Biochemistry Tests with Answers: 50 Questions for Exam Preparation

Hydrophobic amino acid residues in the globin pocket shield the heme iron from oxidation by water in aqueous solution.

Denaturation disrupts the weak bonds stabilizing the secondary, tertiary, and quaternary structures, whereas the strong covalent peptide bonds of the primary structure remain intact.

At the isoelectric point, the number of positive and negative charges on the amphoteric protein molecule balance out, making its net charge equal to zero.

Adult human hemoglobin (HbA) is a tetrameric protein consisting of two α- and two β-polypeptide chains, each bound to a heme group.

Acinar cells of the exocrine pancreas synthesize and secrete the inactive enzyme precursor, trypsinogen, which is subsequently activated in the lumen of the duodenum.

Folic acid is essential for the synthesis of purines and thymidylate (dTMP), supporting the division of rapidly proliferating hematopoietic cells.

According to the Michaelis–Menten equation, the Michaelis constant is numerically equal to the substrate concentration at which the reaction rate is half of the maximum rate.

Pepsin is the main proteolytic enzyme of gastric juice, adapted to function in a strongly acidic environment.

Phosphoprotein phosphatases hydrolytically cleave the phosphoric acid residue from modified amino acid residues of proteins, returning them to a dephosphorylated state.

Specific activity reflects the degree of enzyme purification and is calculated as the ratio of total enzyme activity to the total mass of protein in the preparation (in milligrams).

The insulin receptor belongs to the receptor tyrosine kinase family and autophosphorylates tyrosine residues upon hormone binding.

Gq protein-coupled plasma membrane receptors and IP3 receptors on the endoplasmic reticulum span the lipid bilayer, making them integral proteins.

Calcitriol (the active form of vitamin D) is a steroid hormone that crosses the cell membrane and interacts with intracellular receptors, regulating target gene transcription.

Phosphodiesterase hydrolyzes the cyclic 3',5'-phosphodiester bond in the cAMP molecule to form inactive 5'-AMP, terminating intracellular signal transduction.

Membrane lipids create a specific microenvironment required for the proper conformation and function of membrane-bound enzymes, and some of them act as secondary messengers.

Being a lipophilic weak acid, 2,4-dinitrophenol transports protons across the mitochondrial lipid bilayer, disrupting the proton gradient.

During the reduction of FAD, electrons and protons attach to the nitrogen atoms at positions N-1 and N-5 of the isoalloxazine ring.

ETC stands for electron transport chain (the mitochondrial respiratory chain).

ADP acts as an allosteric activator of key regulatory enzymes of the TCA cycle, including citrate synthase, signaling the cell's need to replenish ATP stores.

The rate of cellular respiration is controlled by the mechanism of respiratory control, which directly depends on the ATP/ADP ratio reflecting the current cellular energy demand.

Chylomicrons are assembled in the mucosa of the small intestine and deliver exogenous triglycerides to adipose tissue and skeletal muscles.

Microfloral enzymes catalyze 7α-dehydroxylation of primary bile acids: cholic acid is converted to deoxycholic acid, and chenodeoxycholic acid to lithocholic acid.

Acetyl-CoA cannot directly cross the mitochondrial membrane, so it is transported into the cytoplasm as citrate (via the citrate shuttle).

Lecithin-cholesterol acyltransferase (LCAT) catalyzes the transfer of a fatty acid residue from phosphatidylcholine (lecithin) to free cholesterol, forming a cholesterol ester and lysophosphatidylcholine.

By accepting lipids (including triacylglycerols from VLDL in exchange for cholesteryl esters), small dense HDL₃ particles increase in size, become enriched in lipids, and are converted into larger and lighter HDL₂.

Phenylalanine hydroxylase uses tetrahydrobiopterin as an electron donor for the incorporation of an oxygen atom into the aromatic ring.

Sulfonamides competitively inhibit bacterial dihydropteroate synthase; therefore, an excess of the substrate (PABA) displaces the drug from the active site of the enzyme.

The main pathway of histidine degradation via the formation of urocanic acid involving histidase and urocanase takes place in liver hepatocytes.

The bulk of ammonia formed during amino acid catabolism is detoxified in the liver via urea synthesis, while a portion is excreted by the kidneys as ammonium salts to maintain acid-base balance.

Angina pectoris is characterized by reversible myocardial ischemia without cardiomyocyte necrosis; therefore, intracellular enzymes (AST and ALT) are not released into the blood. In infarction, hepatitis, and muscle injury, cell destruction leads to hyperenzymemia.

OMP (orotidine-5'-monophosphate) is the immediate precursor of uridine monophosphate in the de novo pyrimidine nucleotide synthesis pathway.

The enzyme thioredoxin reductase uses reducing equivalents from NADPH to reduce the disulfide bonds of oxidized thioredoxin.

IMP dehydrogenase catalyzes the oxidation of inosine monophosphate to form xanthosine monophosphate (XMP) in the pathway of guanylic nucleotide synthesis.

Glutamine-PRPP amidotransferase transfers the amide group of glutamine to 5-phosphoribosyl-1-pyrophosphate, releasing inorganic pyrophosphate and forming 5-phosphoribosyl-1-amine and glutamate.

UMP synthase is a bifunctional enzyme possessing orotate phosphoribosyltransferase and OMP decarboxylase activities, sequentially catalyzing the formation of orotidine monophosphate (OMP) and its decarboxylation to uridine monophosphate (UMP).

Phenobarbital is a classical inducer of microsomal oxidation and conjugation enzymes in the liver, including UDP-glucuronosyltransferase.

The enzyme porphobilinogen synthase (ALA dehydratase) catalyzes the condensation of two molecules of 5-aminolevulinic acid to form one molecule of the monopyrrole porphobilinogen.

Blood drains from the intestine via the portal venous system, through which absorbed substances, including urobilinogen, return to the liver.

5-Aminolevulinic acid is synthesized in the mitochondrial matrix, then exits into the cytosol, where cytoplasmic ALA dehydratase (porphobilinogen synthase) condenses its two molecules into porphobilinogen.

In hepatocytes, the enzyme UDP-glucuronosyltransferase attaches glucuronic acid residues to toxic unconjugated bilirubin, forming a water-soluble and less toxic conjugated (direct) bilirubin.

Elastin molecules lack ordered, rigid, regular structures and, due to their disordered regions, are able to easily stretch and return to their initial state.

Pyridinoline cross-links form mature collagen cross-links, and their excretion increases during intensive degradation of connective tissue and bone matrix in collagenoses and oncological diseases.

Due to their high hydrophilicity and ability to bind large amounts of water, proteoglycans provide elasticity and cushioning function to cartilage tissue.

The abbreviation GAG refers to glycosaminoglycans — long unbranched polysaccharide chains of repeating disaccharide units that are part of the proteoglycans of connective tissue.

Tropocollagen molecules are arranged in parallel and displaced relative to each other by approximately a quarter of their length (about 67 nm), which creates the characteristic cross-striation of collagen fibrils.

Aflatoxins are produced by microscopic fungi of the genus Aspergillus and have pronounced hepatotoxic and carcinogenic effects.

The transfer of a glucuronic acid residue from UDP-glucuronate to the substance being detoxified is carried out by the liver microsomal enzyme UDP-glucuronyltransferase.

The transfer of a sulfate group from the active form of sulfate (PAPS) to xenobiotics and endogenous toxic metabolites is carried out by sulfotransferases.

The insertion of a single oxygen atom into a hydrophobic molecule to form an alcohol or phenolic group represents a microsomal hydroxylation reaction in phase I of xenobiotic biotransformation.

The cytochrome P450 system is monooxygenase and uses NADPH as an electron donor required for the activation of molecular oxygen and substrate hydroxylation.