The article is old but here is the excerpt....
Barley and Wheat
The most striking physical difference between barley and wheat is that wheat lacks barley’s familiar husk. The kernel is also a different shape — more rounded than barley, with a pronounced groove. Chart 1 illustrates the essential differences in composition. Wheat is significantly lower in cellulose and lipids, and higher in protein and starch than barley.
The varying protein level is the most significant difference. For one thing, wheat offers higher yields. Typical pale malt extracts run from 80 to 83 percent, while wheat malt extracts run from 84 to 88 percent.
The higher protein levels of wheat greatly enhance its contribution to foam stability - which means a better, longer-lasting head on the beer - and increase the beer’s mouthfeel (its sensation of fullness in the drinker’s mouth). The downside, and there always seems to be a downside, is that the higher protein levels can create problems in the brewhouse and make the beer hazy. Filtering out the haze can lessen the finished beer’s stability.
Not only are the levels of proteins different between barley and wheat, but the proteins themselves are different. Glutens, which are so critical to the formation of a good bread loaf, are virtually non-existent in barley yet make up 80 percent of wheat proteins. These high-molecular-weight proteins, which allow a loaf to stretch and then hold its shape, are the same proteins that can build a dense head in beer.