Comprehensive Biochemistry Exam Study Notes

Enzyme Inhibition and Kinetic Parameters

Enzyme inhibition can be categorized into three primary types based on the inhibitor's binding site and its effect on kinetic parameters such as $K_m$ and $V_{max}$.

Competitive inhibition occurs when an inhibitor competes with the substrate for the active site. This results in an increase in the Michaelis constant ($K_m$), as more substrate is required to achieve half-maximal velocity, while the maximum velocity ($V_{max}$) remains unchanged.

Non-competitive inhibition involves the inhibitor binding to an allosteric site rather than the active site. In this case, the inhibitor does not compete with the substrate. The $K_m$ remains unchanged because the affinity for the substrate is not directly affected at the active site, but the $V_{max}$ decreases because the enzyme's catalytic efficiency is reduced.

Uncompetitive inhibition is characterized by the inhibitor binding only to the enzyme-substrate (ES) complex after the substrate has already bound. This leads to a decrease in both $K_m$ and $V_{max}$.

Carbohydrates: Classification, Structure, and Linkages

Carbohydrates are classified into several categories based on their functional groups and size. Aldoses, such as glucose and ribose, contain an aldehyde group, while ketoses, such as fructose and dihydroxyacetone, contain a ketone group.

Monosaccharides are simple sugars like glucose and fructose. Disaccharides consist of two sugars, such as maltose, lactose, and sucrose. Polysaccharides are complex chains, including starch, glycogen, and cellulose. In ring structures, carbohydrates are classified as pyranose (a $6$-membered ring, like glucose) or furanose (a $5$-membered ring, like fructose).

The orientation of the hydroxyl group on the anomeric carbon (the carbon formed after ring closure) determines the anomer. In the $\alpha$-anomer, the $OH$ group is below the ring, while in the $\beta$-anomer, the $OH$ group is above the ring (e.g., $\alpha$-D-glucose and $\beta$-D-glucose). Stereochemistry is further defined by the position of the hydroxyl group on the penultimate carbon in a Fischer projection: the D-form (Dextro) has the $OH$ group on the right, and the L-form (Lexo) has the $OH$ group on the left.

Glycosidic linkages differ between storage and structural polysaccharides. Starch is a storage polysaccharide found in plants that uses $\alpha(1\rightarrow 4)$ and $\alpha(1\rightarrow 6)$ linkages; it is digestible by humans. Glycogen is a similar storage molecule using $\alpha(1\rightarrow 4)$ and $\alpha(1\rightarrow 6)$ linkages. Cellulose is a structural polysaccharide found in plant cell walls that utilizes $\beta(1\rightarrow 4)$ linkages, making it indigestible by humans.

Lipid Biochemistry: Functions, Structures, and Nomenclature

Lipids serve several critical roles in biology, including energy storage, cell membrane formation, and acting as signaling molecules. Cholesterol, a key lipid, maintains membrane fluidity and serves as a precursor for steroid hormones, vitamin D, and bile salts.

Fatty acids are categorized as saturated or unsaturated. Saturated fatty acids have no double bonds, possess straight chains, and have high melting points, appearing as solid fats. Examples include palmatic acid ($16:0$) and stearic acid ($18:0$). Unsaturated fatty acids contain one or more double bonds (bent chains), have lower melting points, and appear as liquid fats. Examples include oleic acid ($18:1$, having $18$ carbons and $1$ double bond), linoleic acid ($18:2$), and linolenic acid ($18:3$).

Lipids can also be classified by polarity. Non-polar lipids, such as tricylgylcerols (fats and oils), are entirely hydrophobic. Polar lipids, such as phospholipids, are amphipathic, containing both hydrophilic and hydrophobic regions.

Membrane Dynamics and Transport Mechanisms

The fluid mosaic model describes the cell membrane as a phospholipid bilayer where hydrophilic heads face outward and hydrophobic tails face inward. Proteins float within this bilayer, creating a fluid and dynamic structure that also includes cholesterol and carbohydrates.

Transport across these membranes occurs via different mechanisms. Passive transport requires no $ATP$, as molecules move down their concentration gradient through simple diffusion or facilitated diffusion. Active transport requires energy. The sodium-potassium dump (pump) uses $ATP$ to move $3\,Na^+$ ions out of the cell and $2\,K^+$ ions into the cell per cycle. This process is essential for maintaining membrane potential, facilitating nerve impulses, and supporting muscle concentration.

Cellular Metabolism: Glycolysis and the Citric Acid Cycle

Metabolism involves anabolic pathways, which build complex molecules and require energy (e.g., protein synthesis), and catabolic pathways, which break down molecules to release energy (e.g., glycolysis).

Glycolysis occurs in the cytoplasm, where glucose is converted into $2$ pyruvate. This process generates a gross total of $4\,ATP$, but since $2\,ATP$ are used, the net yield is $2\,ATP$. Additionally, $2\,NADH$ are produced. Important enzymes in this pathway include nexokinase (converting glucose to glucose-$6$-phosphate), PFK-1 (converting fructose-$6$-phosphate to fructose-$1,6$-bisphosphate), and pyruvate kinase (converting $PEP$ to pyruvate).

Under aerobic conditions (oxygen required), pyruvate is converted to acetyl-CoA, and the cell produces $30\text{--}32\,ATP$. The citric acid cycle takes place in the mitochondrial matrix. For every acetyl-CoA, the cycle produces $3\,NADH$, $1\,FADH_2$, and $1\,GTP (ATP)$. Isocitrate dehydrogenase is the rate-limiting enzyme of this cycle. Under anaerobic conditions (no oxygen), pyruvate is converted to lactate by the enzyme lactate dehydrogenase using the reaction: $Pyruvate + NADH \rightarrow lactate + NAD^+$. This pathway yields only $2\,ATP$.

In the electron transport chain, $NADH$ generates approximately $2.5\,ATP$, and $FADH_2$ generates approximately $1.5\,ATP$. ATP synthase is the enzyme responsible for producing $ATP$ using the proton gradient.

Enzyme Structure and the Mechanism of Activation Energy Reduction

Enzymes are biological catalysts made mostly of proteins. Their structure is organized hierarchically. The primary structure is the sequence of amino acids linked by peptide bonds. The secondary structure consists of $\alpha$-helices and $\beta$-pleated sheets formed by hydrogen bonds. The tertiary structure is the three-dimensional folding of a single polypeptide chain, while the quaternary structure involves the association of two or more polypeptide subunits. The active site is the specific region where the substrate binds and catalysis occurs.

Enzymes increase the reaction rate by lowering the activation energy ($E_a$). They achieve this by bringing substrates close together, correctly orienting substrates for the reaction, stabilizing the transition state, providing an alternative reaction pathway, and forming temporary enzyme-substrate complexes. Ultimately, enzymes speed up reactions without being consumed and without changing the overall free energy of the reaction.

Quantitative Analysis of Enzyme Inhibition and Lineweaver-Burk Plots

Kinetic values can be determined from the intercepts of a Lineweaver-Burk plot. The $V_{max}$ is the reciprocal of the $y$-intercept ($V_{max} = 1/y\text{-}intercept$), and the $K_m$ is calculated using the $x$-intercept ($K_m = -1/x\text{-}intercept$).

In a provided example where the $y$-intercept is $0.05$ and the $x$-intercept is $-0.2$: $V_{max} = \frac{1}{0.05} = 20\,units$ $K_m = \frac{-1}{-0.2} = 5\,mM$

Comparative Metabolism and Bioenergetic ATP Calculations

The total $ATP$ yield for the complete oxidation of one glucose molecule is calculated based on the following equivalents: $1\,NADH = 2.5\,ATP$, $1\,FADH_2 = 1.5\,ATP$, and $1\,GTP = 1\,ATP$.

1. Glycolysis: Yields $2\,ATP$ ($2 \times 1 = 2\,ATP$) and $2\,NADH$ ($2 \times 2.5 = 5\,ATP$).

2. Pyruvate Oxidation: Yields $2\,NADH$ ($2 \times 2.5 = 5\,ATP$).

3. Citric Acid Cycle: Yields $2\,ATP/GTP$ ($2 \times 1 = 2\,ATP$), $6\,NADH$ ($6 \times 2.5 = 15\,ATP$), and $2\,FADH_2$ ($2 \times 1.5 = 3\,ATP$).

Total $ATP$ calculation: $2 + 5 + 5 + 2 + 15 + 3 = 32\,ATP$.

Aerobic respiration involves complete oxidation of glucose and occurs in the mitochondria, whereas anaerobic respiration involves partial oxidation, occurs in the cytoplasm, and produces lactate. Aerobic respiration is significantly more efficient than anaerobic respiration.