Exam 2 Biochem

Carbohydrates are polyalcohols containing either a ____ or an _____ group.

keto; aldehyde

Monosaccharides

the building blocks of all carbohydrates

Basic structure of a monosaccharide

a chain of carbons with a hydroxyl group at each carbon except one, which forms a carbonyl group

Aldoses

contain an aldehyde group

Ex: glucose and glyceraldehyde

Ketoses

contain a keto group

Ex: fructose and dihydroxyacetone

Triose

3 carbons

Ex: D-glyceraldehyde and dihydroxyacetone

Tetrose

4 carbons

Pentose

5 carbons

Ex: Ribose

Hexose

6 carbons

Ex: Glucose

Heptose

7 carbons

The most important monosaccharide because it serves as the primary metabolic fuel for cellular respiration

D-glucose

Epimers

monosaccharides that differ in the orientation of substituents around one of their asymmetric carbons

D-Mannose

D-Glucose

D-Galactose

Fischer projection structure

Haworth projection

When rings form, carbon 1 of glucose becomes

asymmetric

In glucose, carbon 1 (the aldehyde carbon) is this and in ketoses, the keto carbon (usually carbon 2) is this

anomeric carbon

D and L convention

refers to the orientation of the substituents at the asymmetric carbon farthest from the carbonyl group.

The most prevalent orientation

D-isomers

Isomer diversity

D-glucose, an aldohexose, has 4 asymmetric carbons (C2, C3, C4, C5), allowing for 16 possible optical isomers

D-Mannose is a ____ epimer of glucose

C2

D-Galactose is a ___ epimer of glucose

C4

Hemiacetal

formed from an aldehyde and an alcohol

Hemiketal

formed from a ketone and an alcohol

Pyranose

A 6-membered ring (favored by D-glucose)

Furanose

A 5-membered ring (favored by D-fructose)

Mutarotation

when anomers interconvert spontaneously

Mutarotation is greatly accelerated in the presence of

acids or bases

Upon ring closure, the carbonyl carbon (C1 in aldoses & C2 in ketoses) becomes a new asymmetric center called

anomeric carbon

Monosaccharides combine into larger molecules by forming

glycosidic bonds

Once the glycosidic bond is formed, _____ is no longer possible

mutarotation

Disaccharides

two monosaccharides

Ex: maltose, lactose, sucrose

Oligosaccharides

a few (3-6) monosaccharides

Polysaccharides

many (7+) monosaccharides

Ex: amylose, glycogen, cellulose

Glycoproteins

contain carbohydrate covalently bound to amino acid side chains

Glycolipids

carbohydrate is covalently bound to a lipid

O-glycosidic

if the sugar binds its partner through an oxygen atom

N-glycosidic

if the sugar binds its partner through a nitrogen atom

Carbohydrate behavior is determined by

size and functional groups

____ are highly water-soluble because they can form multiple hydrogen bonds with water

monosaccharides, disaccharides, and oligosaccharides

Many polysaccharides are insoluble because

their large size increases opportunities for intermolecular interactions

The ___ group of a monosaccharide has reducing properties (it can donate electrons)

free carbonyl

A sugar is a reducing sugar only if it has a ___ not involved in a glycosidic bond.

free anomeric carbon

____ is a notable non-reducing sugar because both of its anomeric carbons are used in the glycosidic bond

sucrose

Which of the following is considered the simplest aldose?

D-Glyceraldehyde

How many asymmetric carbons are found in the open-chain form of D-glucose?

4

What is the primary difference between D-glucose and D-galactose?

They are C-4 epimers

The process by which 𝛼-D-glucose and 𝝱-D-glucose interconvert in water is called: 

Mutarotation

Why is sucrose classified as a non-reducing sugar?

Both anomeric carbons are involved in the glycosidic bond

Which polysaccharide serves as the most abundant biomolecule on earth and contains 𝝱(1-4) glycosidic bonds?

Cellulose

---

serves as the primary transported carbohydrate in the human body

Glucose

Glucose uptake from the blood and extracellular fluids occur via

facilitated diffusion

Facilitated diffusion is mediated by

the GLUT family of carriers

Energy requirement for facilitated diffusion

no energy required

Facilitated diffusion driving force

high concentrations of glucose in the blood and extracellular fluids naturally drive glucose into the cells through transporter proteins

For the generation of a metabolic energy, all major nutrients are degraded to

acetyl-CoA

Where is GLUT 1 expressed?

most tissues

Where is GLUT 2 expressed?

liver, intestine, pancreatic B cells

Where is GLUT 3 expressed?

brain

Where is GLUT 4 expressed?

muscle, adipose tissue, heart

Where is GLUT 5 expressed?

intestine

What is the function of GLUT 1?

Basal glucose uptake

What is the function of GLUT 2?

High-capacity glucose uptake

What is the function of GLUT 3?

Neuronal glucose uptake

What is the function of GLUT 4?

Insulin-dependent glucose uptake

What is the function of GLUT 5?

fructose transport

____ is the only insulin-dependent transporter

GLUT 4

Post-Prandial Response

after a carbohydrate-rich meal, insulin levels rise. insulin binds to receptors on the surface of muscle and adipose tissue

Signaling cascade

1. insulin binding: insulin attaches the the insulin receptor

2. autophosphorylation: receptor undergoes tyrosine-phosphorylation

3. Akt signaling: a signaling cascade (involving Akt) is triggered

4. vesicle fusion: carrier-containing vesicles fuse with the plasma membrane, increasing the number of active GLT4 transporters

Metabolic shift

increases the Vmax of glucose transport. During fasting, when insulin is low, these tissues minimize glucose uptake to ensure the limited supply is redirected to glucose-dependent tissues like the brain and erythrocytes.

Which glucose transporter is specifically responsible for high-capacity uptake in the liver and pancreatic 𝝱-cells?

GLUT 2

Why is the transport of glucose into most cells considered “cost-free” in terms of energy?

There is a high concentration of glucose (100 mg/dL) maintained in the blood

In the absence of insulin, where are the majority of GLUT 4 transporters located in a muscle cell?

Within the membranes of intracellular vesicles

Which of the following is the first step in the insulin signaling cascade that leads to increased glucose uptake?

Insulin receptor autophosphorylation

During fasting, which tissues reduce their glucose uptake to preserve the supple for the brain?

Muscle and adipose tissue

Some individuals are born with a partial deficiency of pyruvate dehydrogenase in all tissues. What tissue suffers most from this abnormality?

Brain

Anaplerotic reaction

a reaction that “fills up” or replenishes metabolic intermediates, such as those in the TCA cycle

Glycogen

the stored form of carbohydrate in the human body

Heteroplasmy

the presence of more than one type of organellar genome (mitochondrial DNA) within a single cell or individual.

Insulin

a hormone produced by the pancreas that regulates the amount of glucose in the blood by promoting its uptake into skeletal muscle and adipose tissue.

Vmax

the maximum velocity of an enzymatic or transport reaction when the system is saturated with substrate.

____ is the cytoplasmic catabolic pathway that converts one molecule of glucose (6 carbons) into two molecules of pyruvate (3 carbons). All cells in the human body are capable of performing this process

glycolysis

Glycolysis Step 1:

Intermediate: Glucose → Glucose-6-Phosphate

Enzyme: Hexokinase

Type: Irreversible (ATP-dependent)

Glycolysis Step 2:

Intermediate: Glucose-6-Phosphate → Fructose-6-Phosphate

Enzyme: Phosphoglucose Isomerase

Type: Reversible

Glycolysis Step 3:

Intermediate: Fructose-6-Phosphate → Fructose-1,6-Bisphosphate

Enzyme: Phosphofructokinase-1

Type: Irreversible (committed step)

Glycolysis Step 4:

Intermediate: Fructose-1,6-Bisphosphate → DHAP + G3P

Enzyme: Aldolase

Type: Reversible (Cleavage)

Glycolysis Step 5:

Intermediate: DHAP ⇌ Glyceraldehyde-3-Phosphate (G3P)

Enzyme: Triose Phosphate Isomerase

Type: Reversible

Glycolysis Step 6:

Intermediate: G3P → 1,3-Bisphosphoglycerate

Enzyme: G3P Dehydrogenase

Type: Reversible (Yields NADH)

Glycolysis Step 7:

Intermediate: 1,3-Bisphosphoglycerate → 3-Phosphoglycerate

Enzyme: Phosphoglycerate Kinase

Type: Reversible (Yields ATP)

Glycolysis Step 8:

Intermediate: 3-Phosphoglycerate → 2-Phosphoglycerate

Enzyme: Phosphoglycerate Mutase

Type: Reversible

Glycolysis Step 9:

Intermediate: 2-Phosphoglycerate → Phosphoenolpyruvate (PEP)

Enzyme: Enolase

Type: Reversible

Glycolysis Step 10:

Intermediate: PEP → Pyruvate

Enzyme: Pyruvate Kinase

Type: Irreversible (Yields ATP)

Hexokinase

the first step, which traps glucose in the cell as glucose-6-phosphate. It is inhibited by its product (glucose-6-phosphate) in most tissues

PFK-1

the "committed step." Once fructose-1,6-bisphosphate is formed, the molecule is destined for glycolysis. It is the most important regulated enzyme, inhibited by high energy (ATP), citrate, and low pH, while being stimulated by AMP, ADP, and insulin

Pyruvate Kinase

the final step, inhibited by ATP in the liver

Glycolysis is divided into two distinct functional phases:

 the preparatory (investment) phase and the payoff (generation) phase

Energy Investment (Preparatory) Phase

involves the first three steps of the pathway, where the cell "spends" energy to prime the glucose molecule for cleavage.

• ATP Consumption: Two molecules of ATP are used (one by Hexokinase and one by PFK-1).

• Purpose: Phosphorylation prevents glucose from leaving the cell and creates a symmetric molecule (fructose-1,6-bisphosphate) that can be cleaved into two three-carbon sugars.

Energy Generation (Payoff) Phase

occurs after the six-carbon fructose is split into two three-carbon molecules (G3P). Because there are now two molecules moving through the pathway, all products are doubled.

• ATP Production: Four molecules of ATP are produced via substrate-level phosphorylation (two by Phosphoglycerate Kinase and two by Pyruvate Kinase).

• NADH Production: Two molecules of NADH are produced by G3P Dehydrogenase.

• Net Yield:

  • 2 ATP (4 generated minus 2 invested).

  • 2 NADH (In aerobic conditions, these proceed to the respiratory chain).

  • 2 Pyruvate.