BIOLOGICAL MOLECULES

When [S] is saturating (>> Km), all enzyme active sites are occupied, and the reaction rate is solely a function of how fast the enzyme can process substrate (Vmax). Vmax is proportional to the total enzyme concentration, so doubling the enzyme doubles V0 under these conditions.

Metabolism is the sum of all cellular reactions. Catabolism is the breakdown of complex molecules into simpler ones, releasing energy. Anabolism is the synthesis of complex molecules from simpler ones, consuming energy. Their regulation is central to life.

Active transport is the movement of molecules against a concentration gradient. This process is directly coupled to ATP hydrolysis as an energy source, for example, by the Na⁺/K⁺ pump. Passive processes like diffusion and facilitated diffusion are driven by the gradient itself and do not require ATP directly.

A phospholipid is a substituted triglyceride. Lecithin (phosphatidylcholine) consists of glycerol esterified to two fatty acids and a phosphate group, which is in turn esterified to the nitrogenous alcohol choline. Complete hydrolysis breaks all these ester bonds.

Linoleic acid (omega-6) and α-linolenic acid (omega-3) are essential fatty acids. Humans and other mammals lack the enzymes (Δ12 and Δ15 desaturases) to insert double bonds at the required positions in the fatty acid chain. They must be ingested in the diet to maintain health.

The conformational change in the induced fit model positions the essential catalytic amino acid side chains in the precise orientation needed to perform chemistry on the substrate. This is in addition to the strain and proximity effects also associated with the model.

The enzymes responsible for the synthesis of phospholipids (the major membrane lipid) and steroids (including cholesterol and steroid hormones) are located primarily in the membrane of the smooth ER. It is therefore a major site of lipid biosynthesis.

Water's dipole nature (O is δ-, H is δ+) allows it to form hydrogen bonds with and dissolve other polar and charged molecules. It forms hydration shells around these molecules (like sugars and amino acids), effectively separating them from their crystal lattice and bringing them into solution.

Heavy metals have high affinity for sulfur. They react with the thiol (-SH) groups of cysteine residues, forming mercaptides. This can block essential catalytic groups, disrupt disulfide bonds (if present), and severely distort the protein's tertiary and quaternary structure, leading to irreversible denaturation.

Protein folding is a cooperative process. The breaking of a few weak interactions in one region of the protein during heating can destabilize neighboring interactions. This leads to a rapid, domino-like collapse of the entire tertiary structure over a very small temperature range.

Condensation (dehydration synthesis) is the universal anabolic reaction for building all major biological macromolecules: monosaccharides to polysaccharides, amino acids to proteins, and nucleotides to nucleic acids. Water is the byproduct of each new bond formed.

Buffers are aqueous systems that resist changes in pH. The bicarbonate system (H2CO3/HCO3⁻) neutralizes small amounts of added acid or base, keeping the blood pH within the narrow physiological range (7.35-7.45) essential for enzyme function and protein stability.

The active site catalytic residues often depend on specific ionization states to function. At the optimum pH, these residues have the correct charge (+ or -) for substrate binding or catalysis. Deviation from this pH alters the ionization, disrupting the interactions and decreasing activity.

Zymogens like pepsinogen, trypsinogen, and chymotrypsinogen are inactive precursors of powerful proteases. They are activated by cleavage only after reaching the gut lumen. This prevents them from hydrolyzing the proteins of the cells that produce them, which would lead to tissue destruction.

Secondary structures are defined by the pattern of hydrogen bonds between the carbonyl oxygen (C=O) and the amide hydrogen (N-H) of the peptide backbone itself. The R-groups are not involved; their interactions define the higher-level tertiary structure.

Uracil is a pyrimidine base found in RNA. Like thymine (its counterpart in DNA), its structure is complementary to adenine, and it forms two hydrogen bonds with adenine during base pairing. Guanine pairs with cytosine.

While all levels contribute, the precise 3D shape of an antigen-binding pocket is a feature of the protein's tertiary structure. It is formed by the folding and precise juxtaposition of R-groups from different parts of a single polypeptide chain (in heavy and light chains).

Adipose tissue, rich in triglycerides, serves as a padding that protects vital organs from physical shock. It also functions as a thermal insulator in subdermal layers, reducing heat loss from the body. Energy storage is its primary role, but the options highlight these secondary roles.

The amino acid is covalently attached to the 3' acceptor stem of its cognate tRNA molecule. During elongation, the peptidyl transferase center of the ribosome catalyzes the nucleophilic attack of the amino group of the incoming aminoacyl-tRNA on the ester bond of the peptidyl-tRNA.