The document discusses different classes of α/β protein domains. The main classes are TIM barrel, Rossman fold, and horseshoe fold. The TIM barrel consists of an alpha-beta barrel with beta sheets forming the core. The Rossman fold has an open twisted beta sheet with helices on both sides. The horseshoe fold contains curved parallel beta sheets with external alpha helices. Many important enzymes use these α/β domain structures, including those involved in glycolysis and protein synthesis.

2. The most frequent of the domain structures are the
α/β domains
 a central parallel or mixed β sheet surrounded by
α helices
 binding crevices are formed by loop regions.
do not contribute to the structural stability of the
fold
but participate in binding and catalytic action.

3. CLASSES OF α/β PROTEINS
 All glycolytic enzymes are α/β structures
 3 main classes
 TIM BARREL
 ROSSMAN FOLD
 HORSE SHOE FOLD

4. TIM BARREL
 Barrel of beta sheets
 Surrounded by alpha helices
 Beta sheets form the core
 Eg: Triosephosphate isomerase

5. ROSSMAN FOLD
 Michel Rossman, 1970
 Lactate dehydrogenase
 Open twisted sheet with
helices on both sides

6. HORSESHOE FOLD
 Leucine rich motifs
 Curved parallel beta sheets
with
all alpha helices on the outside
 Eg: Ribonuclease inhibitor

7. ACTIVE SITE IN BARRELS
 Formed by loops at one end of the α/β barrel
 Active site
Situated in the bottom
Funnel-shaped pocket
Created by 8 loops
Connect the C terminal of beta sheets with N terminal of
alpha helices
(Ref toozen and branden page 53)

8. POSITIONS OF ACTIVE SITES
 General relationship b/w structure & function of
alpha/beta structures
 Active site @ same position- w r t their common
structure- inspite of having diff. functions & diff. aa
sequences
 Barrel structures – funnel shaped active site
 Open α/β structures- crevices at the edge of beta
sheets.
 Crevice- formed by 2 adjacent loop regions that
connect the 2 strands with alpha helices on opp.sides
of beta sheet.

9. Two β-α-β motifs can be joined into a
four-stranded parallel β sheet in two
different ways

10. EXAMPLES
 Some of the crucial enzymes formed by α/β structures
Flavodoxin- has binding site for FMN(flavin
mononucleotide)
Adenylate kinase- AMP binding site
Tyrosyl tRNA synthetase- protein synthesis
Carboxypeptidases- hydrolysis of polypeptides at C
terminal position
Arabinose binding protein-transport of sugars, aa,
ions…in Gram negative bacteria