Dairy Chemistry And Biochemistry

204 DAIRY CHEMISTRY AND BIOCHEMISTRY

(30-40 Res )

Binding sianal Of to the a ribophorins growing polypeptide

putatrve receptor of Removal of the signal

a transient

proteinaceous tunnel

Figure 4.32 Schematic representation of ribosomes attached to mRNA showing the growing
polypeptides and a proposed mechanism for cotranslational crossing of the RER membrane
(from Mercier and Gaye, 1983).

second for the appropriate tRNA. The specificity of the tRNAs is deter-
mined by the sequence of the anticodon which recognizes and hydrogen
bonds with the complementary codon of the mRNA. Interaction between
the tRNA and the appropriate amino acid occurs in the cytoplasm but the
remaining reactions in protein synthesis occur in the ribosomes, which are
complex structures of rRNA and a number of proteins (including enzymes,
initiators and controlling factors). The ribosomes of animal cells have
diameters of about 22nm and a sedimentation coefficient of 80s; they
consist of two principal subunits: 60s and 40s. mRNA passes through a
groove or tunnel between the 60s and 40s subunits; while in the groove,
mRNA is protected from the action of ribonuclease (Figure 4.32).
The information for the amino acid sequence is contained in the mRNA.
Synthesis commences at the correct codon of the mRNA because a special
amino acid derivative, N-formyl methionine:

H

c=o

NH

I

I

I

I

H,CSCH,CH$-COOH

H

is bound to a specific special codon and it forms the temporary N-terminal
residue of the protein; N-formyl methionine is later hydrolysed off, together