Re: What is the chemistry between vinegar, sugar, and rice in sushi rice?

Let's break this into three questions with answers following:
1) When vinegar is first added to warm rice, it gets more slippery  and 
then it sticks up when it cools.  Is it the proteins being  denatured in 
rice with a H+ catalyst from vinegar?  

No.  Good speculation but, I believe the major cause of slipperiness is 
simply the dilute water-starch mix at the cooked rice surface. Think paste. 
 Did you ever make flour paste?  The water and flour are quite slippery 
when dilute. Try heating corn starch with 3 volumes of water; it makes a 
dandy lubricant - until it dries a bit; then it becomes sticky - just like 
sushi rice.  
Among our chemists, there was some speculation that the vinegar could cause 
 acid hydrolysis of starch producing slippier pectins and oligiosaccharides 
(but I doubt it'd be significant). 
I think the role of the vinegar is primarily for flavor and as a spoilage 
retardant.  For instance, the "tezu" used to keep the rice from sticking to 
unwanted parts is made from 1 cup of water with 2 tablespoons of rice 
vinegar and ½ teaspoon of salt.  That's a 1:8 ratio of vinegar to water -- 
pretty dilute vinegar.

2) Why is it  necessary to add sugar?
It's not according to some of the web pages I checked.  Again, I believe 
the sugar is simply for flavor.  The combination of sweet and sour (even 
dilute) is popular in many cuisines. 

3) why does the short rice coated with glucose provide stickier results 
than with brown or long grain? 
I dunno the "why" but short grained rice is more glutenous than longer 
grain rice.  I suspect it's a surface to volume ratio phenomenon.  For 
brown rice, the bran is highly linked starch e.g. cellulose.  The bran 
doesn't hydrate and thus, doesn't become slippery when wet nor sticky when 
dry. 

Here's a few web pages I ran into researching your question: 

http://sushiref.com/glossary/sushi:sushi.html 
http://ftp.neosoft.com/recipes/rice/sushi-rice02.html http://www.stickyrice.com/html/rice.html http://ociialf.org.chemie.tu-muenchen.de/people/ralph/cooking/Sushi.html http://www.netscum.com/~spider/food/sushi.html