section 22.3

Endocrine Pancreas and Pancreatic Hormones

491

F I G U R E 2 2 - 6

Structures of human proinsulin and insulin. Insulin is derived from proinsulin by cleavage at the dipeptides Arg-Arg and

Lys-Arg to give A and B chains held together by disulfide bonds. In the pig, B30 is Ala. In the cow, A8 is Ala, A10 is

Val, and B30 is Ala. Bovine and porcine insulins are used extensively in clinical practice.

insulin

and has the same biological activity as normal

human insulin. However, unlike normal human insulin,

lispro insulin has a reduced capacity for self-association

and thus begins its biological activity within 15 minutes

of subcutaneous administration; it has become the insulin

of choice in many patients with diabetes (discussed later).

Insulin biosynthesis resembles that of other export pro-

teins: gene transcription, processing and maturation of

precursor mRNA, translation of mature RNA, translo-

cation of preproinsulin to cisternae of the endoplasmic

reticulum with removal of the N-terminal leader or signal

sequence of 23 amino acids, folding of proinsulin and for-

mation of the proper disulfide bridges, packaging of proin-

sulin into secretory granules, and conversion of proinsulin

to equimolar amounts of insulin and connecting peptide

(C-peptide) by site-specific enzymatic cleavages. The hu-

man insulin gene is located on the short arm of chromo-

some 11 and consists of a 5' untranslated region split by an

intron; an exon that codes for the leader peptide; a region

coding for the B chain; a region coding for C-peptide that

is split by an intron; a region coding for the A chain; and

a 3' untranslated region.

Conversion of proinsulin to insulin and C-peptide in

secretory granules involves site-specific cleavages at the

Arg-Arg and Lys-Arg sequences (Figure 22-6); these

serve as signals for proteolytic processing of many

other proteins. Cleavage occurs at the C-terminal end

of each pair by trypsin-like enzymes and is followed by

removal of the basic residues by a carboxypeptidase B-like

enzyme.

Insulin monomers undergo noncovalent dimerization by

formation of antiparallel /f-plcated sheet associations be-

tween monomers involving the C-terminal portion of the

B chain. As discussed earlier, lispro insulin, in which the

B28 and B29 is reversed from the normal prolyl and lysyl

sequence, does not dimerize. Three insulin dimers subse-

quently self-associate to form hexamers in the presence of

Zn2+. The Zn2+ hexameric array of insulin probably gives

the /

1

-cell granule its unusual morphologic characteristics.

Zn2+ is released when insulin is secreted. Conversion

of proinsulin to insulin in the secretory granule is not com-

plete and some proinsulin is also released upon secretion

of insulin. Proinsulin has less than

5%

of the biological

activity of insulin. The C-peptide has no physiological

function but assay of C-peptide helps distinguish between

endogenous and exogenous sources of insulin.

Like genes for other proteins, the insulin gene may un-

dergo mutation and produce an abnormal product. This

process may be suspected in individuals who exhibit

hyperinsulinemia

without hypoglycemia or evidence of

insulin resistance. Three abnormal insulins have been doc-

umented: one in which Ser replaces Phe at B24, another

in which Leu replaces Phe at B25, and a third in which

the Lys-Arg basic amino acid pair is replaced by Lys-X

(X = nonbasic amino acid). The former two mutations oc-

cur in the nucleotide sequence that codes for an invariant