Biochem Exam 3 Big Concepts

Why can tiny differences in carbohydrate structure have major biological consequences?

• Carbohydrates can have the same formula but different 3D arrangements

• A small change in OH orientation changes their shape.

• Enzymes, receptors, and transport proteins recognize specific shapes, so they may interact differently with each sugar.

How are monosaccharides classified?

• By their carbonyl group and number of carbons.

• Aldoses contain an aldehyde; ketoses contain a ketone.

• The carbon count gives names such as triose (3), pentose (5), and hexose (6).

• Example: glucose is an aldohexose.

What makes a sugar chiral, and how many stereoisomers can it have?

• A chiral carbon is attached to four different groups.

• Different arrangements create stereoisomers.

• If a molecule has n chiral centers, its maximum number of stereoisomers is 2ⁿ. Glucose has 4, so 16 are possible.

How do you determine whether a monosaccharide is D or L?

• Use its Fischer projection.

• Put the carbonyl group near the top, then find the chiral carbon farthest from it.

• OH on the right = D; OH on the left = L.

What are epimers, and how do they differ from other stereoisomers?

• differ in configuration at exactly ONE chiral carbon.

• All other chiral centers have the same configuration. For example, glucose and galactose differ at carbon 4.

How do monosaccharides form cyclic structures?

• In solution, a sugar can exist in an open-chain or cyclic form.

• Its carbonyl group reacts with an OH group within the same molecule, forming a ring.

• The carbonyl carbon becomes a new chiral center called the anomeric carbon.

What are anomers, and how do alpha and beta glucose differ?

• Anomers are a special type of epimer that differ at the anomeric carbon.

• In D-glucose, this is carbon 1.

• Alpha: its OH is opposite the CH₂OH group.

• Beta: its OH is on the same side.

• They form when glucose cyclizes.

What does a Fischer projection show, and why does it preserve stereochemistry?

• A Fischer projection shows a sugar in its linear form and preserves the arrangement of its chiral centers.

• Horizontal bonds point toward you

• Vertical bonds point away from you

• This lets you determine the sugar's stereochemical configuration, including D or L.

How are Haworth projections and chair conformations different?

• Both represent cyclic sugars.

• A Haworth projection shows the ring as a simplified flat shape, with groups above or below it; it helps identify alpha and beta forms.

• A chair conformation shows the ring's more realistic 3D shape and the spatial positions of its bonds and groups.

How can monosaccharides be chemically modified, and why does this matter?

• Cells can add or remove chemical groups.

• Phosphorylation adds a phosphate group.

• Reduction converts a carbonyl group into an alcohol.

• Removing an OH group is another possible modification.

• These changes give sugars different properties and functions.

What is a glycosidic bond, and how does it form?

• Connects two sugar units or a sugar to another molecule.

• Involves the anomeric carbon of a sugar.

• The configuration at the anomeric carbon becomes fixed when the glycosidic bond forms.

• The linkage can be α or β.

What are disaccharides, and how does the type of glycosidic linkage affect them?

• Disaccharides contain two monosaccharides.

• Different sugars can connect through different glycosidic linkages.

• Lactose = galactose + glucose, β(1→4).

• Sucrose = glucose + fructose, α(1→2)β.

• Changing the linkage changes the carbohydrate's structure and potentially its properties.

What is the difference between reducing and nonreducing sugars? Why does it matter?

• Reducing sugars have a free anomeric carbon.

• Nonreducing sugars do not have a free anomeric carbon available.

• Reducing sugars are more reactive.

• They can be detected experimentally and participate in the Maillard reaction.

• Sucrose is nonreducing because both anomeric carbons are involved in its bond.

What are polysaccharides, and what are their major functions?

• Polysaccharides are long chains of sugar units.

• They can be used for energy storage or structural support.

• Storage examples: starch and glycogen.

• Structural examples: cellulose and chitin.

• Homoglycans contain one type of sugar; heteroglycans contain different types.

What is the difference between amylose and amylopectin?

• Both are components of starch and are made of glucose.

• Amylose is linear and unbranched.

• Amylose uses α(1→4) bonds and can form a helix.

• Amylopectin is branched.

• Amylopectin uses α(1→4) bonds in its main chains and α(1→6) bonds at branch points.

How is glycogen structured, and why is branching important?

• Glycogen stores glucose in animals.

• Main chains use α(1→4) bonds.

• Branches use α(1→6) bonds.

• Branches occur approximately every 12 glucose residues.

• It is more highly branched than amylopectin and forms a compact storage structure.

How does cellulose differ from starch and glycogen?

• Cellulose is structural rather than a glucose-storage molecule.

• It is made of glucose connected by β(1→4) bonds.

• It forms long, straight chains and extended fibers.

• Hydrogen bonds between strands help provide strength and rigidity.

• Most animals cannot digest cellulose because their amylases cannot break β linkages.

What is chitin, and how does it compare to cellulose?

• Both are structural polysaccharides.

• Both have β(1→4) linkages.

• Cellulose is made of glucose.

• Chitin is made of N-acetylglucosamine.

• Both have similar hydrogen-bonding patterns between adjacent strands.

How can different glycosidic linkages create different carbohydrate structures and functions?

• Sugars can have the same basic building blocks but different connections.

• α and β linkages affect the shape of the resulting carbohydrate.

• α linkages are found in storage carbohydrates like starch and glycogen.

• β linkages are found in structural carbohydrates like cellulose and chitin.

• The way sugars connect helps determine their biological role.

Why do starch and cellulose have different functions even though both are made of glucose?

• Starch uses α(1→4) linkages, which allow glucose chains to form storage structures.

• Amylopectin also has α(1→6) branches.

• Cellulose uses β(1→4) linkages, creating straight chains.

• Cellulose chains form hydrogen bonds with neighboring chains, creating strong fibers.

• Their different linkages produce different structures and biological functions.

How are catabolism and anabolism connected to the overall purpose of metabolism?

• Metabolism is the collection of chemical reactions that happen in cells.

• Catabolism breaks down larger molecules into smaller ones and releases energy.

• Anabolism uses energy to build larger molecules.

• Together, they allow cells to break down nutrients, obtain energy, and use that energy to build and maintain cellular components.

Why is metabolism organized into multiple enzyme-catalyzed steps instead of one large reaction?

• Multiple steps allow energy to be released in smaller, manageable amounts.

• Cells can capture some of that energy in molecules like ATP and NADH instead of losing it all at once.

• Each step is controlled by a specific enzyme.

• The steps also create useful intermediates that can enter other metabolic pathways.

How can a metabolic pathway maintain steady-state concentrations while increasing its metabolic flux?

• Steady state means the rate at which an intermediate is produced equals the rate at which it is consumed.

• Its concentration stays relatively constant even though molecules are continuously moving through the pathway.

• Metabolic flux is the rate at which molecules move through a pathway.

• Flux can increase if both production and consumption increase at the same rate. This means the pathway moves faster without changing the intermediate's concentration.

How do cells regulate metabolic pathways, and why might some forms of regulation be faster than others?

Cells regulate pathways in three main ways:

• Substrate availability: controlling how much starting material is available.

• Enzyme abundance: changing how much of an enzyme the cell produces or breaks down. This is generally slower.

• Enzyme activity: changing the activity of enzymes that already exist. This can happen more quickly.

Why can't adding more enzyme make a thermodynamically unfavorable reaction favorable?

• ΔG tells us whether a reaction is thermodynamically favorable under specific conditions.

• Reactant and product concentrations affect the actual ΔG.

• Enzymes speed up reactions by lowering the activation energy.

• However, enzymes cannot change the overall ΔG or make an unfavorable reaction favorable.

How does ATP hydrolysis drive reactions that would otherwise be unfavorable?

• Some cellular reactions require energy and are not favorable on their own.

• ATP hydrolysis releases energy that can be coupled to these reactions.

• This makes the overall combined reaction favorable.

• For example, hexokinase uses ATP to transfer a phosphate group directly to glucose, forming glucose-6-phosphate.

How do cells regulate metabolic pathways, and why might some forms of regulation be faster than others?

• Cells regulate pathways through substrate availability, enzyme abundance, and enzyme activity.

• Changing enzyme activity is usually faster because it modifies enzymes that already exist.

• Changing enzyme abundance takes longer because it requires producing or breaking down enzymes.

How does ATP hydrolysis drive a reaction that would otherwise be unfavorable

• ATP hydrolysis releases free energy.

• Cells couple this favorable reaction to an unfavorable reaction.

• The combined reaction becomes energetically favorable.

• Example: hexokinase uses ATP to transfer a phosphate directly to glucose.