Dictionary of Chemistry [6th Ed.]

pentlanditeA mineral consisting

of a mixed iron–nickel sulphide,

(Fe,Ni) 9 S 8 , crystallizing in the cubic

system; the chief ore of nickel. It is

yellowish-bronze in colour with a

metallic lustre. The chief occurrence

of the mineral is at Sudbury in On-

tario, Canada.

pentoseA sugar that hasÜve car-

bon atoms per molecule. See mono-

saccharide.

pentose phosphate pathway

(pentose shunt)A series of biochemi-

cal reactions that results in the con-

version of glucose 6-phosphate to

ribose 5-phosphate and generates

NADPH, which provides reducing

power for other metabolic reactions,

such as synthesis of fatty acids. Ri-

bose 5-phosphate and its derivatives

are components of such molecules as

ATP, coenzyme A, NAD, FAD, DNA,

and RNA. In plants the pentose phos-

phate pathway also plays a role in

the synthesis of sugars from carbon

dioxide. In animals the pathway oc-

curs at various sites, including the

liver and adipose tissue.

pentyl group (pentyl radical)The

organic group CH 3 CH 2 CH 2 CH 2 CH 2 –,

derived from pentane.

pepsinAn enzyme that catalyses

the breakdown of proteins to

polypeptides in the vertebrate stom-

ach. It is secreted as an inactive pre-

cursor, pepsinogen.

peptideAny of a group of organic

compounds comprising two or more

amino acids linked by peptide bonds.

These bonds are formed by the reac-

tion between adjacent carboxyl

(–COOH) and amino (–NH 2 ) groups

with the elimination of water (see il-

lustration). Dipeptidescontain two

amino acids, tripeptidesthree, and so

on. *Polypeptides contain more than

ten and usually 100–300. Naturally

occurring oligopeptides(of less than

ten amino acids) include the tripep-

tide glutathione and the pituitary

hormones vasopressin and oxytocin,

which are octapeptides. Peptides also

result from protein breakdown, e.g.

during digestion.

A

• Information about IUPAC nomenclature
  of peptides

peptide mapping (peptideÜnger-

printing)The technique of forming

two-dimensional patterns of peptides

(on paper or gel) by partial hydrolysis

of a protein followed by electro-

phoresis and chromatography. The

peptide pattern (or Üngerprint) pro-

duced is characteristic for a particu-

lar protein and the technique can be

used to separate a mixture of pep-

tides.

peptidoglycanA macromolecule

that is a component of the cell wall

of bacteria; it is not found in eukary-

otes. Consisting of chains of amino

sugars (N-acetylglucosamine and N-

acetylmuramic acid) linked to a

tripeptide (of alanine, glutamic acid,

and lysine or diaminopimelic acid), it

confers strength and shape to the

cell wall.

per- PreÜx indicating that a chemi-

pentlandite 402

p

amino acid 1 amino acid 2 dipeptide

peptide bond

HR O

HNCCOH

H

+

HR′ O

HNCC

H

OH

HR O

HNCC

H

HR′ O

NC C

H

OH+H 2 O

water

Peptide bond