which pathway (gluconeogenesis or glycolysis) is active

activates PFK-1 and inhibits FBPase1

ketone bodies

become the major fuel source of the brain after several days fasting

ONLY H2O soluble: acetoacetate, beta hydroxybutarate

liver mitochondria

main source of ketone bodies

1. during fasting, CAC slows and fatty acids generate ATP

2. buildup of acetyl CoA stimulates conversion to ketone bodies

• 2 acetyl CoA

• 7 NADH

• 2 FADH2

Each molecule of beta hydroxybutarate results in: 

facilitated diffusion (passive transport)

Glucose transporters, like GLUT1-14, work by ___.

slow

High Kt = (fast/slow) velocity of transport

pancreatic beta cell

pancreatic cell that senses blood glucose and stimulates insulin release

ATP gated K+ channel

in pancreatic beta cell

closes in the presence of increased ATP 

ligand gated

voltage gated Ca2+ channel

in pancreatic beta cell

opens in the presence of increased blood glucose

depolarization event signals it to open

ion channel

provides aqueous path across membrane through which inorganic ions diffuse at high rates

most have a gate

most have some specificity for an ion

RTK

family of plasma membrane receptors with protein (tyrosine) kinase activity

have extracellular ligand binding domain and cytoplasmic tyrosinase kinase domain

have inherent dimer activity

ex: insulin receptor

insulin receptor

RTK in which binding of this hormone activates enzyme via auto phosphorylation

1. Stimulation of movement of glucose transporter to plasma membrane’s surface via vesicles that fuse to membrane

2. Activation of PP1 that activates enzyme in lipid/glycogen synthesis

How does insulin activated kinase signaling cascade evolve?

PP1

activates enzymes in lipid synthesis and glycogen synthesis

inactivates enzymes in glucose production (gluconeogenesis/glycogenolysis)

1. G6P → G1P

2. G1P + UTP → UDP glucose

3. glucose transferred onto glycogen, releasing UDP

Glycogen synthesis mechanism

activated carrier

small molecule that stores energy or chemical groups in form donated to different metabolic reaction

glycogenin

enzyme that starts glycogen synthesis (only involved at the beginning)

adds glucose molecules to self

glycogen synthase

in glycogen synthesis, catalyzes synthesis of glycogen chains

only synthesizes alpha 1,4 linkage

transfers glucose residue of UDP glucose to non-reducing end of glycogen branch

branching enzyme

in glycogen synthesis, catalyzes synthesis of alpha 1,6 linkage forming glycogen branches

removes oligosaccharide of ~7 residues

• synthesis: removing phosphate

• breakdown: removing phosphate

How does PP1 stimulate glycogen synthesis and breakdown?

• allosterically: glucose binding decreases activity

• phosphate binding from PP1 increases activity

How is glycogen phosphorylase regulated?

PPP

takes place in cytosol

oxidative and non oxidative reactions

oxidative PPP

unidirectional

1. generates ribose 5 phosphate (for making nucleotides)

2. generates NADPH, source of biosynthetic reducing power helps in detoxification of O free radical

non oxidative PPP

reversible

1. generate glycolytic intermediates for ATP

2. generate pentose phosphate for nucleotide synthesis

G6P dehydrogenase

catalyzes 1st slow step of oxidative phase PPP (regulated)

oxidation reaction

forms NADP

1. 5C + 5C → 3C + 7C

2. 3C + 7C → 4C + 6C

3. 4C + 5C → 6C +3C

What is the non oxidative phase PPP C count? (3 steps)

glycolysis and non oxidative PPP

R5P needs exceed NADPH: which fate of G6P?

oxidative PPP only

NADPH and R5P needs are balanced: which fate of G6P?

oxidative PPP, non oxidative PPP, and gluconeogenesis

More NADPH needed than R5P: which fate of G6P?

oxidative PPP, non oxidative PPP, and glycolysis

NADPH and ATP both required: which fate of G6P?

• inhibited by NADPH

• activated by NAP+

How is G6P dehydrogenase regulated?

G6PD deficiency

in individuals near the equator mostly for malaria resistance

ingestion of certain foods (ex: fava beans) cause overwhelming oxidative stress, lysing red blood cells and causing serious medical problems

NADPH and glutathione

Which two compounds protect cells against reactive O species by reducing them?

glutathione

used to eliminate peroxides (ROS)

reduced form required to prevent against cellular damage

lower GSH (reduced glutathione), and higher numbers of ROS

What are the consequences of lower NADPH levels in a cell?

T1 diabetes

insufficient production of insulin

autoimmune destruction of beta cells that develops in early life

used to be called “juvenile __”

T2 diabetes

insulin resistance (cells don’t respond appropriately to insulin)

develops in late adulthood, associated with obesity

more common than the alternative

• glucose is underutilized

• all energy is derived from fats → forms ketone bodies

  • can lower blood pH and lead to ketoacidosis

Physiologically, what happens when T1 diabetes goes untreated?

diet and exercise to manage blood glucose and reduce obesity

How is T2 diabetes treated?

SGLT2 inhibitor

drug to treat T2 diabetes

inhibits the secondary active transporter with sodium (symporter), which prevents the transport of glucose into the cell

alpha glucosidase inhibitor

drug to treat T2 diabetes

intervenes before glucose transport

inhibits enzymes that break down carbs, which decreases the glucose release from starch

sulfonylurea drugs

drug to treat T2 diabetes

closes K+ channel artificially and stimulates insulin secretion

GLP-1 agonist

drug to treat T2 diabetes

modified, more stable version of a compound that regulates blood glucose by decreasing appetite and slowing digestion

ex: ozempic

acetyl CoA

All lipids are derived from ____.

1. Transfer of acetyl CoA out of mitochondria into cytoplasm as citrate

2. Carboxylase enzyme synthesizes malonyl CoA (irreversible, requires ATP)

3. Receptive addition and reduction of 2C units to synthesize C16 fatty acid, palmitate

3 stages of fatty acid synthesis

ATP, NADPH, citrate

Which high energy molecules slow down energy producing pathways, causing build up of intermediates to be transported out of the cell?

biotin

vitamin cofactor required for the function of carboxylase enzymes

1. Carboxylation of biotin cofactor (CO2 added)

2. Carboxyl group carried by biotin to different active site to be transferred to malonyl CoA

How does the formation of malonyl CoA occur?

(2 substeps of step 2: fatty acid synthesis)

fatty acid synthase

catalyzes the assembly of fatty acids in cytosol by repeating 4 step sequence that elongates fatty acyl chain by 2C each cycle

1. Condensation - releases CO2, combines 2 groups

2. Reduction - carbonyl group reduced

3. Dehydration - O removed, resulting in alkene

4. Reduction - double bond reduced

How does the addition of 2C units to fatty acid occur?
(4 substeps of step 3: fatty acid synthesis)