BI5.1-9 | Chemistry & Metabolism of Proteins and Immunology — Glossary

An organic molecule with an amino group (-NH2), a carboxyl group (-COOH), and a variable R-group (side chain) bonded to a central alpha-carbon; the building block of proteins

A molecule carrying both positive and negative charges simultaneously; amino acids exist as zwitterions at physiological pH 7.4 with -NH3+ and -COO-

A covalent bond formed by condensation between the carboxyl group of one amino acid and the amino group of another, with loss of water; rigid and planar due to partial double bond character

The linear sequence of amino acids in a polypeptide chain from N-terminus to C-terminus, encoded by DNA

A right-handed coiled secondary structure stabilised by hydrogen bonds between the C=O of residue n and the N-H of residue n+4, with 3.6 residues per turn

A secondary structure consisting of extended polypeptide strands (parallel or antiparallel) held together by inter-strand hydrogen bonds

The arrangement of two or more polypeptide subunits into a functional multi-subunit protein, stabilised by non-covalent interactions; e.g. haemoglobin (alpha2-beta2)

The most abundant protein in the body (~30% of total protein), a fibrous structural protein with Gly-X-Y repeating sequence forming a triple helix, providing tensile strength to bones, tendons, and skin

An endopeptidase active in the stomach (pH 1.5-2.5) that cleaves proteins at bonds next to aromatic amino acids; derived from pepsinogen activated by HCl

A pancreatic serine endopeptidase that cleaves after Arg and Lys residues; activated from trypsinogen by enterokinase; activates other pancreatic zymogens

The most abundant plasma protein (~60% of total, 3.5-5.0 g/dL), synthesised in the liver; maintains oncotic pressure and transports bilirubin, fatty acids, drugs, and hormones

A Y-shaped glycoprotein produced by B-cells/plasma cells, consisting of 2 heavy + 2 light chains with variable regions for antigen binding and constant regions for effector functions

The most abundant immunoglobulin class (75% of serum Ig); the only Ig that crosses the placenta; main antibody of the secondary immune response

The largest immunoglobulin (pentamer); the first antibody produced in a primary immune response; best complement activator; includes natural ABO blood group antibodies

The hepatic metabolic pathway that converts toxic ammonia to urea for renal excretion; consists of five enzymes starting with CPS-I (rate-limiting) in the mitochondria

The mitochondrial enzyme catalysing the rate-limiting step of the urea cycle: NH3 + CO2 + 2ATP → carbamoyl phosphate; activated by N-acetylglutamate

The transfer of an amino group from an amino acid to an alpha-keto acid, catalysed by aminotransferases (ALT, AST) using pyridoxal phosphate (B6) as cofactor

An autosomal recessive inborn error of amino acid metabolism caused by deficiency of phenylalanine hydroxylase, leading to phenylalanine accumulation and intellectual disability if untreated

A prosthetic group consisting of a porphyrin ring (protoporphyrin IX) with a central Fe2+ ion; found in haemoglobin, myoglobin, cytochromes, and catalase

The rate-limiting enzyme of haem synthesis that catalyses condensation of succinyl-CoA + glycine to form delta-aminolevulinic acid (ALA); requires pyridoxal phosphate (B6); inhibited by haem (feedback)

The yellow breakdown product of haem; unconjugated (indirect) bilirubin is lipid-soluble and albumin-bound; conjugated (direct) by UDP-glucuronyltransferase in the liver for biliary excretion

The oxygen-carrying protein in red blood cells; adult HbA is a tetramer (alpha2-beta2) with four haem groups, each capable of binding one O2 molecule

An autosomal recessive haemoglobinopathy caused by a Glu→Val substitution at position 6 of the beta-globin chain (HbS), causing RBC sickling upon deoxygenation

A group of inherited disorders of haemoglobin synthesis characterised by reduced or absent production of alpha or beta globin chains, leading to microcytic anaemia

Haemoglobin with glucose non-enzymatically attached to the N-terminus of beta chains; reflects average blood glucose over 2-3 months; used for diabetes monitoring (target <7%)