Tamil Nadu HSE Second Year Biochemistry key terms

A simple sugar that cannot be hydrolysed into a smaller sugar unit, containing three to seven carbon atoms; glucose, fructose and ribose are examples, and they serve as the basic building blocks of all larger carbohydrates.

A covalent ether-like linkage formed between the anomeric carbon of one sugar and a hydroxyl group of another, joining monosaccharides into disaccharides and polysaccharides; the alpha or beta configuration of this bond determines whether starch or cellulose results.

A sugar whose anomeric carbon remains free, so it can open to an aldehyde and reduce mild oxidising agents such as Benedict's or Fehling's reagent; glucose, fructose, maltose and lactose are positive, while sucrose is not.

A large polymer of many monosaccharide units joined by glycosidic bonds; storage forms such as starch and glycogen are branched and easily mobilised, while structural forms such as cellulose and chitin are unbranched and resistant to human digestive enzymes.

A polyunsaturated fatty acid that the human body cannot synthesise because it lacks the enzymes to introduce double bonds beyond carbon nine; linoleic acid and alpha-linolenic acid must therefore be supplied in the diet.

A neutral lipid formed by esterifying glycerol with three fatty acid molecules; it is the most concentrated form of stored metabolic fuel and is transported in blood within chylomicrons and very low density lipoproteins.

A molecule carrying both a hydrophilic polar head group and a hydrophobic hydrocarbon tail, which causes it to orient at the boundary between water and fat; phospholipids and cholesterol are examples and form the bilayer of cell membranes.

An ion that bears both a positive and a negative charge at the same time, as amino acids do at their isoelectric point where the alpha-amino group is protonated and the alpha-carboxyl group is deprotonated.

An amide linkage formed by condensation between the alpha-carboxyl group of one amino acid and the alpha-amino group of the next, releasing water; it is planar and partially double-bonded, which restricts rotation in the polypeptide backbone.

The specific pH value at which a molecule such as an amino acid or protein carries no net electrical charge because positive and negative charges balance exactly; at this pH the molecule does not migrate in an electric field.

The exact linear sequence of amino acid residues joined by peptide bonds in a polypeptide chain, determined by the gene; this sequence dictates all higher levels of folding and therefore the final biological function of the protein.

Loss of the native three-dimensional conformation of a protein caused by heat, extremes of pH, urea or detergents, disrupting hydrogen bonds and hydrophobic interactions without breaking peptide bonds; the protein usually loses biological activity and solubility.

The monomer unit of nucleic acids, composed of a nitrogenous base, a five-carbon sugar and one or more phosphate groups; the base may be a purine or a pyrimidine, and the sugar is ribose in RNA and deoxyribose in DNA.

The covalent linkage in which a phosphate group joins the 3-hydroxyl of one sugar to the 5-hydroxyl of the next, forming the sugar-phosphate backbone of DNA and RNA and giving each strand a defined direction from five-prime to three-prime.

The specific hydrogen bonding between adenine and thymine or uracil, and between guanine and cytosine, that holds the two strands of the double helix together and ensures accurate copying of genetic information during replication and transcription.

A biological catalyst, almost always a protein and occasionally an RNA molecule, that accelerates a specific chemical reaction by lowering its activation energy without being consumed or permanently altered in the process.

The small region of an enzyme, formed by a few amino acid residues brought together by folding, where substrate binds through non-covalent forces and where catalysis takes place; its shape determines substrate specificity.

The substrate concentration at which an enzyme-catalysed reaction proceeds at exactly half of its maximum velocity; a low value indicates high affinity of the enzyme for its substrate, and it is denoted Km.

A reversible form of enzyme inhibition in which an inhibitor resembling the substrate binds the free active site, raising the apparent Michaelis constant while leaving maximum velocity unchanged because sufficiently high substrate displaces the inhibitor.

Control of enzyme activity by a small molecule binding at a site distinct from the active site, changing the enzyme's conformation and hence its affinity for substrate; such enzymes often show sigmoidal kinetics and are typical of committed steps in metabolism.

Rapid, reversible regulation of enzyme activity by addition or removal of a chemical group such as phosphate, catalysed by kinases and phosphatases; glycogen phosphorylase and glycogen synthase are controlled in this way in response to hormones.

The process in the inner mitochondrial membrane in which electrons passing down the respiratory chain drive proton pumping, and the resulting proton gradient powers ATP synthase to phosphorylate ADP into ATP.

An ordered series of membrane-bound carriers, including flavoproteins, iron-sulphur proteins, ubiquinone and cytochromes, that pass electrons from NADH and FADH2 to molecular oxygen, releasing energy used to pump protons.

The principle that the energy released by electron transport is stored as an electrochemical proton gradient across the inner mitochondrial membrane, and that the return of protons through ATP synthase drives ATP synthesis.

Synthesis of ATP by direct transfer of a phosphate group from a high-energy metabolic intermediate to ADP, without involvement of the respiratory chain; it occurs at two steps of glycolysis and one step of the citric acid cycle.

The cytoplasmic pathway that converts one molecule of glucose into two molecules of pyruvate through ten enzyme-catalysed steps, generating a net gain of two ATP and two NADH and functioning without oxygen.

The mitochondrial cyclic pathway in which acetyl coenzyme A condenses with oxaloacetate and is oxidised to two molecules of carbon dioxide, yielding reduced coenzymes and one GTP per turn; it is also called the Krebs or tricarboxylic acid cycle.

The synthesis of glucose from non-carbohydrate precursors such as lactate, glycerol and glucogenic amino acids, occurring mainly in liver and kidney and using four unique enzymes to bypass the irreversible steps of glycolysis.

The cyclic exchange in which exercising skeletal muscle converts glucose to lactate by anaerobic glycolysis, and the liver takes up the lactate and resynthesises glucose by gluconeogenesis for return to muscle.

A cytoplasmic alternative route of glucose metabolism, also called the pentose phosphate pathway, that produces NADPH for reductive synthesis and ribose-5-phosphate for nucleotide formation without generating ATP.

The mitochondrial process in which a fatty acyl coenzyme A is repeatedly cleaved two carbons at a time from its carboxyl end, each cycle producing one acetyl coenzyme A, one NADH and one FADH2.

Formation of acetoacetate, beta-hydroxybutyrate and acetone from acetyl coenzyme A in liver mitochondria, occurring when fatty acid oxidation is rapid and oxaloacetate is scarce; ketone bodies serve as fuel for extrahepatic tissues.

A spherical particle with a hydrophobic core of triacylglycerol and cholesteryl ester surrounded by phospholipid, free cholesterol and apolipoproteins, which transports lipids in plasma; chylomicrons, very low density, low density and high density lipoproteins are the main classes.

Transfer of an alpha-amino group from an amino acid to an alpha-keto acid, catalysed by aminotransferases with pyridoxal phosphate as coenzyme, producing a new amino acid and a new keto acid; alanine aminotransferase and aspartate aminotransferase are clinically important examples.

The hepatic pathway that converts the toxic ammonium ion into urea for excretion, combining ammonia with carbon dioxide and aspartate in five enzyme-catalysed steps; its overall equation yields one urea and one fumarate per turn.

The difference between the nitrogen taken in through dietary protein and the nitrogen lost in urine, faeces and sweat; a positive balance occurs in growth and pregnancy, a negative balance in starvation, trauma and wasting disease.

The mode of DNA duplication in which the double helix unwinds and each parental strand acts as a template, so every daughter molecule contains one old and one newly synthesised strand.

Synthesis of an RNA molecule from a DNA template, catalysed by RNA polymerase, in which the base sequence of one strand is copied into a complementary RNA transcript that may be messenger, ribosomal or transfer RNA.

The ribosomal process in which the nucleotide sequence of messenger RNA is read in codons and converted into a specific sequence of amino acids, with transfer RNA delivering residues and peptide bonds forming between them.

The entire process by which information in a gene is used to produce a functional product, usually a protein, controlled mainly at the level of transcription initiation and modulated by promoters, enhancers and repressor proteins.

A deficiency disease caused by lack of thiamine, vitamin B1, producing peripheral neuropathy with weakness and numbness in the dry form and heart failure with oedema in the wet form; it is common where polished rice is the staple.

The disease resulting from niacin deficiency, classically showing the three Ds of dermatitis on sun-exposed skin, diarrhoea and dementia; it occurs where maize is the staple food because its niacin is poorly available.

A small non-protein organic molecule that associates with an enzyme and is required for its catalytic activity, often derived from a vitamin; examples include NAD from niacin and pyridoxal phosphate from vitamin B6.

A polypeptide hormone secreted by the beta cells of the pancreatic islets in response to rising blood glucose, promoting uptake and storage of glucose, fat and protein and lowering blood glucose; its deficiency or inaction causes diabetes mellitus.

A polypeptide hormone from the alpha cells of the pancreatic islets, secreted when blood glucose falls, that raises glucose by stimulating hepatic glycogenolysis and gluconeogenesis and by promoting lipolysis and ketogenesis.

Haemoglobin to which glucose has become irreversibly attached non-enzymatically, reflecting average blood glucose over the preceding two to three months; it is measured as HbA1c and used to assess long-term diabetic control.

A metabolic emergency in which excessive production of ketone bodies lowers blood pH, seen most often in uncontrolled type 1 diabetes; it presents with hyperglycaemia, ketonuria, dehydration and deep rapid breathing.

A technique that separates charged molecules such as proteins in a buffer-soaked support under an applied electric field, their mobility depending on charge, size and shape; it is used for serum protein patterns and haemoglobin variants.

A highly branched polymer of glucose residues linked mainly by alpha-1,4 bonds with alpha-1,6 branches at intervals, stored in liver and skeletal muscle; liver glycogen buffers blood glucose while muscle glycogen supplies local fuel.

A member of a family of twenty-carbon cyclic fatty acid derivatives formed from arachidonic acid by cyclooxygenase, acting as a local hormone in inflammation, pain, fever and smooth muscle contraction.